Chemistry 2.3

Front 1

Explain that some chemical reactions are accompanied by enthalpy changes, which can be exothermic or endothermic

Back 1

Enthalpy - the heat content that is stored within a chemical system.

When bonds break within a reaction, the chemical energy of the atoms (energy stored within the chemical bonds) is altered. When bonds are broken, heat energy may be supplied to the system in order to provide the energy for transfer into chemical energy which is required in the breaking of these chemical bonds. When bonds are made, chemical energy may be released as heat.

Due to the law of conservation of energy, heat loss from a chemical system is equal to the heat gained by the surrounding. The same is true the other way around.

If the products of the reaction have more chemical energy than the reactants and therefore a higher enthalpy, there will be an overall heat loss from the surroundings (and a gain within the chemical system) as more energy is required to break the bonds of the products than is released in making the bonds of the reactants.

change in H = H(products) - H (reactants)

Therefore these reactions will have a positive enthalpy change.

These are endothermic reations.

If the products of the reaction have less chemical energy than the reactants, and therefore a smaller enthalpy, there will be an overall heat gain by the surroundings (and loss from the chemical system) as the energy required to break the bonds of the reactants is less than that released when making the bonds of the products.

Therefore these reactants will have a negative enthaply change (change in H = H(products) - H(reactants).

These reactions are exothermic.

Front 2

Describe the importance of oxidation as an exothermic process

Back 2

Oxidation of fuels -

This is done by combustion and provides essential energy

e.g combustion of petrol for cars/buses/trains

combustion of kerosene in aircrafts.

In such reactions, energy is released as heat because the products have less enthalpy than the reactants.

However, CO2 is a product of such reactions which can have a negative effect on the climate as it is a greenhouse gas.

Respiration -

During this process, sugars e.g glucose are oxidised to form CO2 and water, this is an essential process for life.

C6H12O2 + 6O2 --> 6CO2 + 6H2O.

Front 3

Explain that endothermic processes require an input of heat energy

Back 3

In endothermic reactions, the products have greater enthalpy than the reactants therefore heat energy must be supplied to provide this. This heat energy is taken in from the surroundings into the chemical system.

Thermal decomposition of CaCO3 (limestone) -

This process produces CaO which has a number of uses e.g in the manufacture of cement and for treatment of acid soils.

CaCO3 --> CaO + CO2

This reaction has a positive enthalpy change.

Photosynthesis -

During this process, sugars e.g glucose are made from carbon dioxide and water (opposite of respiration). It is an essential process for life.

6CO2 + 6O2 --> C6H12O2 + 6O2

Energy for this reaction is provided by light from the sun

Front 4

Construct a simple enthalpy profile diagram for a reaction to show the difference in the enthalpy of the reactants compared with that of the products - exothermic reactions

Back 4

Enthalpy profile diagrams compare the enthalpy of the reactants to that of the products.

The image shows an enthalpy profile diagram for an exothermic reaction.

Front 5

Construct a simple enthalpy profile diagram for a reaction to show the difference in the enthalpy of the reactants compared with that of the products - endothermic reactions

Back 5

The image shows an enthalpy profile diagram for an endothermic reaction. The activation energy required is far greater than that for an exothermic reaction.

Front 6

Explain the term activation energy using enthalpy profile diagrams

Back 6

Activation energy is the minimum energy required to start a reaction by breaking the bonds of the reactants.

Even exothermic reactions require an input of energy (activation energy) to start the reaction, however, once the reaction has begun, the net output of energy provides sufficient energy for the reaction to continue such that it becomes a self-sustaining reaction.

The activation energy is shown on an enthalpy profile diagram by the increase in energy from that of the reactants to the maximum energy in the reaction.

Front 7

Define and use the terms:

-Standard conditions

-Enthalpy change of formation

-Enthalpy change of combustion

-Enthalpy change of reaction

Back 7

Standard conditions = 1 atm pressure, 298 K temperature and 100 mol dm-3 concentration.

(The standard state of a substance is its physical state under standard conditions e.g Mg = solid)

The standard enthalpy change of combustion - the enthalpy change that occurs when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states. (Note: the substance being combusted must have no balancing number in front of it in an equation of standard combustion)

The standard enthalpy change of formation - the enthalpy change that occurs when one mole of a compound is formed from its constituent elements in their standard states under standard conditions. (Note: the compound must have no balancing number in an equation of standard formation)

All elements have a standard enthalpy change of formation of 0kJmol-1.

