Chapter 10

Question 1

What is meant by the rate of a chemical reaction?

Answer 1

How fast a reactant is being used up or how fast a product is being made. (change in their concentration over time)

Question 2

What is the equation for rate?

Answer 2

rate = change in conc/ time

Question 3

What are the units for rate?

Answer 3

moldm-3s-1

Question 4

What factors can change the rate of reaction?

Answer 4

-concentration
-temperature
-catalyst
-surface are of solid reactants

Question 5

What does the collision theory state?

Answer 5

Two reacting particles must collide for a reaction to occur.

Question 6

What two conditions must be met for a collision to be effective?

Answer 6

• collide with the correct orientation
• collide with enough energy to overcome the activation energy

Question 7

How does increasing concentration affect the rate of reaction?

Answer 7

The more concentrated a solution is, the greater the number of particles in a given volume of solvent

An increase in concentration causes in an increased collision frequency and therefore an increased rate of reaction

Question 8

How does increasing pressure affect the rate of reaction?

Answer 8

An increase in pressure in reactions that involve gases has the same effect as an increase in the concentrations of solutions

When the pressure is increased, the molecules have less space in which they can move

This means that the number of effective collisions increases due to an increased collision frequency

An increase in pressure therefore increases the rate of reaction

Question 9

What is a catalyst?

Answer 9

a substance that increases the rate of reaction by providing the particles with an alternative mechanism with a lower activation energy.

catalysts are regenerated.

Question 10

What is a homogeneous catalyst?

Answer 10

Homogeneous means that the catalyst is in the same phase as the reactants (same state)

Question 11

What is a heterogeneous catalyst?

Answer 11

Heterogeneous means that the catalyst is in a different phase to the reactants (different state)

Question 12

What are some benefits of catalysts?

Answer 12

• speed up the rate of reaction, lower temp and pressures can be used. (save energy costs + fewer CO2 emissions from burning fossil fuels)
• They are often enzymes and generate specific products at room temp/pressure.
• they can enable different reactions to be used so better atom economy/ reduced waste/ fewer undesired products.

Question 13

What is the spread of molecular energies in gases known as?

Answer 13

The Boltzmann distribution curve

Question 14

How is a Boltzmann distribution curve drawn?

Answer 14

y axis = number of molecules with a given energy
x axis = energy
Ea = activation energy (only a few molecules can overcome this)

line goes up to a peak then curves down.

Question 15

What is the effect of a higher temperature on a Boltzmann distribution graph?

Answer 15

More molecules have an energy greater than or equal to the activation energy. So more collisions cause a reaction which increases the rate of reaction (the peak is less high and more curvey)

Question 16

What is the effect of a catalyst on a Boltzmann distribution graph?

Answer 16

Lowers the activation energy line, so more molecules overcome it.

Question 17

What does dynamic equilibrium mean?

Answer 17

The rate of the forwards reaction is equal to the rate of the reverse reaction and the concentrations of the reactants and products dont change.

Question 18

For a reaction to remain in equilibrium what must be true?

Answer 18

The system must be closed.

Question 19

What is a closed system?

Answer 19

Isolated from its surroundings, so the temperature, pressure and concentrations of reactants and products are unaffected by outside influences.

Question 20

What does le Chateliers principle state?

Answer 20

if a change is made to a system in dynamic equilibrium, the position of the equilibrium moves to counteract this change

Question 21

What does the position of equilibrium mean?

Answer 21

The position of the equilibrium refers to the relative amounts of products and reactants in an equilibrium mixture

When the position of equilibrium shifts to the left, it means the concentration of reactants increases

When the position of equilibrium shifts to the right, it means the concentration of products increases

Question 22

How does equilibrium shift when the concentration of a reactant increases?

Answer 22

Equilibrium shifts to the right to reduce the effect.

Question 23

How does equilibrium shift when the concentration of a reactant decreases?

Answer 23

Equilibrium shifts to the left to reduce the effect

Question 24

How does equilibrium shift when the concentration of a product increases?

Answer 24

Shifts to the left to reduce the effect.

Question 25

How does equilibrium shift when the concentration of a product decreases?

Answer 25

Shifts to the right to reduce the effect.

Question 26

How does equilibrium shift when there is an increase in pressure?

Answer 26

Equilibrium shifts to the side with the fewer number of moles. To decrease the pressure again.

Question 27

How does equilibrium shift when there is a decrease in pressure?

Answer 27

Equilibrium shifts to the side with the larger number of moles. To increase the pressure again.

Question 28

How does equilibrium shift when there is a decrease in temperature?

Answer 28

Equilibrium shifts in the exothermic direction to reverse the change.

Question 29

How does equilibrium shift when there is an increase in temperature?

Answer 29

Equilibrium shifts in the endothermic direction to reverse the change.

Question 30

What’s the effect on equilibrium if a catalyst is added?

Answer 30

There is no effect on equilibrium as it increases the rate of the forward and backwards reaction equally.

Catalysts only cause a reaction to reach equilibrium faster

Catalysts therefore have no effect on the position of the equilibrium once this is reached

Question 31

What system do we use to investigate changes to the equilibrium position with concentration?

Answer 31

The equilibrium between aqueous chromate ions, CrO42-, and dichromate ions, Cr2O72- is sensitive to changes in acid concentration.

2Cr2O72- + H+. —> Cr2O72- + H2O
yellow orange

adding dilute H2SO4 = increases concentration of H+. Equilibrium shifts to the right to oppose this change and makes more product.

adding aqueous NaOH = Decreases the concentration of H+ as the OH- reacts with it and forms water. This causes equilibrium to shift to the left so more H+ is made.

Question 32

What system do we use to investigate changes to the equilibrium position with temperature?

Answer 32

[Co(H2O)6]2+ + 4Cl- ⇌ [CoCl4]2- + 6H2O
pink blue

The forward reaction in this process is endothermic and the backward reaction is exothermic.

By heating up the solution we can increase the amount of heat energy in the system. This causes the equilibrium to shift to minimise the change.

Equilibrium shifts to the right favouring the endothermic reaction (ΔH is positive).

This allows the system to take heat energy in and minimise the increase in temperature.
The solution turns blue as more CoCl42- (aq) is formed.

Cooling down the solution removes the heat energy from the system. This again causes the equilibrium to shift to minimise the change.

Equilibrium shifts to the left favouring the exothermic reaction (ΔH is negative).
This allows the system to release heat energy and minimise the decrease in temperature
The solution turns pink as more Co(H2O)62+ (aq) is formed.

Question 33

What is the Haber process?

Answer 33

The Haber process involves the synthesis of ammonia according to:
N­2 (g) + 3H2 (g) ⇌ 2NH3 (g) ΔHr = -92 kJ mol-1

Question 34

How can we get the best yield of ammonia in the Haber process?

Answer 34

Increase in pressure would shift equilibrium to the right (increase products). But very high pressures are expensive to produce therefore a compromise pressure of 200 atm is chosen.

Since the forward reaction is exothermic, the lower the temperature the more it shifts to the right. But at a low temperature the gases won’t have enough kinetic energy to collide and react and therefore equilibrium would not be reached therefore compromise temperature of 400-450C is used in the Haber process

The process also ends an iron catalyst.

Question 35

What is Kc?

Answer 35

The equilibrium constant.

Question 36

How is Kc written?

Answer 36

Concentration of products / concentration of reactants

each number squared by the stoichiometry number.

Question 37

What is the only change in condition that causes Kc to change?

Answer 37

Change in temperature