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175 Cards in this Set
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Enthalpy Changes What happens when chemicals react to form new substances? |
Bonds break in the reactants and new bonds are formed as products are made. This process changes the chemical energy of the atoms. |
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Enthalpy Changes What is Enthalpy? |
H, the heat content stored in a chemical system. |
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Enthalpy Changes How can you measure enthalpy? |
It is impossible to measure the enthalpy of the reactants or products directly. Instead we measure the energy absorbed and released into the surroundings during a chemical change. |
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Enthalpy Changes Heat loss in a chemical system = ? |
Heat gain to surroundings (increase in temp) |
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Enthalpy Changes Heat gain in a chemical system? |
Heat loss from surroundings (Temperature decrease) |
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Enthalpy Changes What is ΔH? |
Enthalpy change: the heat exchange with the surroundings during a chemical reaction, at a constant pressure; the difference between the Δ products and Δ reactants
Δ H = H products - H reactants |
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Enthalpy Changes What is an exothermic reaction? |
A reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings
^H = -ve |
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Enthalpy Changes Draw a graph showing the enthalpy changes in an exothermic reaction |
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Enthalpy Changes What is an endothermic reaction? |
A reaction in which the enthalpy of the products is greater than the enthalpy of the reactions, resulting in heat gain from the surroundings. ^H= +ve |
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Enthalpy Changes Draw a graph showing the enthalpy changes in an exothermic reaction |
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Enthalpy Changes What are the two exothermic reaction examples you need to know? |
Respiration Oxidation of fuels |
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Enthalpy Changes Explain the oxidation of methane |
A common example of the oxidation of fuels for combustion is the oxidation of Methane to CO2 and H2O. The products have less enthalpy than the reactants so it is exothermic and excess energy is released into the surroundings as heat. |
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Enthalpy Changes Write the equation for the combustion of methane |
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Enthalpy Changes Why is the oxidation of fuels important? |
We rely on the combustion of petrol and diesel in cars, buses and trains and the combustion of kerosene in aircraft. |
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Enthalpy Changes Explain what happens in respiration and give the full equation |
Sugars such as glucose are oxidised to CO2 + H20 C6H12O6 (aq) + 6O2 (g) -> 6CO2 (g) + 6H2O (l) |
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Enthalpy Changes Why is respiration important ? |
Without respiration, there would be no life. |
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Enthalpy Changes What are the two examples of endothermic reactions? |
Thermal decomposition of limestone Photosynthesis |
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Enthalpy Changes What does limestone contain? |
CaCO3 |
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Enthalpy Changes
Explain the thermal decomposition of CaCO3. Include the reaction. |
The decomposition of CaCO3 by heat is an important endothermic reaction used to make CaO (lime) |
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Enthalpy Changes Uses of lime? |
manufacture of cement |
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Enthalpy Changes Explain what occurs in photosynthesis - include the reaction. |
Sugars such as glucose are made from CO2 + H20. Light from the sun provides the energy for photosynthesis |
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Enthalpy Changes What is the importance of photosynthesis? |
Without it, there would be no life. |
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Enthalpy Changes What do endothermic processes require? |
A heat energy input |
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Enthalpy Changes What is an enthalpy profile diagram? |
A diagram for a reaction to compare the enthalpy of the reactants and products |
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Enthalpy Changes Draw the enthalpy profile diagrams for an endothermic and exothermic reaction. |
Exo = H prod < H reac = Heat loss to surroundings Endo = H prod > H reac = Heat gain from surroundings |
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Enthalpy Changes What is activation energy? |
The minimum energy required to start a reaction by the breaking of bonds |
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Enthalpy Changes Draw enthalpy profile diagrams for exothermic and endothermic reactions with activation energy. |
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Enthalpy Changes Define standard conditions |
A pressure of 100kPa (1atm), a stated temperature (usually 298K = 25 degrees C) and a concentration of 1moldm-3 (for reactions with aqueous solutions) |
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Enthalpy Changes For a standard enthalpy change, what must a substance be in? Define. |
Standard state- the physical state of a substance under the standard conditions of 1atm and 25 degrees C |
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Enthalpy Changes Define standard enthalpy change of reaction. |
^Hr* - the enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states.
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Enthalpy Changes Define standard enthalpy change of combustion |
^Hc* 0 the enthalpy change that takes place when 1 mol of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states
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Enthalpy Changes Define standard enthalpy change of formation |
^Hf* 0 the enthalpy change that takes place when one mol of a compound is formed from its constituent elements in their standard states under standard conditions.