The standard enthalpy change of reaction - 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. (Note: the enthalpy change shown after a given equation gives the enthalpy change for the molar quantities stated in the equation)

Front 8

Calculate enthalpy changes directly from experimental results

Back 8

We can measure the enthalpy change in a system by measuring it heat exchange with the surroundings due to the laws of conservation of energy -

heat lost from chemical system = heat gained by surroundings (as well as the other way around)

Q = mc(delta)T

Where -

Q is the heat energy exchanged with the surroundings in joules (J)

m is the mass of surroundings (e.g the mass of the solution) in grams (g)

c is the specific heat capacity of the surroundings

(delta)T is the change in temperature in degrees celcius (C) or in Kelvin (K).

An exothermic reaction will show a positive temperature change

An endothermic reaction will show a negative temperature change.

To find the enthalpy change from Q, it must be divided by the moles of substance. Remember to convert from J to kJ.

To determine enthalpy change of combustion experimentally:

- Measure a known volume of water into a beaker and measure its initial temperature

- Weigh the burner containing the fuel

-Light the burner and burn for a reasonable amount of time such that the temperature has risen by a significant amount

-Measure the new temperature of the water and calculate the temperature change

-Reweigh the burner and calculate the mass of fuel that has been burnt. (used to calculate moles of substance used.)

-From these results it is possible to calculate the enthalpy change of combustion.

- The answer must always have a NEGATIVE sign in front of it, as the energy gained by the water/surroundings = the energy lost from the system.

The experimental value here is likely not to exactly match the theoretical value which may be due to two reasons

- heat loss to surroundings (not used to heat water)

- incomplete combustion

More accurate measurements can be obtained using a bomb calorimeter which eliminates these issues.

Front 9

Define and use the term average bond enthalpy

Back 9

Bond enthalpy - the enthalpy change that occurs when breaking by homolytic fission, one mole of a given bond in the molecules of a gaseous species

Average bond enthalpy - the average enthalpy change that occurs when breaking by homolytic fission, one mole of a given bond in the molecules of a gaseous species.

Front 10

Explain exothermic and endothermic reactions in terms of enthalpy changes associated with the breaking and making of chemical bonds

Back 10

Bond breaking requires energy so is an endothermic process with a positive enthalpy change. Bond making releases energy so is an exothermic reaction with a negative enthalpy change.

The same amount of energy is released when a bond breaks as when it forms. Therefore, in a reaction:

If the bonds that are formed are stronger than those which are broken the reaction will be exothermic because the bond enthalpy of the products will be greater and therefore will release more energy in their formation than was required to break the bonds of the products, resulting in an overall loss of energy from the chemical system.

If the bonds that are formed are weaker than those which are broken the reaction will be endothermic because the bond enthalpy of the products will be less and therefore will release less energy than was taken in in the breaking of the bonds so there will be an overall gain of energy by the chemical system.

Front 11

Calculate an enthalpy change of reaction from average bond enthalpies

Back 11

Enthalpy change of reaction = sum of bond enthalpies of bonds broken - sum of bond enthalpies of bonds made.

Therefore an exothermic reaction will have a negative enthalpy change and an endothermic reaction will have a positive enthalpy change

(Note: Enthalpy not the same as bond enthalpy)

Front 12

Use Hess' law to construct enthalpy cycles

Back 12

Hess' law - if a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route.

This is used when it is not possible to measure the enthalpy change of a reaction directly.

e,g if the activation energy is to high or the rate of reaction is too slow or more than on reaction is taking place.

Using enthalpy change of combustion values - -arrows point towards CO2 + H2O (intermediate step)

- enthalpy change = (reactant) + (reactant) - (product)

Using enthalpy change of formation values -

- intermediate step is constituent elements of compounds taking part in reaction

- arrows point away from constituent elements

- enthalpy change = - (reactant) - (reactant) + (product) + (product)

This concept may be applied to any energy cycle.

Front 13

Describe the factors effecting rate of reaction

Back 13

The rate of reaction is the change in concentration of a reactant or product in a given time.

Rate = change in concentration / time

The units of rate of reaction are mol dm-3 s-1

For the majority of reactants, the rate of reaction will be greatest at the start of the reaction as the concentration of reactants is the greatest.

As the reaction proceeds this concentration will decrease as reactants are converted into products therefore the rate of reaction will decrease.

When the concentration of at least on of the reactants reaches zero the rate of reaction will also be zero therefore the reaction will stop.

Factors affecting rate of reaction:

Temperature - if the temperature of a system increases, the molecules of the reactants will gain kinetic energy and therefore will start to vibrate with more energy so the molecules will collide more frequently and with greater force, therefore there will be more frequent collisions with energy greater than the activation energy increasing the rate of reaction.

Concentration - increasing the concentration means that there are more molecules in a given volume therefore collisions will be more frequent as the molecules are closer together. Therefore there will me more collisions with energy greater than the activation energy in a given time so the rate of reaction will increase.

