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.

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.

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

Endothermic reactions absorb heat. EXAMPLES decomposition reaction some precipitation reactions

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

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

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.

EXAMPLE OF ENTHALPY CALCULATIONS

EXAMPLE OF ENTHALPY CALCULATIONS