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Enthalpy Changes
Write ^Hc* for ethane |
C2H6 (g) + 3 1/2 O2 (g) ---> 2CO2 (g) + 3H2O (l) -ve. |
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Enthalpy Changes Write the ^Hf* for water |
H2 (g) + 1/2 O2 (g) ---> H2O (l)
0ve. |
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Enthalpy Changes What is the ^Hf* for an element ? |
0kj mol-1 Always. You're forming an element from an element so there is no chemical change. |
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Enthalpy Changes How can you determine enthalpy? |
You cannot determine the enthalpy of reactants or products directly, but determine the heat exchange with surroundings.
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Enthalpy Changes What do you need to know to determine the heat exchange during a reaction? |
M = mass of surroundings involved in the heat exchange C = specific heat capacity of surroundings ^T = change in temperature of surroundings (Tfinal - Tinitial) |
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Enthalpy Changes Define specific heat capacity |
c,
The energy required to raise the temperature of 1g of a substance by 1 degree |
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Enthalpy Changes How do you calculate heat exchanged with the surroundings? |
Q = mc^T |
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Enthalpy Changes An excess of Mg is added to 100cm3 of 2mol dm-3 CuSO4 (aq). Temperature increases from 20->65. Find the enthalpy change. Mg (s) + CuSO4 (aq) -> MgSO4 (aq) + Cu (s) SHC = 4.18 Jg-k-1 Density of solution= 1gcm-3 |
Q=mc^T mass = 100 x1 = 100g. C = 4.18 Change in T = 65 -20 = 45 100 x 4.18 x 45 = -18,810 100/1000 x 2 = for 0.2 mols CuSO4. x5 = 1 mol. 18,810 x 5 = 94,050= -94.05 kJ mol-1 |
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Enthalpy Changes How can you determine ^Hc for a liquid fuel experimentally? |
- Burn a known mass of substance in air - Heat a known mass of water - Measure the temperature change of the water |
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Enthalpy Changes
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1) measure a volume of water into a beaker 2) 1cm-3 water = 1g 3) Weigh spirit burner and the fuel 4) Measure the initial temperature of the water 5) Light the burner and heat the water until the temperature of the water has risen a reasonable amount. measure the final temperature of the water. 6) Extinguish flame and re-weigh burner. |
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Enthalpy Changes During combustion, 1.5g of propan-1-ol heated 250 cm 3 water by 45 degrees. Find ^Hc* for propan-1-ol. SHC = 4.18Jg-1k-1 Density = Igcm-3 |
q=mc^t m = 250 c= 4.18 t = 45 250x45x4.18 = 47,025 1.5g propan 1-ol = ? mols? CH3CH2CH2OH = 12+3+12+2+12+2+16+1 = 60gmol-1 1.5/60 = 0.025 mols 1/0.025 = 40 so 47,025 x 40 = change for 1 mol= 1,881,000 -1881 kJmol-1 |
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Enthalpy Changes Why might an experimental value for ^Hc* differ from a data book? |
X may have been incomplete combustion X may have been heat loss to surroundings |
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Enthalpy Changes How can you improve experimental results? |
Use better apparatus that - cuts down on heat loss - ensures complete combustion |
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Enthalpy Changes Define bond enthalpy |
The enthalpy change that takes place when 1 mol of a given bond in the molecules of a gaseous species is broken by homolytic fission. |
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Enthalpy Changes Energy is need to... .... makes energy. |
Break bonds (ENDO)
Making bonds (EXO) |
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Enthalpy Changes Define average bond enthalpy |
The average enthalpy change that takes place when breaking 1 mol of a given type of bond in a molecule of a gaseous species by homolytic fission i.e H-C bonds occur in many different molecules and the strength of this bond varies depending on its environment - so an avg is calculated. |
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Enthalpy Changes In an exothermic reaction, the bonds FORMED are _______ than bonds BROKEN |
STRONGER |
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Enthalpy Changes In an endothermic reaction, the bonds FORMED are _________ than the bonds BROKEN |
WEAKER |
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Enthalpy Changes Calculate ^H from CH4 (g) + 2O2 (g) --> CO2 (g) + 2H2O (g) C-H = +413 O=O = +497 C=O = +805 O-H= +463 |
Bonds broken: CH4 + 2O2 C-H x 4 = (413 x 4) O=O x 2 = (497 x 2) Bonds formed: CO2 + 2H2O C=O x 2 ( 805 x 2) H-O x 4 (463 x 4) Sum of (bonds broken) - (bonds formed) = -816kJ mol-1 |
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Enthalpy Changes Why might you not be able to measure ^H directly? |
-High Ea -Slow rate -More than one reaction taking place |
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Enthalpy Changes What is Hess' law? |
Hess' law states that if a reaction can take place by more than one rout and the initial and final conditions are the same, the total enthalpy change is the same for each route. |
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Enthalpy Changes What should a hess cycle look like when using values of ^Hc*? |
Reactants -----------------> Products \ / Combustion products (Arrows down) |
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Enthalpy Changes What should a hess cycle look like when using values of ^Hf*? |
Reactants ---------------------> products \ / Elements (arrows up) |
