Module 3: Chapter 10 (Reaction rates and equilibrium)

Question 1

What is the rate of chemical reaction?

Answer 1

How fast a reactant is used up OR how fast a product is formed

Question 2

On a progress curve why is it the steepest at the start?

Answer 2

The rate is the fastest at the start of a reaction as each reactant is at its highest concentration

Question 3

What are the factors that affect the rate of a chemical reaction?

Answer 3

• Concentration (or pressure if reactants are gases)
• Temperature
• Use of a catalyst
• Surface area of solid reactants
• Radiation

Question 4

Describe Collision Theory?

Answer 4

For a reaction to occur reactant particles must come together and collide with:

• the correct orientation
• energy greater than the activation energy for the reaction

Question 5

How does increasing the concentration affect the rate?

Answer 5

Low concentration - less particles in a given volume so less frequent collisions and therefore a slower rate of reaction

Higher concentration - more particles in a given volume so more frequent collisions and therefore a faster rate of reaction

Question 6

How can the progress of a reaction be monitored?

Answer 6

By monitoring the removal of a reactant or monitoring the formation of a product, for example:

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

Question 7

What is a catalyst?

Answer 7

1. A catalyst is a substance that changes the rate of a chemical reaction without undergoing any permanent change itself
   · not used up itself
   · may react with the reactant to form an intermediate or may provide a surface for the reaction to happen on
   · at the end of the reaction it is always regenerated
2. A catalyst provides an alternative reaction pathway with a lower activation enthalpy

Question 8

What is a homogenous catalyst?

Answer 8

A homogenous catalyst has the same physical state as the reactants

E.g. ozone depletion
Cl· radicals from CFC’s catalyse the breakdown
2O3(g) <=Cl·(g)=> 3O2(g)

Question 9

What is a heterogenous catalyst?

Answer 9

A heterogenous catalyst has a different physical state from the reactants

Question 10

Describe the process of how catalysts work?

Answer 10

1. The reactant molecules form weak bonds with the catalyst surface - this is called adsorption
2. Bonds with the reactant molecules break
3. New bonds form
4. Product molecules leave the catalyst surface - this is known as desorption

Question 11

How are catalysts used in industry?

Answer 11

• Catalysts increase the rate of many industrial chemical reactions by providing an alternative reaction pathway with a lower activation enthalpy
• This lowers the temperature needed for the reaction
• Lower temperatures means less electricity from combustion of fossil fuels used
• Less combustions leads to a reduction of CO2 emissions and other atmospheric pollutants
• Examples: Rhodium, Platinum and Palladium

Question 12

What does a Boltzmann distribution curve show?

Answer 12

Shows the spread of molecular energies in a sample of gas molecules

Question 13

Draw and label and label a Boltzmann distribution curve

Answer 13

• No molecules have zero energy, so the curve starts at the origin
• The area under the curve equals the total number of molecules
• There is no maximum energy for a molecule so the curve never meets the x-axis
• A line of Ea and shaded area shows the proportion of molecules with an energy greater than the activation energy

Question 14

Describe the line on the diagram is it was at a higher temperature

Answer 14

The peak is lowered and shifted to the right

A greater of proportion of molecules have an energy greater than the activation energy

Question 15

What happens at a higher temperature?

Answer 15

• more molecules have an energy greater than/equal to the activation energy
• a greater proportion of collisions will lead to a reaction, increasing the reaction rate
• collisions will be more frequent as the molecules are moving faster, which also increases the reaction rate

Question 16

What happens to in the presence of a catalyst?

Answer 16

• A greater proportion of molecules exceeds the new lower activation energy
• more molecules collide with an energy greater than the activation energy
• more effective collisions, therefore faster rate of reaction

Question 17

What happens in an equilibrium system?

Answer 17

• the rate of the forwards reaction is equal to the rate of the reverse reaction
• the concentrations of the reactants and products remain constant
• system must remained closed

Question 18

What is Le Chatelier’s principle?

Answer 18

When a system in equilibrium is subjected to an external change the system readjusts itself to minimise the effects of that

· the position of equilibrium indicates how far the reaction has progressed when equilibrium is established

Question 19

Describe what happens to the equilibrium for changing concentrations

Answer 19

A + B ⇌ C + D

· An increase in conc of A or B, or a decrease in conc in C or D causes the equilibrium position to move to the right (e.g. Add an acid, concentration of H+ ions increases so equilibrium position moves to the right/left)

· An increase in conc of C or D, or a decrease in conc of A or B the equilibrium concentration moves to the left

Question 20

Describe what happens to the equilibrium for changing pressure

Answer 20

only affects gases into gases
2NO2(g) ⇌ N2O4(g)
brown colourless

· an increase in pressure equilibrium position move to the side with fewer gaseous molecules (here it would move to the right, colourless colour)

· a decrease in pressure equilibrium position moves to side with more gaseous molecules

Question 21

Describe what happens to the equilibrium for changing temperature

Answer 21

• a decrease in temperature, equilibrium favours the exothermic process
• an increase in temperature, equilibrium will favour the endothermic process

Question 22

What happens to equilibrium position when a catalyst is added?

Answer 22

• doesn’t change the position of equilibrium
• it speeds up the rates of the forwards and reverse reaction equally
• it will increase the rate at which equilibrium is established

Question 23

How is the equilibrium constant calculated?

Answer 23

[C]^c [D]^ [products]
Kc = —————- = —————–
[A]^a [B]^b [reactants]

Question 24

What does Kc tell us?

Answer 24

the magnitude of Kc tells us the relative proportions of reactants and products in the equilibrium system