3.2 - Physical Chemistry

Question 1

Define the term enthalpy change.

Answer 1

The energy exchange that takes place with the surroundings at constant pressure during a reaction. Exothermic reactions will have a negative ΔH. Endothermic reactions require an input of thermal energy and will have a positive ΔH.

Question 2

What does exothermic mean?

Answer 2

A reaction in which energy is given out from the system to the surroundings.

Question 3

What does endothermic mean?

Answer 3

A reaction in which energy is taken in to a system from the surroundings.

Question 4

Give examples of endothermic and exothermic enthalpy changes.

Answer 4

Endothermic - photosynthesis, thermal decomposition of calcium carbonate.
Exothermic - oxidation. It is used in the combustion of fuels. Carbohydrates such as glucose are oxidised in respiration.

Question 5

Describe the term activation energy.

Answer 5

The minimum amount of energy required for a reaction to take place.

Question 6

What are the standard conditions for an enthalpy change?

Answer 6

Temperature - 298K.
Pressure - 100KPa.
Solutions - conccentration of 1.0 mol dm-3
Reactants and products in normal physical state for these conditions.

Question 7

Define enthalpy change of a reaction (ΔrH).

Answer 7

The enthalpy change when amounts of reactants react together under standard conditions of 298K and 100KPa, to give products in their standard states.

Question 8

Define standard enthalpy change of formation (ΔfH).

Answer 8

The enthalpy change when 1 mole of a compound is formed from its elements under standard conditions of 298K and 100KPa.

Question 9

Define standard enthalpy change of combustion (ΔcH).

Answer 9

The enthalpy change when 1 mole of a substance reacts completely with oxygen under standard conditions of 298K and 100KPa in their standard states.

Question 10

Give the formula needed to calculate enthalpy changes from experimental results.

Answer 10

ΔH = mcΔt.
ΔH = enthalpy change in Joules.
m = the mass of water that surrounded the reaction in grams.
Δt = the temperature change.

Question 11

How do you make sure you have the correct sign for the enthalpy change?

Answer 11

When you have calculated the enthalpy change you must give it a negative sign if the reaction was exothermic, but positive if the reaction was endothermic.
If the temperature increased - it’s exothermic.
If the temperature dropped - it’s endothermic.

Question 12

How do you calculate the enthalpy change for the reaction?

Answer 12

Calculate the number of moles used.
Divide the energy change (in kJ) by the number of moles used to find the energy change per mole.
kJ/mol.

Question 13

Why might the value calculated for an enthalpy change via experiment be different to the real value?

Answer 13

Energy loss to the surroundings during the experiment.
Not standard conditions.
If a combustion reaction is involved then there might have been some incomplete combustion.

Question 14

Define average bond enthalpy.

Answer 14

The enthalpy change when 1 mole of gaseous covalent bonds is broken.

Question 15

Explain, using bond forming and bond breaking, why a reaction can be exothermic.

Answer 15

Bond breaking absorbs energy while bond forming releases energy. More energy is released as bonds are formed than taken in when bonds are broken.

Question 16

Explain, using bond forming and bond breaking, why a reaction can be endothermic.

Answer 16

Bond breaking absorbs energy while bond forming releases energy. More energy is absorbed as bonds are broken than released when bonds are formed.

Question 17

How can bond enthalpies be used to calculate enthalpy changes?

Answer 17

ΔH = bonds broken - bonds formed.

Question 18

Why will the enthalpy change calculated from average bond enthalpies be different from the actual enthalpy change value?

Answer 18

Average bond enthalpies are calculated based on the bond in a variety of different environments.
This will be different to the bond enthalpies for the actual environment.

Question 19

What is Hess’ Law?

Answer 19

The total enthalpy change for a reaction is independent of the route by which that reaction takes place provided the initial and final conditions are the same. I.e. whatever route you look at, the enthalpy change is the same.

Question 20

Describe the techniques and procedures used to determine enthalpy changes directly and indirectly.

Answer 20

Directly: Set up the reaction to take place in an insulated container (e.g. polystyrene cup). Add a thermometer to record the temperature change. This process works well for reactions that take place in solutions.
Indirectly: Indirect methods are often needed for measuring the energy given out by a fuel. A known mass of water is heated by the fuel and the temperature change of the water is recorded. A copper or bomb calorimeter is often used.

Question 21

Describe the key features of collision theory.

Answer 21

Particles must collide in the correct orientation with enough energy in order to react.

Question 22

Describe and explain the effect of concentration on the rate of reaction.

Answer 22

Increasing the concentration of a substance can increase the rate of reaction. It increases the number of particles in the same volume. This increases the number of collisions per second. This results in more reactions per second.

Question 23

Describe and explain the effect of pressure on the rate of reaction.

