Chapter 10: Reaction Rates and Equilibrium

Question 1

1. What is meant by rate of reaction?
2. Describe the general rate of reaction at different times.

Answer 1

1. The rate of a chemical reaction measures how fast a reactant is being used up or how fast a product is being formed. The rate of reaction can be defined as the change in conentration of a reactant or a product in a given time.
   rate = change in concentration / time (units mol dm^–3 s^–1)
2. The rate of reaction is fastest at the start of the reaction, as each reactant is at its highest concentration. It slows down as the reaction proceeds, because the reactants are being used up and their concentrations decrease. Once one of the reactants have been completely used up, the concentrations stop changing and the rate of reaction is zero.

Question 2

1. What factors change the rate of chemical reaction?
2. What is the collision theory?

Answer 2

1. The factors affecting the rate of reaction are:
   concentration (or pressure when reactants are gases),
   temperature,
   use of catalyst,
   surface area of solid reactants.
2. The collision theory states that two reacting particles must collide for a reaction to occur. Usually only a small proportion of collisions result in a chemical reaction. In most collisions, the molecules collide but then bounce off each other and remain chemically unchanged.

Question 3

Why are some collisions effective and others ineffective?

Answer 3

An effective collision is one that leads to a chemical reaction. A collision will be effective if two conditions have been met:

• the particles collide with the correct orientation
• the particles have sufficient energy to overcome the activation energy barrier of the reaction.

Question 4

How does increasing the concentration affect the rate of reaction?

Answer 4

When the concentration of a reactant is increased, the rate of reaction generally increases. An increase in concentration increases the number of particles in the same volume. The particles are closer together and collide more frequently. In a given time, there will therefore be more effective collisions (correct orientation and sufficient energy) and an increased rate of reaction.

Question 5

How does increasing the pressure of a gas affect the rate of reaction?

Answer 5

When a gas is compressed into a smaller volume the pressure of a gas is increased and the rate of reaction increases. The concentration of the gas molecules increases as the number of gas molecules occupy a smaller volume. The gas molecules are closer together and collide more frequently, leading to more effective collisions in the same time.

Question 6

How can the progress of a chemical reaction be followed?

Answer 6

The progress of a chemical reaction can be followed by:

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

The method chosen will depend on the properties and physical states of the reactants and products in the reaction. In addition to conentration, measurable properties that might change as the reaction proceeds include gas volume, mass of reactants or products, and colour.

Question 7

What methods can be used to determine the rate of reaction for a reaction that produces a gas?

Answer 7

If a reaction produces a gas, two methods that can be used to determine the rate of the reaction are:

• monitoring the volume of gas produced at regular time intervals using gas collection
• monitoring the loss of mass of reactants using a balance.

Volume of gas produced and mass loss are both proportional to the change in concentration of a reactant or product. So the change in volume with time or the mass loss with time both give a measure of the rate of reaction.

Question 8

Describe the method used to monitor the production of a gas using gas collection and consequently determining the rate of reaction.

Answer 8

Question 9

How can the loss of reactants be monitored using a balance? Use an example to explain your answer.

Answer 9

The rate of reaction between calcium carbonate and hydrochloric acid can also be determined by monitoring the loss in mass of the reactants over a period of time.

The carbonate and the acid are added to a conical flask on a balance. The mass of the flask and its contents is recorded initially and at regular time intervals. The reaction is complete when no more gas is produced so no more mass is then lost. A graph of mass lost against time is plotted.

Question 10

What is a catalyst? Explain.

Answer 10

A catalyst is a substance that changes the rate of a chemical reaction without undergoing any permanent change itself.

• The catalyst is not used up in the chemical reaction.
• The catalyst may react with a reactant to form an intermediate or may provide a surface on which the reaction can take place.
• At the end of the reaction the catalyst is regenerated.

A catalyst increases the rate of a chemical reaction by providing an alternate reaction pathway of lower activation energy.

Question 11

What is a homogenous catalyst? Give two examples.

Answer 11

A homogenous catalyst has the same physical state as the reactants. The catalyst usually reacts with the reactants to form an intermediate. The intermediate then breaks down to givethe product and regenerates the catalyst.

Question 12

What is a heterogenous catalyst?

Answer 12

A heterogenous catalyst has a different physical state from the reactants. Heterogenous catalysts are usually solids in contact with gaseous reactants or reactants in solution. Reactant molecules are absorbed (weakly bonded) onto the surface of the catalyst, where the reaction takes place. After reaction, the product molecules leave the surface of the catalyst be desorption.

