1. HEAT IN CHEMICAL REACTIONS
Lesson Outcome:
To understand the concepts of endothermic and exothermic reactions.
To be able to calculate the heat given off or needed by a chemical reaction.

2. Exothermic and Endothermic Reactions
Joule: The SI unit for energy and heat.
Bond breaking vs. bond forming.
Combustion is a process in which a self-sustaining chemical reaction occurs at temperatures above those of the surroundings.

3. Exothermic reactions release heat. EXAMPLES:
direct combination reactions
reactions of metals with water and acids
reactions of acids with bases

4. Endothermic reactions absorb heat. EXAMPLES
decomposition reaction
some precipitation reactions

5. Enthalpy (H)
ENTHALPY is a measure of the total energy possessed by a substance or group of substances.
The change in enthalpy for a chemical reaction, ΔH, is defined as the heat absorbed (per mole of specified reactant or product) when the reaction occurs at constant pressure.
ΔH = enthalpy of products – enthalpy of reactants

6. Heat transfer and the sign of the enthalpy change:
When ΔH is Positive
Endothermic
Heat absorbed
Eg: +95 KJ/mol
When ΔH is Negative
Exothermic
Heat released
Eg: – 56KJ/mol

7. Enthalpy of Reactions ΔHrxn = Hproducts - Hreactants
The enthalpy change (ΔH) for a reaction is equal to the heat absorbed or gained during the reaction.
The amount of heat absorbed / released is dependent on the quantity (mole).
Standard enthalpy change (ΔHo): at 1atm and 250C.

8. EXAMPLE OF ENTHALPY CALCULATIONS

9. EXAMPLE OF ENTHALPY CALCULATIONS