Chapter 10

Question 1

How does concentration/ pressure affect the rate of reaction?

Answer 1

• The higher the concentration/ pressure, the more molecules per unit volume, which increases the frequency of collisions

Question 2

What are 3 ways of monitoring the rate of reaction of reactions that produce gas?

Answer 2

• Use a gas syringe to measure the volume of gas produced
• Place the conical flask (with wool at the top to prevent any splashing) on a balance, and record the change in mass
• Use an inverted measuring cylinder and trough filled with water, and record the volume of displaced water

Question 3

How is rate of reaction usually shown on a graph?

Answer 3

• Time (s) against concentration/ mass/ volume of gas, etc.
• The rate of reaction is the gradient

Question 4

What do catalysts do?

Answer 4

• A catalyst provides an alternative reaction route with a lower activation energy
• This causes a greater proportion of the molecules to have the activation energy or more
• This increases the rate of the reaction
• They themselves are not used up

Question 5

What needs to be included on an enthalpy profile (including the use of a catalyst)?

Answer 5

• x-axis: progress of reaction
• y-axis: enthalpy
• ΔH
• Ea
• Ec

Question 6

What is a heterogeneous catalyst?

Answer 6

• A catalyst that has a different physical state from the reactants

Question 7

What is a homogeneous catalyst?

Answer 7

• A catalyst that has the same physical state as the reactants

Question 8

What are 2 benefits of using a catalyst?

Answer 8

• Lower temperatures and pressures can be used
• This decreases the combustion of fossil fuels and therefore carbon dioxide emissions

Question 9

What does the Boltzmann distribution show?

Answer 9

• The spread of molecular energies in gases

Question 10

When drawing the Boltzmann distribution, what are 7 things to include?

Answer 10

• x-axis: energy
• y-axis: number of molecules
• Starts at origin
• Peak
• Lower gradient on right
• Ea
• Never meets x-axis

Question 11

Why does the graph cross at the origin?

Answer 11

• None of the molecules have 0 energy

Question 12

What does the area under the graph show?

Answer 12

• The total number of molecules

Question 13

Why does the curve never meet the x-axis?

Answer 13

• There is no maximum energy of a molecule- the curve would need to reach infinite energy for it to meet the x-axis

Question 14

How does temperature affect a Boltzmann distribution curve?

Answer 14

• The peak is lower and to the right
• The curve still has a higher gradient on the left side rather than the right side, and the Ea is in the same position
• The area under the curve is the same

Question 15

How does increasing the temperature increase the rate of reaction?

Answer 15

• At higher temperatures, more molecules have energy greater than or equal to the activation energy

Question 16

How does a catalyst affect a Boltzmann distribution curve?

Answer 16

• There is a new, lower activation energy; Ec

Question 17

When does dynamic equilibrium exist?

Answer 17

• In a closed system where the rate of the forwards and backwards reactions are equal, and the concentrations of the products and reactants don’t change

Question 18

What is Le Chatelier’s principle?

Answer 18

• The position of a dynamic equilibrium shifts to minimise the effect of any change

Question 19

How do changes in concentration affect equilibrium?

Answer 19

• If there is an increase in the concentration of the reactants or a decrease in the concentration of the products, equilibrium shifts to the right (so that more of the products are formed)
• If there is an increase in the concentration of the products or a decrease in the concentration of the reactants, equilibrium shifts to the left

Question 20

How does temperature affect the position of equilibrium?

Answer 20

• Increasing the temperature shifts the position of equilibrium towards the products of the endothermic reaction
• Decreasing the temperature shifts the position of equilibrium away from the products of the endothermic reaction

Question 21

How does pressure affect equilibrium?

Answer 21

• Increasing pressure shifts equilibrium towards the side with fewer moles of gas
• Decreasing the pressure does the opposite

Question 22

How does a catalyst affect equilibrium?

Answer 22

• It doesn’t have an effect on the position of equilibrium
• It increases the rate of both the forward and backward reactions by the same amount

Question 23

What are some limitations when choosing conditions for reversible reactions in industry?

Answer 23

• High pressures are expensive and dangerous
• Low temperatures can sometimes increase the yield of product, but the rate of reaction would be too slow

Question 24

What is Kc, and what does it show?

Answer 24

• An equilibrium constant
• The position of equilibrium in homogeneous reactions

Question 25

What do square brackets mean?

Answer 25

• The concentration of whatever is inside the brackets

Question 26

How is the equilibrium constant calculated?

Answer 26

• If your reaction is aA + bB <-> cC + dD then Kc=[C]^c[D]^d / [A]^a[B]^b

Question 27

What does the calculated value of Kc tell us?

Answer 27

• If it less than 1, the position of equilibrium is towards the reactants
• If it is equal to 1, it is halfway between the reactants and the products
• If it greater than 1, it is towards the products