Answer 23

Increasing the pressure can increase the rate of a reaction. It reduces the space between the molecules. As the molecules are closer together there will be more collisions per second. This results in more reactions per second.

Question 24

How do you calculate the rate of reaction from the gradient of a graph?

Answer 24

Y axis should be the change in mass or concentration. X axis should be time in seconds. Take a tangent for the time or concentration you are trying to find the rate for. Calculate the gradient for the tangent.

Question 25

What is a catalyst?

Answer 25

A substance that increases the rate of reaction without being consumed by the overall reaction.

Question 26

Explain why a catalyst works.

Answer 26

They provide a different route for the reaction which involves a lower activation energy.

Question 27

What is the difference between homogeneous and heterogeneous catalysis?

Answer 27

Homogeneous catalysts are in the same state as the reactants.
Heterogeneous catalysts are in a different state to the reactants.

Question 28

What is the importance of catalysts (in relation to sustainability)?

Answer 28

Catalysts affect the conditions needed for a reaction. This is often a reduction in temperature. This reduces the energy needed and so reduces the amount of fossil fuel combustion. This reduces the CO2 emissions from burning fossil fuels.
Catalysts enable different reactions to be used (reactions that initially would have needed too much energy) to make a product. These reactions may have a better atom economy and therefore reduced waste products.
Enzymes are sometimes used as catalysts. These generate very specific products and work effectively close to room temperatures and pressures.

Question 29

Give some examples of the economic importance of catalysts.

Answer 29

Iron is used as a catalyst in ammonia production. A Ziegler-Natta catalyst is used in polythene production. A platinum/palladium/rhodium catalyst is used in catalytic converters.

Question 30

How can the rate of reaction be measured using measurement of mass?

Answer 30

If the reaction being studied produces a gas you can measure its formation using a mass balance. A conical flask is laced on the mass balance and the reactants are added. Start a timer when the reaction starts. Take regular mass measurements at regular time intervals. The reaction is finished when the mass stops decreasing. If toxic gases are used this should be performed in a fume cupboard.

Question 31

How can the rate of reaction be measured using measurement of gas volumes?

Answer 31

If the reaction being studied produces a gas you can measure its formation in a gas syringe. A conical flask is set up that is connected via a delivery tube to a gas syringe. Start a timer when the reaction starts an record the measurement on the gas syringe at regular time intervals. No gas escapes in this experiment so it is a good method to use if toxic gases are being made.

Question 32

What are the features of a dynamic equilibrium?

Answer 32

Forward and reverse reactions occur at equal rates. Closed system. Macroscopic properties remain constant - concentration of reactants and products does not change.

Question 33

What is Le Chatelier’s principle?

Answer 33

When any of the conditions affecting the position of a dynamic equilibrium are changed, the position of the equilibrium will shift to minimise that change.

Question 34

Explain the effect of increasing and decreasing the following factors on the equilibrium: temperature, pressure, concentration.

Answer 34

Note: You must state ‘the equilibrium will be pushed to the right/left’ to get the mark.
Temperature: Increase = pushes equilibrium in the endothermic direction. Decrease = pushes equilibrium in the exothermic direction.
Pressure (you must look at the equation to work out the number of molecules on each side): Increase = pushes equilibrium in direction with fewer molecules. Decrease = pushes equilibrium in direction with more molecules.
Concentration: Increase = equilibrium will move away from the substance that increased (i.e. the substance with increased concentration reacts more). Decrease = Equilibrium will move towards the substance that decreased (i.e. the substance with decreased concentration is produced more).

Question 35

What is the importance of a compromise between equilibrium and reaction rate?

Answer 35

Industrially, you want to produce product as fast as possible - this means you want to increase the reaction rate as much as possible via increased temperature and pressure.
Sometimes increasing the pressure or temperature within a reaction at equilibrium may favour the backwards reaction - this means it would push the equilibrium towards the reactants and reduce the yield of product.
In these cases, the chemical industry must compromise between a rate that is not too slow, but does not significantly reduce the product yield.

Question 36

How does a catalyst effect the position of an equilibrium?

Answer 36

A catalyst increases the rate of the forward and the backward reaction by the same amount. The position of the equilibrium will be unchanged.

Question 37

Describe the techniques and procedures used to investigate changes to the position of equilibrium for changes in concentration or temperature.

Answer 37

Colour changes are often used to qualitatively measure the changes to an equilibrium. If the reactants and products produce a different colour then you can assess the movement of the equilibrium by the shift in colour when temperature/concentrations are changed. Titrations can also be used to assess the concentrations of each substance at equilibrium and what happens when changes are made.

Question 38

Describe the term equilibrium constant (Kc).

Answer 38

The ratio of the concentrations of the products and reactants at equilibrium. If Kc is greater than 1 then the equilibrium lies on the right hand side. If Kc is smaller than 1 then the equilibrium lies on the left hand side.