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81 Cards in this Set
- Front
- Back
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What are the 4 main types of energy? |
Light/solar Electricity Thermal Chemical |
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What is chemical energy? |
Energy stored in chemical bonds in compounds |
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What is the first law of thermodynamics? |
Energy can be converted from one form to another and cannot be created or destroyed |
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What do many processes involve? |
An energy change or conversion |
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What energy changes occur in photosynthesis? |
Solar to chemical |
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What energy changes occur in the combustion of petrol? |
Chemical to mechanical, heat, light and kinetic |
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What energy changes occur in respiration? |
Chemical to kinetic and heat |
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What energy changes occur in a battery? |
Chemical to electrical |
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What is enthalpy and what represents it? |
A measure of the thermal energy of a system. H |
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What can we calculate and what can't we calculate? |
We can't measure the enthalpy of a system, but we can measure the enthalpy change when heat is added/released from a system. |
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What represents enthalpy change? |
A triangle (capital delta) H |
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What is an enthalpy change? |
The amount of heat released (or absorbed) by a chemical reaction, carried out at a constant pressure. |
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What are the units for enthalpy change? |
KJ mol-1 |
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What do enthalpy changes involve? |
An exchange of heat energy between a system and its surroundings/ |
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What can enthalpy changes be? |
Endothermic or exothermic |
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What is the chemical system? |
The reactants and products (molecules reacting) |
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What are the surroundings? |
Things outside the chemical system |
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What happens when enthalpy change negative? |
When the system releases energy into the surroundings. Enthalpy of the system decreases. Temperature of the surroundings increases. Enthalpy change is exothermic |
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What happens when enthalpy change is positive? |
The system absorbs thermal energy from the surroundings Enthalpy of the system increases Temperature of the surroundings decreases Enthalpy change is endothermic |
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What symbol represents enthalpy change? |
∆HR
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How do we calculate enthalpy change (∆HR) in a reaction? |
Enthalpy of the products (HP) - enthalpy of the reactants (HR) |
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What is an exothermic reaction? |
A reaction where thermal energy is released to the surroundings. The enthalpy change for an exothermic reaction has a negative sign. |
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How do you draw an enthalpy profile diagram for exothermic reactions? |
Draw a horizontal line on the graph and write the reactants above. Then, draw an arrow pointing downwards and write what the enthalpy change equals (-ve) next to it with units. Next, draw a horizontal line originating from the arrow and write the products above. |
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What are some examples of exothermic reactions? |
Combustion (oxidation) of fuels Respiration (oxidation of carbohydrates) |
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What is an endothermic reaction? |
A reaction where thermal energy is absorbed from the surroundings. The enthalpy change for an endothermic reaction has a positive sign. |
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How do you draw an enthalpy profile diagram for an endothermic reaction? |
Draw a line and write the reactants above it. The draw an arrow pointing upwards and write what the enthalpy change equals (+ive). Next, draw a line originating from the arrow and write the products above it. |
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What must always be included in an enthalpy profile diagram? |
The X axis must be labelled reaction pathway The Y axis must be labelled enthalpy H The arrow must touch both lines (and point to the products) State symbols mus be used The enthalpy change figure must be included. |
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What is an example of an endothermic reaction? |
Photosynthesis |
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What is bond fission? |
Breaking a chemical bond |
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What are the 2 types of bond fission? |
Homolytic and heterolytic |
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What happens in homolytic fission? |
The bond breaks equally, so an electron goes t each atom. This creates 2 radicals (a species with an unpaired electron). |
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What happens in heterolytic fission? |
The bond doesn't break equally and both electrons go to one of the atoms. This creates a positive and a negative ion. |
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What does a chemical reaction involve? |
A molecular chemical reaction involves breaking covalent bonds in the reactant molecules and forming new covalent bonds in the product molecules. |
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When is energy absorbed? |
When bonds break. This is endothermic and bond enthalpy is positive. |
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When is energy released? |
When bonds are formed. This is exothermic and bond enthalpy is negative. |
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Why is breaking a covalent bond endothermic? |
You have to overcome the forces of attraction between the bonding pair of electrons and the nuclei of the bonded atoms. |
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What does average bond enthalpy involve? |
The breaking of one mole of bonds in gaseous molecules. |
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Why is an average bond enthalpy needed? |
Types of bonds exist in many different molecules. Their strength varies depending on the environment it's found in. |
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How do you calculate the enthalpy change (∆HR) of a reaction in terms of bonds? |
∆H breaking bonds + ∆H making bonds |
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When can enthalpy changes for reactions be compared? |
Only if they're measured under the same set of conditions i.e. standard conditions |
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What are the standard conditions? |
Temperature- 25 degrees c (298K) Pressure- 1atm (100 kPa) Solutions must have a concentration of 1.0 mol dm-3 |
