1. 1
   A chemistry teacher draws an energy level diagram on the board to describe an exothermic reaction. The diagram shows a horizontal line for the reactants, a curved line rising to a peak and then falling to a horizontal line for the products, which is lower than the reactants line.

   (a)

   A teacher describes an energy level diagram for an exothermic reaction, in which the line representing the products is drawn lower down the energy axis than the line representing the reactants. What does this represent?

   [1 mark]

   • AThe products have less energy than the reactants
   • BThe products have more energy than the reactants
   • CThe products and reactants have exactly the same energy
   • DThe reaction has no activation energy

   (b)

   What does the peak in the curved line on this energy level diagram represent?

   [1 mark]

   • AThe overall energy change of the reaction, $\Delta H$
   • BThe activation energy of the reaction
   • CThe energy released by the reaction
   • DThe temperature of the surroundings

   (c)

   Describe how the overall energy change, $\Delta H$, for this exothermic reaction is shown on the energy level diagram, and state the sign convention that is used to represent this value.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student draws an energy level diagram for a thermal decomposition reaction, which is known to be endothermic. The diagram includes a horizontal reactants line, a curved line rising to a peak, and a horizontal products line.

   (a)

   A student draws a different energy level diagram, this time for an endothermic reaction. Where should the products line be drawn relative to the reactants line?

   [1 mark]

   • ABelow the reactants line
   • BAt exactly the same height as the reactants line
   • CAbove the reactants line
   • DThe height of the products line cannot be determined for an endothermic reaction

   (b)

   Which feature would show the effect of adding a catalyst to this reaction on the same energy level diagram?

   [1 mark]

   • AA wider gap between the reactants and products lines
   • BA lower reactants line
   • CA higher products line
   • DA lower peak (lower activation energy), following a different pathway between the same reactants and products lines

   (c)

   Explain why the peak of the curved line represents the activation energy, and state what would happen to the rate of this reaction if the activation energy were lowered by using a catalyst.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A textbook chapter compares energy level diagrams for a reversible reaction, showing the forward reaction (exothermic) and the reverse reaction (endothermic) as separate diagrams, in order to illustrate how the position of reactants and products lines can change.

   (a)

   A textbook shows two energy level diagrams side by side for the same reversible reaction: one for the forward (exothermic) reaction and one for the reverse (endothermic) reaction. Describe how the reactants and products lines would differ between the two diagrams.

   [3 marks]

   (b)

   Explain why the activation energy for the reverse (endothermic) reaction is different from the activation energy for the forward (exothermic) reaction, describing how each activation energy is measured on its respective diagram.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A revision guide asks students to practise sketching and correctly labelling energy level diagrams for both exothermic and endothermic reactions, and to explain the effect a catalyst has on such a diagram.

   (a)

   Describe fully how you would draw and label an energy level diagram for an exothermic reaction, including the reactants line, products line, activation energy and overall energy change, and explain how each labelled feature relates to the chemistry of the reaction.

   [6 marks]

   (b)

   Describe fully how the same energy level diagram for this exothermic reaction would need to be redrawn to show the effect of a catalyst, and explain why a catalyst changes the rate of reaction but does not change the overall energy change, $\Delta H$.

   [6 marks]

   Total for question 4: 12 marks

End of questions