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Rates and Equilibrium What is the rate of reaction? |
The change in concentration of a reactant or a product in a given time. rate = Change in conc/time units = mol dm-3/s = moldm-3s-1 |
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Rates and Equilibrium Describe what happens as a reaction progresses |
- At the start of the reaction, each reactant has its greatest concentration. The rate of the reaction is at its fastest. - As the reaction proceeds, the concentrations of the reactants decrease - the rate of reaction slows down - When one of the reactants is used up the rate becomes 0 and the reaction stops. |
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Rates and Equilibrium What can alter the rate of reaction? |
-Catalysts -Temperature -Concentrations -Pressure (when reactants are gaseous) -Surface area |
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Rates and Equilibrium Simply, what is collision theory? |
The statement that a chemical reaction can only take place when the reacting molecules collide. |
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Rates and Equilibrium What are the two conditions required for a reaction to occur? |
-MOLECULES MUST HAVE SUFFICIENT ENERGY TO OVERCOME THE ACTIVATION ENERGY OF THE REACTION. - MOLECULES MUST ALSO COLLIDE IN THE CORRECT ORIENTATION |
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Rates and Equilibrium Describe the effect of concentration on reaction rate |
If the concentrations of the reactants are increase, the rate of reaction also increases. |
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Rates and Equilibrium Explain how concentration effects the rate |
-Increased concentration = more molecules in the same volume -The molecules will be closer together and there is a greater chance of the molecules colliding - Collisions are more frequent so there are more with energy greater than the activation energy. |
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Rates and Equilibrium Describe the effect of pressure on reaction rate |
Increase in pressure of a gas = increased rate of reaction |
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Rates and Equilibrium Explain the effect of pressure on the reaction rate |
-Molecules are pushed closer together = the same number of molecules occupy a smaller volume - More collisions occur and more occur with energy greater than the activation energy. More frequent collisions = increased rate. |
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Rates and Equilibrium Define catalyst |
A substance that increases the rate of a chemical reaction without being used up |
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Rates and Equilibrium How do catalysts work? Draw a reaction pathway diagram to demonstrate this. |
The lower the activation energy of the reaction by providing an alternative route for the reaction to follow - the alternative route has a lower energy. |
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Rates and Equilibrium Why are catalysts used in industry? |
-To lower costs - they speed up the process by lowering the activation energy, so less energy is required for molecules to react. -Then, if less energy is required, less fossil fuels are burnt so less CO2 is released into the atmosphere during energy production. -Some catalysts increase the percentage yield of an industrial preparation. -Catalysts can enable different reactions to be used, with better atom economy and with reduced waste. |
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Rates and Equilibrium What are two examples of reactions that require catalysts? |
Haber Process (production of ammonia) Zeigler- Natta process |
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Rates and Equilibrium Describe how catalysts in the Haber process |
N2 (g) + 3H2 (g) ↔ 2NH3 (g) Ammonia is made by reacting Nitrogen and Hydrogen. The triple Nitrogen-nitrogen bond has to be broken, requiring a large energy input resulting in a large Activation energy. Fe is used to catalyse this reaction weakening the N-N bond, lowering the activation energy. |
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Rates and Equilibrium What is uses of ammonia? |
- Used as the basis of fertiliser manufacture, improving crop yields to feed an increasing population |
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Rates and Equilibrium Describe how catalysts are used in cars |
Catalytic converters with platinum/palladium/rhodium improve air quality by reducing toxic emissions from vehicles and preventing photochemical smog. |
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Rates and Equilibrium
Describe what a biological catalyst is. |
Enzymes - large protein molecules that catalyse the reactions of large biological molecules in short periods of time. They operate under mild conditions, such as low temperatures, atmospheric pressure and at an optimum pH value. |
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Rates and Equilibrium What is the importance of the ziegler-natta catlayst? |
Produces the majority of unbranched poly(ethene) |
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Rates and Equilibrium Benefits of using enzymes in industry? |
:) Lower temperatures and pressures can be used than with conventional inorganic catalysts, saving energy and costs :) Enzymes allow a reaction to take place which forms PURE products with no side-reactions. They often remove the need for complex separation techniques, thereby reducing costs. :) Conventional catalysts are often poisonous and can pose disposal problems at the end of their industrial life - enzymes however, are BIODEGRADABLE. |