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What represents standard conditions? |
∆Hᶿ
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What is any enthalpy change under standard conditions called? |
Standard enthalpy change |
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Define standard enthalpy change of a reaction |
The enthalpy change that accompanies a reaction in the molar quantities expresses in a chemical equation under standard conditions |
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What represents standard enthalpy change of a reaction? |
∆HᶿR
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Define enthalpy change of formation |
The enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions |
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What symbol represents enthalpy change of formation? |
∆HᶿF
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Define enthalpy change of neutralisation |
The enthalpy change that accompanies the neutralisation of an aqueous acid by an aqueous base to form 1 mole of H2O (l) under standard conditions |
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What symbol represents enthalpy change of neutralisation? |
∆HᶿN
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Define enthalpy change of combustion |
The enthalpy change when 1 mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states. |
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What must you have in enthalpy change of combustion? |
1 mole of the fuel |
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What is Hess' law? |
It states that 'the enthalpy change of a reaction depends only on the initial and final states and is independent of the route taken. |
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What is this used for? |
It's used to determine enthalpy change that can't be measure directly from experiments. |
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Why may it be difficult to measure enthalpy change directly from experiments? |
The reaction may occur to quickly/ slowly at room temperature. There may be lots of side products |
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What do we use to calculate enthalpy change indirectly? |
An enthalpy cycle |
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What do we write with an enthalpy cycle for formation? |
Reactants on the left, products on the right and constituent elements below. We then draw arrows from the reactants to the products. Each enthalpy change has an arrow. We then calculate the enthalpy change. |
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What equation do we then use to work out an unknown enthalpy change? |
The sum of the clockwise arrows= the sum of the anticlockwise arrows |
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Why do elements which remain elements not have arrows? |
There's no enthalpy change |
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How do we write the enthalpy change cycle for combustion? |
The top left is reactants, the top right is products and the bottom is carbon dioxide and H2O. The reactants and products have arrows down to the CO2 and H2O |
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How can we determine whether an enthalpy change has occurred in a chemical reaction? |
We can measure temperature change |
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What is usually used to measure temperature change? |
A colorimeter |
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What basic equipment can be used? |
A polystyrene cup calorimeter. Includes a polystyrene cup, lid, glass loop stirrer and thermometer |
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What is the sign and type of temperature change experienced in an endothermic reaction? |
Temperature decrease + |
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What is the sign and type of temperature change experienced in an exothermic reaction?
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Temperature increase - |
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What is the first stage of working out the enthalpy change of a reaction? |
Q = m x c x ∆T |
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What is Q and what's it measured in? |
Total heat energy in Joules |
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What is m? |
Mass in grams |
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What is c? |
Specific heat capacity This is 4.18 J g-1 C-1 |
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What is ∆T? |
Temperature change in degrees C |
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What mass is measured? |
The mass of the liquid experiencing the change, not the reactants |
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What is the second stage of working out the enthalpy change of a reaction? |
Find out the moles of substance required. Use either n=mass/Mr or n=cv |
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What is the third stage of working out the enthalpy change of a reaction? |
Enthalpy change(∆H)= Q in kJ / moles |
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How do you convert joules into kilojoules? |
Divide by 1,000 |
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What's the method for finding the enthalpy change of a reaction? |
Put water into the cup and weigh the solid and lid and bottle of the weighing bottle. Put thermometer into cup and measure initial temp. Add the solid and stir. Record the minimum/ maximum temp reached. |
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How do you find the enthalpy change in combustion of an alcohol? |
Record the temp of a known mass of water in a beaker. Put alcohol in a spirit burner and weigh. Burn for set time Record max temp Reweigh spirit burner to determine mass of fuel burnt. |
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What could lead to a procedural/systematic error? |
Unwanted heat exchange with the environment. Not done under standard conditions Evaporation of water Incomplete combustion |
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What could be done to improve this? |
Draught excluder, copper calorimeter, lagging, lid on beaker, higher oxygen concentration, standard conditions, stirring. |
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Would burning the fuel for longer affect the enthalpy change of combustion? |
No as mass burned would increase, so moles would increase but temp change would increase at the same rate, so the end value would be the same. |
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Would using a smaller volume of water change the value? |
No because, a larger mass of water would result in less heat change, so they change by the same amount. |
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What would you do to calculate the enthalpy change of neutralisation? |
The initial temp of acid+alkali are measured and recorded. A known volume of acid is added to a known volume of alkali in a polystyrene cup calorimeter. Stir and record maximum temperature reached. |
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What is different in working out enthalpy change for neutralisation? |
You must find the moles of an acid/alkali and use this to find the moles of water produced. |
