Chapter 10 Reaction Rates And Equilibrium

Question 1

What does the rate of a chemical reaction measure?

Answer 1

How fast a reactant is being used up or how fast a product is made.

Question 2

How can the rate of reaction be measured?

Answer 2

Rate= change in concentration / time

Mol dm^-3 s^-1 = moldm^-3 / s

Question 3

Describe how the rate of reaction changes over time

Answer 3

The rate of reaction is fastest at the start of a reaction, as the reactants are at their highest concentration.
However, the rate then slows as the reaction proceeds because the concentration of the reactants decreases.
The reaction plateaus once one of the reactants has been completely used up so the rate of reaction becomes zero.

Question 4

What factors can alter the rate of reaction?

Answer 4

Concentration/ pressure (when reactants are gases)
Temperature
Catalyst
Surface area of solid reactants

Question 5

What is Collision theory?

Answer 5

For a reaction to occur particles must collide with sufficient energy that can overcome the activation energy and collide with the correct orientation.

Question 6

What is an effective collision?

Answer 6

An effective collision is one that leads to a chemical reaction

Question 7

When is a collision considered effective?

Answer 7

When particles collide with the correct orientation
When the particles have sufficient energy that can overcome the activation energy barrier.

Question 8

How does a change in concentration affect the rate of reaction?

Answer 8

Rate of reaction increases as concentration increases meaning there are more particles per unit volume, so the particles are closer together and more frequent effective collisions can occur per second.

Question 9

How does changing pressure affect the rate of reaction?

Answer 9

Rate of reaction increases as the pressure of gases increases, this is because there are more gas particles per unit volume so the particles are closer together and more frequent effective collisions can occur per second.

Question 10

How does the surface area of a reactant affect the rate of reaction?

Answer 10

Rate of reaction increases as the surface area of reactants increases, this is because there is a greater surface area exposed for collisions to occur from other particles. Therefore, increasing the number of frequent effective collisions occuring per second.

Question 11

How can the progress of a chemical reaction be measured?

Answer 11

Monitoring the removal (decrease in concentration) of the reactant
Following the formation (increase in concentration) of the products

Question 12

How can the progress of a chemical reaction be measured?

Answer 12

Monitoring the removal (decrease in concentration) of the reactant
Following the formation (increase in concentration) of the products

Question 13

What is a catalyst?

Answer 13

A catalyst provides and alternative pathway for the reaction to occur by lowering the activation energy.
Catalysts are not used up in a chemical reaction.

Question 14

What is a Homogenous catalyst?

Answer 14

The catalyst and reactant are in the same physical state.

Question 15

What is a Heterogenous catalyst?

Answer 15

The catalyst and reactants are in different physical states

Question 16

How does a homogenous catalyst work?

Answer 16

The catalyst reacts with the reactants to form an intermediate, the intermediate then breaks down to give the product and then regenerates.

Question 17

How does a heterogenous catalyst work?

Answer 17

The catalyst provides a surface for the reaction to occur as they are usually solids in contact with gaseous reactants or reactants in solution. The reactant molecules are adsorbed onto the surface of the catalyst, the reaction occurs and the products are then desorbed.

Question 18

Why are catalysts sustainable?

Answer 18

Allows processes to operate at lower temperatures
Less fossil fuels are required
Lower carbon emissions are produced
Higher atom economies are sustained
There is less waste produced because there is less energy resources needed to process the waste.

Question 19

What is the Boltzmann distribution curve?

Answer 19

A graph which shows the spread of molecular energies of gases

Question 20

What are the features of the Boltzmann distribution curve?

Answer 20

No molecules will have zero energy- the curve starts at the origin.
The area under the curve is equal to the total number of molecules
There is no maximum energy, the curve does not meet the x axis.

Question 21

How does temperature affect the rate of reaction?

Answer 21

The rate of reaction increases as temperature increases as this causes the particles in the reaction to move faster because they have more kinetic energy. Therefore, a greater proportion of these particles will have energy that exceeds the activation energy, increasing the number of frequent effective collisions per second.

Question 22

How does increasing the temperature affect the Boltzmann distribution curve?

Answer 22

As temperature increases, the average energy of the molecules also increases, this means that the graph is stretched to the right as more molecules have higher energy. There will be a smaller proportion of molecules with lower energy so the peak of the curve is reduced.

Question 23

How does decreasing the temperature affect the Boltzmann distribution curve?

Answer 23

As temperature decreases, the average energy of the molecules decreases, this means that the graph will shift to the left as more molecules have energy lower than the activation energy. There will be a larger proportion of molecules with less energy so the peak of the curve increases.

Question 24

How does a catalyst affect the Boltzmann distribution curve?

Answer 24

In the presence of a catalyst, a greater proportion of molecules will have energy equal to or greater than the activation energy needed for the reaction to occur. Therefore, more frequent effective collisions take place increasing the rate of reaction.