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Rates and Equilibrium What industrial processes use enzymes? |
-food and drink production -Production of dairy, alcoholic drinks, fruit juices - production of detergents, cleaning agents -Ibuprofen |
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Rates and Equilibrium What is the Boltzmann distribution? |
The distribution of energies of molecules at a particular temperature, often shown as a graph |
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Rates and Equilibrium Draw a Boltzmann distribution graph in a gas at constant temperature. |
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Rates and Equilibrium Explain the important features of the Boltzmann distribution |
- The area under the curve is equal to the total number of molecules in the sample. The area does not change with the conditions. - There are no molecules with 0 energy - the curve starts at the origin - There is no maximum energy for a molecule - the curve never touches the energy axis - Only the molecules with an energy greater than the activation energy are able to react. |
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Rates and Equilibrium Draw a Boltzmann distribution curve to show the effect of temperature on the reaction rate |
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Rates and Equilibrium Explain the effect of temperature on reaction rate |
At higher temperatures, the kinetic energy of all the molecules increases. The Boltzmann distribution flattens and shifts to the right. The number of molecules in the system does not change and the area under the curve remains the same. |
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Rates and Equilibrium Why does the rate increase as temperature increases? |
- More collisions take place in a certain length of time -the molecules are moving faster and have more kinetic energy - A higher proportion of molecules have an energy that is greater than the activation energy - more collisions will lead to a chemical reaction - THUS more molecules will overcome the activation energy and there are MORE successful collisions in a certain length of time. |
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Rates and Equilibrium Draw a Boltzmann distribution showing the effect of a catalyst. |
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Rates and Equilibrium Explain how catalysts affect the rate |
Reduces the activation energy so more molecules in the system will overcome the new lower activation energy for the reaction. There are more successful collisions in a length of time and rate will increase. |
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Rates and Equilibrium Describe how the reaction of Magnesium with Sulfuric acid is irreversible. |
Mg (s) + H2SO4 (aq) ---> MgSO4 (aq) + H2 (g) Reactive metal + excess acid --> salt + H2 The reaction stops when all metal has reacted. It has gone to completion '--->' |
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Rates and Equilibrium Describe how the reaction between SO2 and O2 is reversible. |
2SO2 (g) + O2 (g) ↔ 2SO3 (g) It is reversible - at equilibrium '↔' |
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Rates and Equilibrium Define dynamic equilibrium
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The equilibrium that exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction |
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Rates and Equilibrium When is a system in dynamic equilibrium? |
-The concentrations of the reactants and the products remain the same -The rate of the forward reaction is the same as the rate of the reverse reaction |
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Rates and Equilibrium What are the conditions for dynamic equilibrium to exist? |
SYSTEM MUST REMAIN ISOLATED - no materials added or taken away and NO external conditions such as temperature or pressure are being altered. |
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Rates and Equilibrium What can change the position of equilibrium? |
- Concentrations of reactants and products - Pressure in reactions involving gases - temperature |
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Rates and Equilibrium What is le Chatelier's principle? |
states that when a system is in dynamic equilibrium is subjected to a change, the position of equilibrium will shift to minimise the change |
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Rates and Equilibrium CH3CH2OH (l) + CH3CH2COOH (l) ↔ CH3CH2COOCH2CH3 (l) + H2O (l) What would effect of increasing the concentration of a reactant do? |
Cause the equilibrium to shift in the direction that DECREASES this increased reaction concentration - The system opposes the change by decreasing the concentration of the reactant by removing it - The position of the equilibrium shifts to the right hand side- forming more products. |
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Rates and Equilibrium CH3CH2OH (l) + CH3CH2COOH (l) ↔ CH3CH2COOCH2CH3 (l) + H2O (l) What would effect of increasing the concentration of a product do? |
The equilibrium position will move in the direction that decreases this increased product concentration -The system opposes the change by decreasing the concentration of the product by removing it - The equilibrium shifts to the left-hand side, forming more reactants. |
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Rates and Equilibrium When will changing the total pressure of a system change the position of equilibrium? |
If gases are present. |
