Unit 1 - Reaction Profiles

Question 1

Describe an exothermic reaction

Answer 1

Energy is released to the surroundings.
- The temperature rises.

Question 2

Describe an endothermic reaction

Answer 2

Energy is taken in from the surroundings.
- The temperature falls.

Question 3

What does a potential energy diagram show?

Answer 3

The energy pathway for a reaction.

Question 4

What is enthalpy?

Answer 4

The energy content or potential energy of a substance.

Question 5

What is enthalpy change?

Answer 5

The energy difference between products and reactants.
- ∆H

Question 6

What are the units for enthalpy?

Answer 6

Kilojoules (kJ) and Joules (J).

Question 7

What are the units for molar enthalpy?

Answer 7

Kilojoules per mole (kJ mol⁻¹).

Question 8

Sketch the reaction profile for an exothermic reaction

Answer 8

Reaction Progress on x axis.
Energy (kJ mol⁻¹) on y axis.
- Reactants have more energy than products.
- ∆H is negative.
- Activated complex is at the highest point of the reaction pathway.

Question 9

Sketch the reaction profile for an endothermic reaction

Answer 9

Reaction Progress on x axis.
Energy (kJ mol⁻¹) on y axis.
- Products have more energy than reactants.
- ∆H is positive.
- Activated complex is at the highest point of the reaction pathway.

Question 10

What sign does ∆H have for exothermic reactions?

Answer 10

Negative.

Question 11

What sign does ∆H have for endothermic reactions?

Answer 11

Positive.

Question 12

How would you calculate ∆H for a reverse reaction?

Answer 12

It is the same value as for the forward reaction but has the opposite sign (-v or +ve).

i.e. if forward reaction +50 kJ mol⁻¹ then reverse reaction will be -50 kJ mol⁻¹.

Question 13

What is the activation energy?

Answer 13

The minimum energy required by colliding particles to form an activated complex.

Question 14

What is the activated complex?

Answer 14

An unstable arrangement of atoms found at the maximum of a potential energy barrier during a reaction.

Question 15

Sketch a Maxwell-Boltzmann distribution curve for a lower and higher temperature

Answer 15

Reaction Progress on x axis.
Number of particles on y axis.
- 2 curves on graph, one at lower temperature and one at higher temperature.
- Ea is fixed on the y axis.
- The lower temperature curve has only a small proportion of particles with energy greater than Ea.
- The higher temperature curve has a larger proportion of particles with energy greater than Ea. It appears to have moved over to the right compared to the lower temperature graph.
- Both curves have the same area underneath them.

Question 16

What is temperature?

Answer 16

A measure of the average kinetic energy of the particles in a substance.

Question 17

Explain how a small rise in temperature leads to a greater rate of reaction

Answer 17

An increase in temperature means a greater number of molecules with energy greater than the activation energy Ea.
- There are more successful collisions.
- Increased rate of reaction.

Question 18

Describe how use of a catalyst increases rate of reaction

Answer 18

A catalyst provides an alternative pathway with a lower activation energy.

Question 19

Sketch a potential energy diagram to show the effect of a catalyst on activation energy

Answer 19

Reaction Progress on x axis.
Energy (kJ mol⁻¹) on y axis.
- A potential energy diagram should be drawn showing 2 curves.
- The uncatalysed curve should have a higher Ea than the catalysed curve but both should start and end at the same point.