Understanding Energy Changes in Chemistry: Enthalpy and Reaction Profiles

1. Energy & Energy Changes 2AB Chemistry

2. System and surroundings When a reaction occurs there is energy flow between the system and the surroundings. • The components of chemical reaction form the system reactants products • The surroundings is everything else. • If the reaction takes place in solution it is this that forms the surroundings (rather than the outside of the tube).

3. Enthalpy • All substances have stored chemical potential energy called enthalpy (H). • In a reaction, the products and reactants have differing enthalpies. • Enthalpy cannot be measured directly but the enthalpy change DH can be measured experimentally. • Enthalpy change can also be called the heat of reaction.

4. Energy Profile Diagrams • Energy profile diagrams are used to represent changes in enthalpy between reactants and products. • The x axis is labelled reaction progress or reaction coordinate (NOT time) Exothermic Reaction Endothermic Reaction Products Reactants + H -H Enthalpy Enthalpy Products Reactants Reaction Progress Reaction Progress

5. Exothermic Reactions • The enthalpy of reactants is larger than enthalpy of products • energy is released from the system into the surroundings generally in the form of heat • Exothermic reactions usually feel ‘hot’

6. Endothermic Reactions • Enthalpy of products is greater than enthalpy of reactants • Energy is absorbed from the surroundings during the reaction • Endothermic reactions generally feel ‘cold’

7. Summary

8. An exothermic reaction • If zinc metal is added to a solution of copper sulfate an exothermic reaction takes place.

9. An Endothermic reactions Dissolving ammonium nitrate in water is an example of an endothermic reaction. The DH is +18 kJmol-1

10. Activation Energy • The activation energy is the minimum energy necessary for a reaction to occur.