Question 13

Describe four industrial processes involving heterogenous catalysts.

Answer 13

Question 14

Describe and explain the Boltzmann distribution.

Answer 14

• In a substance, some molecules move slowly with low energy and some molecules move fast with high energy. Most molecules move close to the average speed and have close to the average energy.
• This spread of molecular energies in gases is known as the Boltzmann distribution. The graph is marked with a line, E_a, that represents the activation energy of a reaction.

Question 15

1. Describe and explain the effect of temperature on a Boltzmann distribution curve.

Answer 15

Part 1

• As the temperature increases, the average energy of molecules also increases.
• A small proportion of molecules will still have low energy, but more molecules have higher energy. The graph is now stretched over a greater range of energy values and the peak is at a higher energy.
• The no. of molecules is the same, so the area under the curve stays the same.

Part 2

• At higher temperature, more molecules have an energy greater than or equal to the activation energy.
• Therefore a greater proportion of collisions will lead to a reaction, increasing the rate of reaction.
• Collisions will also be more frequent as the molecules are moving faster, but the increased energy of the molecules is much more important than the increased frequency of collisions.

Question 16

Show the effect of a catalyst on a Boltzmann distribution curve. What does this mean?

Answer 16

A catalyst provide an alternative reaction route with a lower activation energy. Compared to E_a, a greater proprtion of molecules now have an energy equal to, or greater than the lower activation energy. On collision, more molecules will react to form products. This leads to an increase in the rate of reaction.

Question 17

Describe a system in equilibrium

Answer 17

In an equilibrium system:

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

Question 18

Why are equilibrium systems described as being dynamic?

Answer 18

Equilibrium systems are dynamic. At equilbrium both the forward and reverse reactions are taking place. As fast as the reactants are becoming products, the products are becoming reactants. Therefore in an equilibrium system the concentrations of the reactands and products remain unchanged even though the forward and reverse reactions are still taking place.

Question 19

Describe how a reaction can remain in equilibrium. Explain.

Answer 19

For a reaction to remain in equilibrium, the system must be closed. A closed system is isolated from its surroundings so the temperature, pressure, and concentrations of reactants and products are unaffected by outside influences.

Question 20

1. What does the position of equilibrium indicate?
2. What is le Chatelier’s principle?

Answer 20

1. The position of equilibrium indicates the extend of the reaction. In a reversible reaction, if the temperature, pressure (for reactions involving gases), or concentration of the reactants of products is changed, then the position of equilibrium may change.
2. le Chatelier’s principle states that when a system in equilibrium is subjected to an external change, the system readjusts itself to minimise the effect of that change.

Question 21

Illustrate what happens when an equilibrium system is subjected to a change such as adding more reactant molecules.

Answer 21

The position of equilibrium shifts to the right. More products are made than reactants until a new equilibrium is established.

Question 22

Describe and explain the effect of concentration changes on equilibrium.

Answer 22

Changing the concentration of a reactant or product in an equilbrium system will change the rate of forward or reverse reactions. The position of equilibrium will then change.

When an equilibrium system adjusts as a result of a change:

• if there are more producs formed, the position of the equilbrium has shifted to the right
• if there are more reactants formed, the position of the equilibrium has shifted to the left.

Question 23

Give an example of an experiment used to investigate changes to the position of equilibrium with concentration.

Answer 23

The equilibrium betweem aqueous chromate ions [CrO₄^2–] and dichromate ions [Cr₂O₇^2–] is sensitive to changes in acid concentration. You can carry out a simple experiment to show shift in equilibrium as the two substances have different colours.

1. Add a solution of yellow potassium chromate [K₂CrO₄] to a beaker.
2. Add dilute sulfuric acid [H₂SO₄] dropwise until there is no further change. The solution turns to an orange colour.
3. Add aqueous sodium hydroxide [NaOH(aq)] until there is no further change. The solution changes back to a yellow colour.

Question 24

Explain the changes in colour when dilute sulfuric acid is added to a solution of yellow potassium chromate [K₂CrO₄].

Answer 24

When you add dilute sulfuric acid, you are increasing the concentration of H⁺(aq) ions. This increases the rate of the forward reaction and so causes the position of equilibrium to shift to minimise the change in H⁺(aq) concentration.

1. This shift decreases the concentration of the added reactant, H⁺(aq).
2. The position of equilibrium shifts to the right of the equation, making more products.
3. A new position of equilibrium is established towards the products.
   