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Rates and Equilibrium Describe the pressure of this: N2 (g) + 3H2 (g) ↔ 2NH3 (g) |
There are 4 moles of gas on the left hand side and 2 on the right : the side with the greater moels = highest pressure |
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Rates and Equilibrium What is the effect of increasing the total pressure? |
The position of equilibrium will move to the side with fewer gas molecules, as this will decrease the pressure. In this equation, it will move to the RIGHT. |
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Rates and Equilibrium What will decreasing the total pressure do? |
Cause the position of equilibrium to move to the side with the greater number of gas molecules, as this will increase the pressure. In this equation, it will move to the LEFT. |
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Rates and Equilibrium What does the effect of changing the temperature depend on? |
The enthalpy sign. |
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Rates and Equilibrium N2 + 3H2 ↔ 2NH3 ^H = -92 kJ mol- 1 Forward reaction =? Reverse reaction = ? |
Exothermic Endothermic |
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Rates and Equilibrium for this equation, what is the effect of increasing the temperature? |
Causes the position of equilibrium to move in the direction that decreases the temperature -The system opposes the change by taking in heat and the position of equilibrium moves to the LEFT -THE POSITION OF EQUILIBRIUM MOVES IN THE ENDOTHERMIC (^H +VE) DIRECTION |
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Rates and Equilibrium for this equation, what is the effect of DECREASING the temperature? |
Causes the position of equilibrium to move in the direction that increases the temperature -The system opposes the change by releasing heat and the position of equilibrium moves to the RIGHT -THE POSITION OF EQUILIBRIUM MOVES IN THE EXOTHERMIC (^H -VE) DIRECTION |
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Rates and Equilibrium What is the effect of a catalyst on the position of equilibrium? |
A catalyst does NOT alter the position of equilibrium or the composition of an equilibrium -It speeds up the rate of the forward and reverse reactions EQUALLY -A catalyst increases the rate at which equilibrium is established |
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Rates and Equilibrium What are two examples of chemical processes that exist as equilibrium systems? |
- The preparation of ammonia from nitrogen and hydrogen in the Haber process -The conversion of sulfer dioxide into sulfur trioxide in the Contact process |
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Rates and Equilibrium What do chemists strive for in industry? |
Highest yield of desired product possible as quickly and cheaply as possible. |
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Rates and Equilibrium Describe the Haber process |
Nitrogen is obtained from the air by fractional distillation, Hydrogen is prepared by reacting together methane from natural gas and water. |
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Rates and Equilibrium What conditions favour ammonia production? |
Ammonia is produced by the forward reaction. - The forward reaction produces FEWER gas molecules (4 -> 2), so a HIGH PRESSURE is favoured - The forward reaction is EXOTHERMIC, so a LOW TEMPERATURE is favoured |
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Rates and Equilibrium What are the drawbacks? |
- Although a low temp should produce a high equilibrium yield, the reaction would take place at a very low rate. At low temps, few N2 and H2 molecules have energy equal to or greater than the required activation energy - A higher pressure increases the concentration of the gases, increasing the rate. So a high pressure should produce a high equilib yield and rate. HOWEVER, large quantities of energy are required to compress gases, adding significantly to the cost of running the process. There are also safety implications - any failure in the systems could potentially allow chemicals to leak into the environment, endangering workers on site. |
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Rates and Equilibrium What does a modern ammonia plant need? |
To produce sufficient yield of ammonia at a reasonable cost and in as short as time as possible. |
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Rates and Equilibrium What compromise is made? |
- Temperature : must be high enough to allow the reaction to proceed at a realistic rate, whilst still producing an acceptable equilb yield. 400-500 IS TYPICALLY USED -Pressure: A high pressure must still be used, but not must be so high that workers are in danger or the environment threatened. 200atm. - Catlayst: Iron catalyst used to speedup the rate, allowing equilibrium to be established faster and for lower temperatures to be used. Less energy is used to generate heat, reducing costs. |
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Rates and Equilibrium What does this compromise ultimately achieve? |
conversion of 15% nitrogen and hydrogen into ammonia. Ammonia produced is then liquefied and removed. Unreacted Nitrogen and Hydrogen gases are passed through the reactor again, until virtually all N2 and H2 has been converted to ammonia. |
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Resources Define the greenhouse effect |
The process in which the absorption and subsequent emission of infrared radiation by atmospheric gases warms the lower atmosphere and the planet's surface. |
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Resources Describe how greenhouse gases exist naturally in the atmosphere |
