4) Reaction Rates and Equilibrium

Question 1

Define the rate of reaction.

Answer 1

The change in concentration of a reactant or a product in a given time.

Question 2

What four factors affect the rate of a chemical reaction?

Answer 2

• Concentration.
• Temperature.
• Use of a catalyst.
• Surface area.

Question 3

What is the effect of concentration on the rate of a reaction in terms of frequency of collisions?

Answer 3

• Increase in concentration increases the number of particles in the same volume.
• The particles are closer together and collide more frequently.

Question 4

What is the effect of the pressure of gases on the rate of a reaction in terms of frequency of collisions?

Answer 4

• The concentration of gas molecules increases as the same number of gas molecules occupy a smaller volume.
• The gas molecules are closer together and collide more frequently, leading to more effective collisions.

Question 5

What two conditions ensure an effective collision?

Answer 5

• Particles colliding with the correct orientation.

- Particles have sufficient energy to overcome the activation energy.

Question 6

How is the reaction rate calculated from a graph measuring how a physical quantity changes with time?

Answer 6

Using gradients.

Question 7

How could the progress of a reaction be followed?

Answer 7

• Monitoring the decrease in concentration of a reactant.

- Monitoring the increase in the concentration of a product.

Question 8

What is a catalyst?

Answer 8

A substance that alters the rate of a reaction by providing an alternative route without being used up.

Question 9

How does a catalyst increase the rate of a chemical reaction?

Answer 9

Provides an alternative reaction pathway with a lower activation energy.

Question 10

Explain what is meant by a homogenous catalyst.

Answer 10

A homogenous catalyst has the same physical state as the reactants.

Question 11

Explain what is meant by a heterogeneous catalyst.

Answer 11

A homogenous catalyst has a different physical state from the reactants.

Question 12

Why do catalysts have great economic importance and benefits for increased sustainability?

Answer 12

• They lower temperatures.

- Therefore reducing energy demand from the combustion of fossil fuels with a resulting reduction in CO2 emissions.

Question 13

What are two features of a Boltzmann distribution?

Answer 13

• No molecules have zero energy - the curve starts at the origin.
• The area under the curve is equal to the total number of molecules.

Question 14

Use ‘Rates & Equilibrium’ to test knowledge of drawing a Boltzmann distribution.

Answer 14

Rate knowledge 1-5.

Question 15

Use ‘Rates & Equilibrium’ to test knowledge of drawing the effect of temperature on a Boltzmann distribution.

Answer 15

Rate knowledge 1-5.

Question 16

Use ‘Rates & Equilibrium’ to test knowledge of drawing the effect of a catalyst on a Boltzmann distribution.

Answer 16

Rate knowledge 1-5.

Question 17

Explain how a higher temperature affects the rate of a chemical reaction.

Answer 17

• More molecules have energy greater than or equal to the activation energy.
• Therefore a greater proportion of collisions will lead to a reaction.

Question 18

What does the symbol “⇌” indicate?

Answer 18

That the reversible reaction is in equilibrium.

Question 19

What is meant by Dynamic Equilibrium?

Answer 19

A dynamic equilibrium exists in a closed system when:

• The rate of the forward reaction is equal to the rate of the reverse reaction.
• The concentrations of reactants and products do not change.

Question 20

If the forward reaction is exothermic (ΔH is negative), how does an increase in temperature affect the position of equilibrium?

Answer 20

• The position of equilibrium is shifted to the left.

- More reactants are made.

Question 21

If the forward reaction is exothermic (ΔH is negative), how does a decrease in temperature affect the position of equilibrium?

Answer 21

• The position of equilibrium is shifted to the right.

- More products are made.

Question 22

If the forward reaction is endothermic (ΔH is positive), how does an increase in temperature affect the position of equilibrium?

Answer 22

• The position of equilibrium is shifted to the right.

- More products are made.

Question 23

If the forward reaction is endothermic (ΔH is positive), how does a decrease in temperature affect the position of equilibrium?

Answer 23

• The position of equilibrium is shifted to the left.

- More reactants are made.

Question 24

How does pressure affect the position of equilibrium?

Answer 24

Increasing the pressure of the system will shift the position of equilibrium to the side with fewer molecules, reducing the pressure of the system.

Question 25

When discussing the changes in pressure in terms of the position of equilibrium, what must you always state?

Answer 25

Whether there are more moles on the left or right side of the equation.

Question 26

Why does a catalyst not affect the position of equilibrium?

Answer 26

A catalyst increases the rate of both forward and reverse reactions in an equilibrium by the same amount.

Question 27

Use ‘Rates & Equilibrium’ to test knowledge of generating an expression for Kc.

Answer 27

Rate knowledge 1-5.

Question 28

What does a Kc value of 1 indicate?

Answer 28

A position of equilibrium that is halfway between reactants and products.

Question 29

What does a Kc value > 1 indicate?

Answer 29

A position of equilibrium that is towards the products.

Question 30

What does a Kc value < 1 indicate?

Answer 30

A position of equilibrium that is towards the reactants.

Question 31

Explain why increasing the concentration of H2O2(aq) increases the rate of decomposition.

Answer 31

• More particles per unit volume.

- Therefore more collisions per second.

Question 32

Explain how the catalyst can increase the rate of decomposition of H2O2(aq).

Answer 32

• The reaction takes an alternative route.

- Activation energy is lowered.

Question 33

High pressures and low temperatures would give the maximum equilibrium yield of methanol. Explain why the actual conditions used in the chemical industry might be different from those above.

Answer 33

-Too expensive and dangerous to use high pressure.

Too slow to use a low temperature.

Question 34

Suggest two ways in which the use of catalysts helps chemical companies to make their processes more sustainable.

Answer 34

• Catalyst not used up in reaction.
• Reactions take place at lower temperatures with lower energy demand and lower activation energy.
• Use less fuel so less carbon dioxide emitted into the atmosphere.
• Different reactions can be used with better atom economy.

Question 35

Use collision theory to explain the changes in the rate of a reaction as it proceeds.

Answer 35

• As the reaction proceeds, the concentration decreases.
• The rate of collision decreases.
• The reaction stops when all of one reagent is used up.

Question 36

Suggest and explain two reasons why catalysts are used in industrial processes.

Answer 36

• To save energy/money by allowing process to run at a lower temperature.
• Goes faster to save time.