Pressure - increasing the pressure of a gas means that there will be a greater number of gas molecules in a given volumes so the frequency of collisions will increase. Therefore there will be a greater number of collisions with energy greater than the activation energy in a given time so the rate of reaction will increase.

Surface area - increasing the surface are of a solid increases the availability of molecules with which the other reactant may collide. therefore, there will be more frequent collisions with energy greater than the activation energy and the rate of reaction will increase.

Front 14

State that a catalyst speeds up a reaction, without being consumed by the overall reaction

Back 14

A catalyst is a substance which increases the rate of reaction without being used up in the process.

The reactants are adsorbed onto active sites on the catalyst which helps them to react together, here the catalyst is forming an intermediate. They then dissociate (are desorbed) such that the catalyst is regenerated, remaining unchanged at the end of the reaction.

Front 15

Explain, using enthalpy profile diagrams, how the presence of a catalyst gives rise to an increased rate of reaction

Back 15

A catalyst provides an alternative route for the reaction which requires a lower activation energy, therefore there will be a greater number of collisions between the molecules with energy greater than the activation energy so the rate of reaction will increase.

Front 16

Explain that catalysts affect the reaction conditions that are needed

Back 16

Catalysts lower the activation energy required for the molecules to react, therefore less energy needs to be inputted into the system.

This external energy usually comes from electricity supplies or the combustion of crude oil.

Therefore, catalysts both save energy costs, making the processes cheaper industrially, and benefit the environment as reduced combustion of fossil fuels means that less CO2 is released into the atmosphere.

Some catalysts can also be used to increase the percentage yield of a reaction.

Front 17

Explain that catalysts enable different reactions to be used, with better atom economy and reduced waste

Back 17

Production of ethanoic acid:

Ethanoic acid used to be produced by oxidation of butane, in which butane was heated in air with a catalyst of manganese/colbalt/chromium ions, to produce the ethanoic acid.

The process gave a low percentage yield.

More recently, the Monsanto process is used -

CH3OH + CO --> CH3COOH

Methanol is reacted with carbon dioxide at 300 degrees celcius and 700 atmospheres of pressure with a catalyst of colbalt and iodide ions. However, this extremely high pressure raised serious safety concerns.

Further research led to the development of rhodium as a catalyst to this process, the use of which lowered the temperature required to 150 degrees celcius and the pressure to approximately 30 atmospheres.

This process also gives 100% atom economy.

The Cativa process is another method, which uses an iridium catalyst, which is cheaper than rhodium, it also releases less CO2 into the environment which has environmental benefits.

Front 18

Explain that catalysts have great economic importance

Back 18

Catalysts reduce the amount of energy required for a reaction to occur therefore reducing the cost of many processes of an industrial scale.

e.g the production of ammonia:

N2 + 3H2 <--> 2NH3

This requires a lot of energy as the strong triple N bond gives a high activation energy. An iron catalyst is used which lowers the activation energy (as well as increasing the rate of reaction)

The Ziegler-Natta catalyst is important for the production of poly(ethene)

Catalytic converters improve air quality by reducing toxic emissions from vehicles.

Front 19

Explain that catalyst are often enzymes

Back 19

Enzymes are large protein molecules which are able to catalyse the reactions of large quantities of biological molecules in short periods of time.

Enzymes operate at low temperatures and pressures (close to RTP) and have an optimum pH value.

Enzymes can also be used industrially with a number of benefits

- lower temperatures and pressures can be used than with conventional catalysts, therefore reducing costs.

- Enzymes often enable the production of pure products, which do not need to undergo separation processes, thereby further reducing costs.

- Enzymes are biodegradable unlike conventional catalysts and therefore do not contribute to landfill/disposal problems.

enzymes are used in a variety of applications

e.g in washing powders and detergents to reduce the temperature required for washing.

e.g in the production of food and drink such as dairy products and alcoholic drinks.

Front 20

Explain the Boltzmann distribution and its relationship with activation energy

Back 20

In a sample of gas or liquid, the molecules move around inside a container as they have kinetic energy, colliding with each other and the container, however, the collisions are assumed to be elastic so no energy is lost from the system.

In a sample, the molecules have differing amounts of energies:

- a small amount have low energy

- a small amount have high energy

- the majority have average energy.

Boltzmann distribution - the distribution of energies of molecules at a particular temperature, often shown as a graph.

On a Boltzmann distribution curve:

- no molecules have no energy so the curve must start at the origin

- the area under the curve is the total number of molecules in the sample, therefore it must stay the same even if conditions change

- There is no maximum energy for a molecule, so the line must not touch the x axis (labelled energy)

- Only molecules with energy greater than the activation energy (which is marked with a line) are able to react - collisions of molecules with energy less than the activation energy will not lead to reaction.