   • The solution turns orange as CrO₇^2– forms.

Question 25

Explain the changes in colour sodium hydroxide is added to a solution of yellow potassium chromate [K₂CrO₄] after adding dilute sulfuric acid.

Answer 25

When you add aqueous sodium hydroxide [NaOH(aq)], the added OH^–(aq) ions react with H⁺(aq), decreasing the concentration of H⁺(aq) ions.

The decreased concentration of the reactant [H⁺(aq)] decreases the rate of the forward reaction and so causes the position of equilibrium to shift to minimise the change in concentration.

1. The shift increases the concentration of the reactant that has been removed [H⁺(aq)].
2. The position of equilibrium shifts to the left, making more of the H⁺(aq) reactant.
3. A new position of equilibrium is established.
   
   • The solution turns yellow as CrO₄^2–(aq) forms.

Question 26

Describe and explain the effect of changig the temperature of a system in equilibrium.

Answer 26

Changing the temperature of a system in equilibrium will result in the position of equilibrium changing. The direction in which the equilbrium shifts depends on the sign of ΔH.

• Forward and reverse directions have the same value for the enthalpy change but the signs are opposite.
• An increase in temperature shifts the equilibrium position in the endothermic direction (ΔH is positive).
• A decrease in temperature shifts the equilbrium position in the exothermic direction (ΔH is negative).

Question 27

Cobalt chloride [CoCl2] dissolves in water to form a pink solution. The dissolving process actually produces an equilibrium between two complexes of cobalt that are different colours.

Describe a simple experiment to show the sensitivity of the equilibrium to changes in temperature.

Answer 27

Question 28

Cobalt chloride [CoCl2] dissolves in water to form a pink solution. The dissolving process actually produces an equilibrium between two complexes of cobalt that are different colours.

Explain the effect of increasing the heat energy of the system. By extension, what is the effect of reducing the energy again?

Answer 28

In the boiling water you are increasing the heat energy of the system. This causes the position of equilibrium to shift to minimise the change.

• As the forward reaction is endothermic (ΔH is positive), the position of equilibrium shifts to the right in the endothermic direction, to take heat energy in and minimise the increase in temperature.
• The solution turns a blue colour.

Decreasing the temperature shifts the position of equilibrium in the opposite direction, in the direction that gives out energy, to the reverse exothermic side (ΔH is negative) on the left.

Question 29

Give a summary of the effect of temperature on the position of equilibrium for exothermic and endothermic reactions.

Answer 29

Question 30

Describe and explain the effect of pressure changes on equilibrium.

Answer 30

Changing the pressure of a system containing gases in equilibrium may result in the position of equilibrium changing, but only if there are more gaseous molecules on one side of the equation than the other.

Question 31

The gases nitrogen dioxide [NO₂(g)] and dinitrogen tetroxide [N₂O₄(g)] have different colours.

Describe and explain the effect of increasing the pressure of the system. By extension, describe the effect of decreasing the pressure.

Answer 31

The pressure of the gas is proportional to its concentration. In the same container, two moles of NO₂(g) would have twice the concentration and twice the pressure as the same container holding one mole of N₂O₄(g).

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

• As there are fewer gaseous moles on the right side of the equilibrium, the position of equilibrium shifts to the right reducing the number of gaseous moles to minimise the increase in pressure.
• More colourless N₂O₄(g) is formed and the brown colour fades.

Decreasing the pressure shifts the position of equilibrium in the opposite direction, to the side with more gaseous moles on the left thus making the brown colour deeper.

Question 32

Describe and explain the effect of a catalyst on equilibrium.

Answer 32

A catalyst does not change the position of equilibrium; it only speeds up the rates of the forward and reverse reactions equally.

A catalyst will, however, increase the rate at which an equilibrium is established.

Question 33

How is the exact position of equilibrium calculated?

Answer 33

Question 34

What does the value of K_c tell us?

Answer 34

The magnitude of Kc indicates the relative proportions of reactants and products in the equilibrium system.

As a rough guide,

• a Kc value of 1 indicates a position of equilibrium that is halfway between reactants and products.
• a Kc value > 1 indicates a position of equilibrium that is towards the products.
• a Kc value < 1 indicates a position of equilibrium that is towards the reactants.

So the larger the value of Kc, the further the position of equilibrium lies to the right and the greater the concentrations of the products compared to the reactants.