Water vapour - from evaporation of lakes and oceans is the most abundant greenhouse gas CO2 is produced by many natural processes -volcanic eruptions -respiration of animals -burning or decay of organic matter such as plants Methane (although there are smaller quantities than CO2 it contributes more to the greenhouse effect). -emitted during the production of coal, natural gas and oil - a product of rotting organic waste in landfill sits -released from certain animals as a by-product of digestion |
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Resources Describe how CO2 absorbs IR. |
CO2 is a linear molecule O=C=O. It can absorb IR, causing the molecule to vibrate. Eventually the vibrating molecule emits some of this energy in the form of radiation. This can then be absorbed by another greenhouse gas molecule or at the earth's surface. |
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Resources What bonds absorb IR in a)H2O b)CH4 ? |
a) O-H b) C-H |
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Resources What does the greenhouse effect of a gas depend on? |
The concentration of the gas in the atmosphere and it's ability to absorb IR. |
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Resources What is GWP? |
Global Warming Potential - the ability of a trace gas to cause global warming (lifetime of a gas in the atmosphere and ability of the gas to absorb IR.) |
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Resources What are the effects of global warming? |
- Ice caps melting = rivers overflow due to excessively heavy rainfall and melting glaciers. -Droughts - water shortages - Tropical areas experience more frequent and destructing storms and floods - Glaciers are disappearing and there are more extreme heat waves -Some areas experience high crop yields, some drought and disease = disastrous harvests -More extreme storms and hurricanes -Increasing temperatures expand the water in the oceans causing sea levels to rise |
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Resources Solutions to minimise global warming? |
International agreement to reduce greenhouse gas emissions Development of non-carbon fuels - wind, solar, tidal and nuclear power Renewable sources of energy Biofuels |
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Resources What is the function of Carbon capture and storage? |
Captures CO2 from power stations and stores it away safely, instead of it being released into the atmosphere. It is an immediate strategy to get rid of CO2. |
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Resources Where can we store unwanted gases? |
Underground porous rock can act as a sponge to store CO2 and keep it from leaking away Old oil and gas fields are some of the best natural containers for CO2 |
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Resources How is the oxidation of methane changed? |
A decarbonised fuel is produced by reforming natural gas into a mixture of H2 and CO2: CH4(g) + 2H2O (G) ---> CO2(g) + 4H2 (g)
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Resources How else can you store carbon? |
In stable minerals. CO2 is trapped by conversion into a carbonate rock. CO2 is reacted with metal oxides: CaO (s) + CO2 (g) ---> CaCO3 (s) This occurs naturally and has produced much of limestone rock as CaCO3. HOWEVER, the natural process is very slow so efforts are being made to increase the rate, but is very energy intensive. |
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Resources What is the role of chemists? |
- to provide scientific evidence to governments to verify that global warming is taking place -investigate solutions to environmental problems, such as carbon capture and storage. |
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Resources What is the Kyoto protocol? |
International agreement that dangerous climate change must be prevented. Chemists monitor the progress of this. |
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Resources What is Ozone? |
O3 - a molecule consisting of three oxygen atoms. |
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Resources When is Ozone a) bad b) good? |
a) ozone near to the Earth's surface in the troposphere is an air pollutant with harmful effects on the respiratory systems of animals b) ozone in the upper atmosphere in the stratosphere protects living organisms by preventing harmful UV light from reaching the Earth's surface |
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Resources What is the trophosphere? |
The lowest layer of the Earth's atmosphere, extending from the Earth's surface up to about 7km (above the poles) and to about 20km (above the tropics) |
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Resources What is the stratosphere? |
The second layer of the Earth's atmosphere, containing the 'ozone layer' about 10km to 50km above the Earth's surface |
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Resources Where is the ozone layer found? |
The Stratosphere |
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Resources What is the function of the ozone layer? |
UV radiation has wavelengths of 270-400nm: the ozone layer filters out the shorter wavelengths. ie. less than 320 nm. These shorter wavelengths would be very damaging to life. |
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Why is the ozone layer hotter than the parts of the upper atmosphere? |
Because it converts UV radiation to heat. |
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Resources What does the action of UV do to ozone? |
Cause it to be continuously formed and broken down in the stratosphere .
