1. Higher Chemistry Unit 1(b) Potential energy diagrams

2. After today’s lesson you should be able to: recognise a potential energy diagram (PED) for an exothermic and endothermic reaction. calculate enthalpy changes from PED include appropriate signs to indicate exothermic and endothermic reactions. calculate the activation energy for a reaction from PED. explain what an activated complex is and how it is related to activation energy. show how a catalyst affects a reaction by use of a PED

3. What is a potential energy diagram? Shows the energy pathway of a reaction as the reactants change into products. Can be used to calculate the activation energy (E A ) and enthalpy change (ΔH) for a reaction.

4. Exothermic reactions An exothermic reaction is one in which the reactants have more energy than the products

5. The enthalpy change for any reaction is equal to the potential energy of the products – the potential energy of the reactants i.e. ΔH = H products – H reactants ΔH always has a negative value for an exothermic reaction. e.g. for the previous reaction ΔH = -969 – (-75) = -894kJ

6. Endothermic reactions An endothermic reaction is one in which the reactants have less energy than the products

7. ΔH always has a positive value for an endothermic reaction. e.g. for the previous reaction ΔH = -1029 – (-1207) = +178kJ

8. Activation energy Most reactants have to have some form of energy supplied to them before they have sufficient kinetic energy to collide and react successfully. This energy is known as the activation energy. The activation energy for a reaction is also represented on a potential energy diagram. The greater the activation energy the higher the maximum point on the graph.

9. E A = the potential energy at the maximum point of the graph – the potential energy of the reactants

10. Activated complex An unstable arrangement of atoms that form when the reactants have sufficient energy to collide successfully. An activated complex exists only for a split second.

11. Reverse reactions Sometimes you are asked to calculate the enthalpy change or activation energy for a reverse reaction i.e. the products changing back into the reactants. For reverse reactions: ΔH = H reactants – H products E A = the potential energy at the maximum point of the graph (the hump of the graph) – the potential energy of the products

12. Catalysts and potential energy diagrams Catalysts lower the activation energy of a reaction. They do not affect the enthalpy of the reactants and products. SCHOLAR - Higher Chemistry : How catalysts work SCHOLAR - Higher Chemistry : How catalysts work

13. Exercise Revision Questions for Higher Chemistry P23 – 25 Exercise 1.7 Q1 - 8