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
• H2O(l) g H2O(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
• H2O(s) + energy g H2O(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 1oC
– specific heat (C) is the amount of energy required to raise 1 g of a
substance 1oC
• ex: it takes 4.184J (1cal) to raise 1 gram of water 1oC
– The heat capacity of 1 gram of the substance is its specific heat
• C = Heat (q) / (mass (g) x change in temp (DT))
–
q/(g x DT)
– unit = J/goC or cal/goC
– The heat capacity of an object depends on the mass and its
composition