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Resources What are the three types of UV radiation? |
- Uv a 320-400 nm - less energy than the shorter wavelengths and is not as damaging, but reaches the Earth surface (5% absorbed) - UV b - 280-320 nm - can cause sunburn and genetic damage which can result in skin cancer (absorbed 95% by ozone) - UV c - 200-280 nm - 100% entirely absorbed by ozone (most harmful) |
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Resources What is the effect of decrease in the ozone layer? |
More UV-b (and UV c) will reach the Earth's surface = increased genetic damage to living organisms. (skin cancer + sunburn) |
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Resources What is the first step in the formation of ozone? |
UV radiation with a wavelength of less than 240nm is absorbed by O2 molecules. Because this radiation is high-energy, the O2 molecule is broken to two oxygen atoms O2 + (radiation <240 nm) -------> 2O |
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Resources What is the second step in the formation of ozone? |
The O atoms then react with O2 molecules to form Ozone molecules O3. This process generates heat. O2+O ---> O3 + heat The heat is absorbed by air molecules in the stratosphere raising its temperature. Ozone is formed mainly in the upper reaches of the stratosphere. |
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Resources What happens to the ozone molecules formed in the above reaction? |
They then absorb UV with wavelengths of 240-310 nm. In this process O3 molecules are converted back to O2 molecules and O atoms. This is the reverse reaction that forms ozone. O3 + (radiation <310nm) ----> O2 + O |
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Resources what happens to the O then produced? |
It immediately reacts with other O2 molecules to reform ozone O2 + O ----> O3 + heat |
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Resources What are the purpose of these steps? |
- the chemical energy released when O and O2 combine is converted to kinetic energy of molecular motion (heat) -the overall effect is to convert penetrating UV radiation into heat, without any net loss of ozone. |
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Resources What does this cycle do? |
Keeps the ozone layer in a stable balance. A natural steady state is being formed at the same rate as it is being broken down. O2 + O↔ O3 It protects living organisms from the harmful effects of high-energy UV radiation. |
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Resources How is ozone removed? |
When an oxygen atom and an ozone molecule combine, they form to O2 molecules: O3 + O -> 2O2 |
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Resources How fast is this? |
Slow since the concentration of O atoms is very low. |
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Resources What is Ozone depletion potential |
The relative amount of breakdown to the ozone leayer caused by a substance, compared with CFCL3 (Trichlorofloromethane), which has a value of 1.0 |
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Resources What can disrupt the natural equilibrium of ozone formation-destruction |
Introduction of new compounds eg Chlorine radicals, caused by human activity. |
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Resources What is the main source of Chlorine radicals in the stratosphere |
CHloroflorocarbons (CFCs) |
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Resources Why are CFCs especially damaging to the ozone layer |
CFCs are very stable, so can only be broken down by highly energetic UV so can reach the stratosphere before being broken down. |
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Resources Draw the initiation step when UV strikes a CFC molecule |
CFCl3 ---> *CFCl2 + Cl* The C-Cl bond breaks forming a Chlorine radical |
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Resources Draw the propagation steps |
Radicals are extremely reactive, once a Chlorine radical has been generated it can react with O3, breaking it down Cl* + O3 ---> ClO* +O2 ClO* + O ---> Cl* + O2 Chlorine is acting as a catalyst, it remains chemically unchanged at the end of the process |
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Resources Draw the overall reaction for the breakdown of Ozone |
O3 + O ---> 2O2 Eventually Chlorine will form a stable compound, it can destroy 100,000 O3 molecules before this occurs. |
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Resources What other radicals can take place in this process? |
*NO (Nitrogen Oxide) from lightning or aircraft engines. *NO + O3 ---> *NO2 +O2 *NO2 + O ---> *NO + O2 |
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Resources What are the benefits of using CFCs |
They are non toxic, non flammable and stable, making them useful in aerosols and refrigerants |
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Resources How are CFCs regulated |
Montreal protocol, controls global emissions of CFCs Important restrictions: -zero production of CFCs by 2000 -Zero production of Tetrachloromethane -Zero production of Halons by 2000 (some exceptions) -Zero production of 1,1,1,trichloromethane by 2005 -HCFCs and HFCs replace CFCs in 15% of applications |
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Resources What pollutants are produced from traffic emissions? |
CO Nitrogen oxides Unburnt hydrocarbons (produces photochemical smog) |
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Resources Describe the properties of CO, where it is produced and how long it exists in the atmosphere. |
- Posionous gas -Emitted from incomplete combustion of hydrocarbons and other organic compounds - From traffic pollution -CO molecules can exist for about one month in the environment before being oxidised to CO2 |
