Enthalpy

1. Enthalpy Calorimetry of Chemistry

2. Enthalpy • During a chemical reactions bonds are broken and/or made. When this occurs we can gain or lose chemical potential energy. • If we lose chemical potential energy then it must go somewhere because energy is never created or destroyed, it just changes form. • chemical potential energy transforms into heat energy.

3. Enthalpy • potential energy of a chemical. • measured in kilojoules per mole (kJ/mol).

4. Reaction profile diagrams show the change in enthalpy over the progress of a reaction. • Note that there is a small energy lump to get over before the reaction can start. This is called an activation energy e.g. a match needs a bit of heat to get it started – this is the activation energy

5. Ea Energy Reactants Products ΔH Reaction Coordinate General Reaction Scheme

6. Enthalpy of Reactions • Enthalpy depends on conditions. • Formula to determine enthalpy: H = Hfinal - Hinitial

7. Where does the Energy go? In the case of a chemical reaction, you need to keep the different types of energy separate in your mind: • Bond energy – energy INSIDE the molecules • Thermal energy (heat) – kinetic energy of the molecules • Energy of the “bath” – kinetic energy of solvent or other molecules in the system

8. Exothermic Reactions • ‘Exo’ means out and ‘thermic’ means heat. So Exothermic means ‘heat (coming) out’. • For example a fire is an exothermic reaction. In other words exothermic reactions are reactions where heat is made. • Exothermic reactions show that • the reactants have lost chemical potential energy (enthalpy) • negative change in enthalpy (- ΔH). • energy is a REACTANT • Bond making reactions are exothermic

9. Ea Energy Reactants Products ΔH Reaction Coordinate Exothermic Reaction – “hot pack”

10. Exothermic energy changes H = Hfinal – Hinitial < 0 Hinitial>Hfinal This energy is internal to the molecule. The excess gets absorbed by the rest of the system as heat causing the molecules to move faster (more kinetic energy) and the temperature to increase.

11. Endothermic Reactions • ‘Endo’ means in, so endothermic means ‘heat (going) in’. • In other words endothermic reactions have a loss of heat. The reaction causes the chemicals to get colder. • An endothermic reaction causes and increase in enthalpy because the • heat energy has been transformed into chemical potential energy. • there has been a positive change in enthalpy (+ ΔH). - Energy is a PRODUCT • Bond breaking reactions are endothermic.

12. Endothermic Reaction – “cold pack” Ea Energy Products Reactants ΔH Reaction Coordinate

13. Endothermic energy changes H = Hfinal – Hinitial > 0 Hinitial<Hfinal This energy is internal to the molecule and must come from somewhere. The additional energy required by the system gets absorbed from the rest of the system as heat causing the molecules to move slower (less kinetic energy) and the temperature to decrease.

14. Clicker Question Consider the following reaction: 2 H2 (g) + O2 (g) 2 H2O (g) If Hrxn < 0, it means: • The products have less energy than the reactants – you could make a hot pack. • The reactants have less energy than the products – you could make a cold pack. • The products have less energy than the reactants – you could make a cold pack. • The reactants have less energy than the products – you could make a hot pack.

15. ENDOTHERMIC Lets compare… Exothermic A change in a chemical energy where energy/heat EXITS the chemical system Results in a decrease in chemical potential energy A change in chemical energy where energy/heat ENTERS the chemical system Results in an increase in chemical potential energy