Chemical Reactions That Involve Heat Objectives: 1. Describe the difference between exothermic and endothermic reactions. 2. Explain the relationship between the heat capacity and the specific heat of a substance. Key Terms: Thermochemistry, exothermic reaction, endothermic reaction, heat capacity, specific heat

Chemical Reactions That Involve Heat The study of the changes in the heat of reactions is called thermochemistry. The changes in energy result from the storage or release of chemical potential energy in the bonds of a substance. The heat (q) of reactions is recorded in Joules since heat is a type of energy. – Heat is the measure of the KE of a substance while temperature is the measure of the average KE of a substance (KE/Mass) – heat always flows from a warmer substance to a cooler substance – reactions will always conform to the law of conservation of energy – heat can either be measured in Joules or cal (1 cal = 4.184J) When chemicals react they involve the transfer of energy (heat). This transfer can result in the heating (exothermy) or the cooling (endothermy) of the environment.

Exothermic Reactions Exothermic reactions are reactions that liberate heat energy. The excess heat in the reaction is the result of: – the formation of new bonds in the products – synthesis of more moles of products than reactants – increasing in order through a phase change H 2 O (l)  H 2 O (s) + energy One of the most obvious types of exothermic reaction is the combustion reaction In an exothermic chemical equation, heat is placed in the products – Reactants contain more energy than products as the energy came from stored chemical potential energy

Endothermic Reactions Exothermic reactions are reactions that absorb heat energy. The gain in heat energy of the reactants is the result of: – the absorption of energy to break bonds in the reactants – Decreasing in order through a phase change H 2 O (s) + energy  H 2 O (l) In an endothermic chemical equation, heat is placed in the reactants – Products contain more energy than reactants as the energy came from the environment and is stored as chemical potential energy

Heat Capacity The heat capacity is the amount of heat energy needed to raise the temperature of a substance 1 o C – specific heat (C) is the amount of energy required to raise 1 g of a substance 1 o C ex: it takes 4.184J (1cal) to raise 1 gram of water 1 o C – The heat capacity of 1 gram of the substance is its specific heat C = Heat (q) / (mass (g) x change in temp (  T)) – q/(g x  T) – unit = J/g o C or cal/g o C – The heat capacity of an object depends on the mass and its composition