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Resources What effect does CO have on humans? |
CO binds strongly to Haemoglobin, reducing the amount of oxygen that can be supplied to tissues and organs. |
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Resources Symptoms of CO poisoning? |
Reduced manual dexterity, disturbed vision, tiredness and an inability to perform complex tasks. |
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Resources How are Nitrogen oxides produced? |
During the burning of fuels in the internal combustion engine, air is drawn into the cylinder along with the fuel. The fuel is burned in the presence of oxygen, generating energy. Nitrogen oxides are also produced during this high-temperature process as some of the nitrogen from the air is oxidised by the oxygen. Two oxides of nitrogen are produced, NO and NO2. |
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Resources What are the environmental impacts of Nitrogen dioxide? |
forms low-level ozone. Much of the NO2 released into the atmosphere is converted into nitric acid - a contributor to acid rain. Nitrogen oxides are respiratory irritants and even low levels affect asthmatics |
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Resources What does VOC stand for? |
Volatile organic compounds |
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Resources Where are they released from? |
Vehicle exhaust gases from unburnt fuels |
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Resources what two compounds are found in petrol? why are they dangerous? |
Benzene and buta-1,3-diene - they are human carcinogens |
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Resources Once released into the atmosphere, what happens to unburnt hydrocarbons? |
They combine with NO2 to form low-level ozone. (The energy for this reaction is provided by sunlight - photochemical smog is produced) Low-level ozone is a serious pollutant which causes breathing difficulties and increasing susceptibility to infections. |
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Resources What is a catalytic converter made from? |
Platinum, Rhodium and Palladium supported on a honeycomb mesh (large SA) |
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Resources How do catalytic converters work? |
Hot exhaust gases pass over the catalytic surface and the harmful gases are converted into less harmful products, which are then released into the atmosphere. |
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Resources What are the two types of catalytic converters? |
- Oxidation catalysts - Three-way catalysts |
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Resources Describe the use of oxidation catalysts |
Used on diesel engines to decrease emissions of CO and unburnt hyrdrocarbons. Combined with complex filter systems, they also remove particulate matter and nitrogen oxides. |
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Resources What reactions take place in a oxidation catalysts? |
2CO (g) + O2 (g) ---> 2CO2 (g) C12H26 (l) + 18 1/2 O2 (g) ----> 12CO2 (g) + 13H2O (l) |
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Resources Describe the use of three-way catalysts |
Fitted to petrol engines. NO reacts with CO to form the non-toxic gases N2 and CO2. 2NO (g) + 2CO (g) ----> N2 (g) + 2CO2 (g) |
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Resources How do catalytic converters work? |
The catalyst provides a surface on which the reaction takes place. -The Co and NO gas molecules diffuse over the catalytic surface of the metal. Some of the molecules are held on to the metal surface by adsorption - Temporary bonds are formed between the catalytic surface and the gas molecules - These bonds hold the gas molecules in the correct position on the metal surface, where they react together -After the reaction, the CO2 and N2 products are desorbed from the surface and diffuse away from the catalytic surface |
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Resources What does sustainability mean for chemists? |
The development of processes that prevent the depletion of natural resources. |
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Resources How can you monitor air pollution? |
Using Infrared spectroscopy.
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Resources What is the purpose of green chemistry? |
Cut down waste, develop more efficient processes, use non-hazardous alternatives, use or make things that are non hazardous, use less energy |
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Resources What are the 12 principles of Green chem? |
1) Prevention 2) Atom economy 3) Less-hazardous chemical synthesis 4) Designing safer chemicals 5) Safer solvents and auxiliaries 6) Design for energy efficiency 7) Use of renewable feedstocks 8) reduced derivatives 9) Catalysis 10) Design for degradation 11) Real-time analysis for pollution prevention 12) Inherently safer chemistry for accident prevention
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Resources Give an example of how renewable resources are used |
Plastic bags from agricultural waste - using renewable resources such as plant-based substances rather than finite resources like fossil fuels |
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Resources How can waste be prevented |
By developing new methods of synthesis with higher atom economy |
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Resources What can CO2 from fermentation of ethanol be used for? |
-CO2 in foam - CO2 as a solvent - CO2 for decaffeinated coffee - CO2 for beer - CO2 for dry cleaning - CO2 for toxic waste treatment -CO2 for chemical synthesis |
