Rates and equilibria

Question 1

What is the rate of reaction?

Answer 1

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

Question 2

What factors affect rate of reaction?

Answer 2

• Temperature.
• Pressure (gaseous reactants).
• Concentration.
• Surface area.
• Presence of catalyst.

Question 3

What does collision theory state?

Answer 3

Particles of reactants can only react together if they collide with each other, and the collision meets these conditions:

1. The particles need to collide with energy greater than or equal to that of the activation energy.
2. The particles need to collide at the correct orientation in order to react.

Question 4

Why does increasing the concentration of reactants increase the rate of reaction?

Answer 4

As the concentration is increased, there are more reactant particles within a given volume. This means that there are more collisions between reactant particles within a given time, meaning that there are more successful collisions within a given time; increasing the rate of reaction.

Question 5

Why does increasing the pressure of gaseous reactants increase the rate of reaction?

Answer 5

As pressure is increased, there are more reactant molecules within a given volume. This means that there are more collisions between reactant molecules within a given time, meaning that there are more successful collisions within a given time; increasing the rate of reaction.

Question 6

Why does increasing surface area (state of division) of solid reactants increase the rate of reaction?

Answer 6

Only exposed particles on the surface of a solid reactant are able to react. As surface area of solid reactant increases, more particles are exposed. This means that there are more collisions between reactant particles within a given time, meaning that there are more successful collisions within a given time; increasing the rate of reaction.

Question 7

Why does increasing temperature increase the rate of reaction?

Answer 7

Increasing the temperature gives the reactant particles more energy which results in 2 things:

1. Particles have more kinetic energy so move around more, resulting in more collisions within a given time.
2. A greater proportion of particles have greater than or equal to the activation energy, so a greater proportion of collisions will have greater than or equal to the activation energy.

Both these things result in more successful collisions within a given time; increasing the rate of reaction.

Question 8

What is the Boltzmann distribution?

Answer 8

The distribution of energies of molecules at a particular temperature, often shown as a graph.

Question 9

What are the important features of the Boltzmann distribution?

Answer 9

• Area under curve is total number of particles and doesn’t change with conditions.
• Graph always start from origin.
• The curve is asymptotic as there is no maximum energy.
• Molecules with greater than or equal to the E_a (activation energy) are able to react.

Question 10

What is a catalyst?

Answer 10

A substance that increases the rate of reaction without being used up in the process.

Question 11

How does a catalyst increase the rate of reactions?

Answer 11

Catalysts increase the rate of reactions by providing an alternative pathway for the reaction to take place with a lower activation energy. This means that more reactant particles have greater than or equal to the activation energy, meaning that there are more successful collisions within a given time; increasing the rate of reaction.

Question 12

What are the environmental benefits of catalysts?

Answer 12

1. Less energy is required for a reaction to take place, meaning that less fossil fuels need to be burnt to produce that energy, reducing carbon dioxide emissions.
2. Catalysts may improve a better yield, meaning that less chemicals are wasted and need to be disposed of, reducing waste production.

Question 13

What are the commercial benefits of catalysts?

Answer 13

1. Less energy is required for a reaction to take place, meaning that less fossils fuels need to be burnt to produce that energy, decreasing energy costs.
2. Faster rates of reaction and improved yields mean that more products can be produced in less time, increasing the stock output and income.

Question 14

What are some examples of catalysts in industry?

Answer 14

• The Haber process is the commercial process for producing ammonia which is used in fertilisers. This process involves breaking a N-N triple bond, a process that would take substantial amounts of energy without the use of an iron catalyst.
• The Ziegler-Natta catalyst is used in the production of polythene.
• Platinum/rhodium based catalytic converters are used for converting potentially toxic exhaust fumes into harmless gases.
• Zeolite is used in catalytic cracking in the oil industry.
• Concentrated sulfuric acid catalyst is used in the hydration of ethene to form ethanol.

Question 15

What are enzymes?

Answer 15

Biological catalysts. Proteins that are shaped specifically to catalyse certain metabolic reactions that only work under specific conditions.

Question 16

What are the benefits of using enzymes as industrial catalysts?

Answer 16

1. Lower temperatures and pressures are required than conventional catalysts, reducing energy costs.
2. Catalysts usually encourage reactions that produce pure products without any waste, removing the need for separation processes.
3. Conventional catalysts may be poisonous and become a disposal problem at the end of their industrial life.

Question 17

What is homogeneous catalysis?

Answer 17

Catalysis of a reaction in which the catalyst and reactants are in the same phase.

Question 18

What is heterogeneous catalysis?

Answer 18

Catalysis of a reaction in which the catalyst and reactants are in different phases.

Question 19

How does a catalytic converter work?

Answer 19

1. Reactants (hydrocarbons, carbon monoxide and nitrogen monoxide) are adsorbed onto the surface of the catalyst by Van der Waal’s interactions.
2. Adsorption disrups electrons in reactants and weakens bonds within the reactants.
3. Chemical reactions occur.
4. Products (carbon dioxide, water and nitrogen) are desorbed from the surface of the catalyst.

Question 20

What reactions does a catalytic converter catalyse?

Answer 20

1. 2CO + 2[O] → 2CO₂
2. 2NO → N₂ + O₂
3. C_xH_y + (2x + y/2) [O] → x CO₂ + y/2 H₂O

Question 21

Why are catalytic converters important?

Answer 21

1. Carbon monoxide is a highly toxic gas that combines with haemoglobin in the blood and prevents it form carrying oxygen.
2. Nitrogen oxides can react with water in the atmosphere to form nitric acid which is the primary cause for acid rain.

Question 22

What is a dynamic equilibrium?

Answer 22

The equilibrium that exists in a closed system when the rate of forward reaction is equal to the rate of the reverse reaction, resulting in no obervable change.

Question 23

What are conditions required for a dynamic equilibrium?

Answer 23

1. The system is closed; no materials are added or removed.
2. The reaction is reversible.

Question 24

What factors affect position of equilibrium?

Answer 24

1. The concentration of reactants and products.
2. Pressure (if gases are involved).
3. Temperature.

Question 25

What is Le Chatelier’s principle?

Answer 25

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

Question 26

What effect would increasing the temperature have on the position of equilibrium?

Answer 26

It would shift the position of equilibrium in the direction of the endothermic reaction which takes in heat from the surroundings in order to reduce the temperature.

Question 27

What effect would increasing the concentration of reactants have on the position of equilibrium?

Answer 27

It would shift the position of equilibrium towards the product side to reduce the concentration of reactants.

Question 28

What effect would increasing the pressure have on the position of equilibrium?

Answer 28

It would shift the position of equilibrium in the direction with the least moles of gases in order to decrease the pressure.

Question 29

What effect does a catalyst have on the position of equilibrium?

Answer 29

It doesn’t affect the position of equilibrium since a catalyst would increase the rate of the forward and reverse reaction equally.

Question 30

What is the haber process?

Answer 30

The industrial process for producing ammonia with the equation:

N₂ + 3H₂ ⇔ 2NH₃ ΔH = -92 KJ.mol^-1

Question 31

What are the ideal conditions for the haber process to take place?

Answer 31

1. High pressures to shift position of equilibrium to the right since there are less moles of gas on the right.
2. Low temperatures to shift position of equilibrium to the right since the forward reaction is exothermic.
3. High temperatures and pressures for fast rate of reaction.

Question 32

What compromises need to be made in the Haber process?

Answer 32

1. The temperature needs to be compromised in order to obtain a decent rate of reaction as well as a decent yield.
2. The pressure needs to be compromised in order to obtain a decent rate of reaction and yield as well as maintaining good safety and reasonable costs.