Front 21

Describe qualitatively, using the Boltzmann distribution, the effect of temperature changes of reaction rate.

Back 21

If the temperature is increased, the kinetic energy of the molecules increases, therefore the number of molecules with energy greater than the activation energy will increase.

On a Boltzmann distribution curve:

- the line flattens and shifts to the right with increased temperature

- the area under the line stays the same

Rate of reaction increases with temperature because the increased kinetic energy of the molecules means that a greater proportion have sufficient energy to overcome the activation energy so there will be a higher frequency of successful collisions.

Front 22

Interpret catalytic behaviour in terms of Boltzmann distributions

Back 22

A catalyst reduces the activation energy of a reaction therefore a greater proportion of molecules will have energy greater than the activation energy so there will be a higher frequency of successful collisions and the rate of reaction will increase.

Front 23

Explain that a dynamic equilibrium exists when the rate of the forwards reaction is equal to the rate of the reverse reaction

Back 23

Reversible reactions are reactions which can take place in either the forward or reverse direction.

Reversible reactions can reach a state of dynamic equilibrium - the equilibrium that exists in a closed system when the rate of forward reaction is equal to the rate of the reverse reaction.

When a reaction is in a state of dynamic equilibrium, there is no observable change within the system, however, in actuality the reactants are being converted into products at the same rate that products are being converted back into reactants.

In dynamic equilibrium:

- the concentration of the products and reactants are constant (but not necessarily the same as each other)

- the rates of the forward and backward reactions are equal.

Front 24

State le Chatelier's principle

Back 24

le Chatelier's princlple - when a system in dynamic equilibrium is subjected to change, the position of equilibrium will shift to minimise the change.

Front 25

Apply le Chatelier's principle to deduce qualitatively, the effect of a change in conditions on a homogeneous system in equilibrium

Back 25

Temperature - if the temperature is increased, the endothermic reaction will be favoured in order to oppose the increased temperature of the surroundings and the position of equilibrium will shift towards the side of the reaction (left/right) with the products of the endothermic reaction. If temperature is decreased, the exothermic reaction will be favoured in order to oppose the decrease in temperature of the surroundings and the position of equilibrium will shift towards the side of the reaction (right/left) with the products of the exothermic reaction.

Concentration - if the concentration of the reactants is increased, the position of equilibrium will shift towards the right hand side (side of the reaction with the products), favouring the forward reaction in order to to oppose this increase in concentration. If the concentration of the products is increased, the position of equilibrium will shift towards the left hand side (side of reaction with reactants), favouring the backward reaction in order to oppose the increased concentration.

Pressure - if the pressure a system is increased, the position of equilibrium will shift towards the side of the reaction fewer gas molecules, favouring the reaction that yields fewer, in order to oppose the increased pressure. If the pressure is decreased, the position of equilibrium will move to the side of the reaction with more gas molecules, favouring the reaction that yields more, in order to oppose the decreased pressure.

(Note: the effect of a change in pressure on the position of equilibrium is only relevant if there are gases present).

Front 26

Explain, from given data, the importance in the chemical industry of a compromise between chemical equilibrium and reaction rate

Back 26

In industry, chemists strive to produce the highest possible yield of a desired product as quickly and cheaply as possible.

The Haber Process -

N2(g) + 3H2(g) <--> 2NH3(g)

Ammonia here is the desired product, and is produced by the forwards reaction which will be favoured when there is a high pressure (as there are fewer gas molecules on the right side of the reaction) and a low temperature (as the forward reaction is exothermic). Therefore a high pressure and low temperature will provide the highest yield of ammonia.

However, a low temperature will decrease the rate of reaction significantly, and a high pressure will (despite increasing the rate of reaction) require a lot of energy to sustain, increasing the cost of the process, and cause safety issues as chemicals may be more likely to leak into the environment.

Therefore, a compromise must be made between yield and rate of reaction.

In modern ammonia plants:

- A temperature of approximately 450 degrees celcius is used, such that there is an acceptable yield of ammonia as well as an appropriate rate of reaction.

- A pressure of approximately 200 atm. is used, which is high enough to produce a good yield and rate of reaction but does not put the workers at significant safety risk.

- An iron catalyst is used in order to speed up the rate of reaction without altering the position of equilibrium (as the rates of the forward and backward reactions are increased equally). This allows lower temperatures to be used, and also saves energy costs as the process requires a smaller input of energy to generate heat.

This process only converts 15% of H2 and N2 into ammonia, however, the ammonia is liquefied and removed and unreacted gases can be recycled to the start of the process and reacted again such that there is very little waste.