Question 25

What is dynamic equilibrium?

Answer 25

A reaction that takes place in a closed system where the rate of the forward reaction is equal to the rate of the reverse reaction so the concentration of the reactants and products remains constant.

Question 26

What is Le Chateliers Principle?

Answer 26

When a system in dynamic equilibrium is subjected to a change in conditions its position will shift to minimise the change.

Question 27

How does changing the concentration of the reactant affect equilibrium?

Answer 27

Increasing the concentration of a reactant will shift the position of equilibrium in the forwards direction so that more products form.
Decreasing the concentration of a reactant will shift the position of equilibrium in the reverse direction so more reactants form.

Question 28

What is homogeneous equilibrium?

Answer 28

An equilibrium in which all the reactants are in the same state.

Question 29

How does changing the temperature of a system affect equilibrium?

Answer 29

Firstly, identify the sign for enthalpy change
Negative ΔH= exothermic
Positive ΔH= endothermic
Increasing the temperature of the system favours the endothermic reaction
Decreasing the temperature of the system favours the exothermic reaction

Question 30

How does changing the pressure of a system affect equilibrium?

Answer 30

Refer to the number of moles of gases on each side Increasing the pressure of a system favours the side with the fewest moles
Decreasing the pressure of the system favours the side with the most moles
Pressure will have no affect on equilibrium if there is the same number of moles on each side

Question 31

How does the addition of a catalyst affect equilibrium?

Answer 31

A catalyst does not change the position of equilibrium, however it will increase the rate of the forward and reverse reactions equally.

Question 32

What is the equation for the equilibrium constant?

Answer 32

Kc= [C]^c x [D]^d /
—————————-
[A]^a x [B]^b

[A] and [B] = concentration of reactants
[C] and [D] = concentration of products
a,b,c,d = number of moles of each reactant/ product

Question 33

What is the shorthand representation of concentration?

Answer 33

[A] = concentration of chemical A
Eg [CO2]= concentration of CO2

Question 34

What does a Kc value of 1 indicate?

Answer 34

The position of equilibrium is halfway between the reactants and products

Question 35

What does a Kc value of greater than 1 indicate?
Kc > 1

Answer 35

The position of equilibrium shifts towards the products/right.

Question 36

What does a Kc value of less than 1 indicate?
Kc < 1

Answer 36

The position of equilibrium shifts to the reactants/ left

Question 37

How can to the position of equilibrium due to concentration be investigated?

Answer 37

1- Add a solution of yellow Potassium chromate, K2CrO4, to a beaker
2- Add dilute sulfuric acid drop wise until no further change can be observed. The solution turns orange.
3- Add aqueous sodium hydroxide until no further change can be observed. The solution changes back to a yellow colour.

Question 38

Explain why the addition of dilute sulfuric acid causes the yellow Potassium chromate to turn orange.

Answer 38

When dilute sulfuric acid is added, the concentration of H+ ions increases. Therefore, this causes the position of equilibrium to shift to right making more products so the solution turns orange as CrO7^2- forms.

Question 39

How can the position of equilibrium due to temperature be investigated?

Answer 39

1- Dissolve Cobalt chloride in water in a boiling tube. Add a small quantity of hydrochloric acid and place the boiling tube in iced water. The solution is a pink colour.
2- Set up a boiling water bath and place the boiling tube into the boiling water. The solution will turn a blue colour.
3- Transfer the boiling tube back to the iced water to change the solution back to pink.

Question 40

Explain why changing the temperature of Cobalt chloride causes a colour change.

Answer 40

When placed into the boiling water, the heat energy being supplied to the system increases causing equilibrium to shift.
As the forward reaction is endothermic, the position of equilibrium shifts to the right to take heat energy in and minimise the temperature rise.
The solution then turns a blue colour.

Question 41

How can the progress of a chemical reaction be monitored?

Answer 41

By monitoring the removal (decrease in concentration) of a reactant
By following the formation (increase in concentration) of a product.

Question 42

How can the progress of a chemical reaction that produces a gas be monitored?

Answer 42

-Monitoring the volume of gas being produced at regular time intervals using gas collection (gas syringe or upturned measuring cylinder filled with water)
-Monitoring the loss of mass of reactants using a balance

Question 43

When bonds are broken, what does this tell you about the reaction?

Answer 43

Bond breaking is an endothermic process because energy is taken in from the surroundings.

Question 44

When bonds are formed, what does this tell you about the reaction?

Answer 44

Bond making is an exothermic process because energy is released to the surroundings

Question 45

How does adding a catalyst affect the rate of reaction?

Answer 45

A catalyst provides an alternative pathway for the reaction to occur lowering the activation energy meaning more particles are able to exceed the activation energy and more frequent effective collisions can occur per second.