Thermochemistry Chemistry 12

What is Thermochemistry? Thermochemistry is the study of heat changes that occur during chemical reactions, phase changes, solution formation and nuclear reactions. Chapters 16 and 17 MHR and Chapters 10 and 11, Nelson Chemistry support this unit. Energy is the capacity for doing work or supplying heat. Heat is a form of energy. Heat flows from a warmer body to a colder body.

Temperature A measure of the average kinetic energy of the particles of a system. The system is the part of the universe on which we focus our attention. The surroundings include everything else in the universe. In thermochemistry, the surroundings are considered to be only the region in the immediate vicinity of the system. Temperature measures the change in the surroundings.

Surroundings System

First Law of Thermodynamics AKA, the Law of Conservation of Energy - the energy of the universe remains constant. Heat flows from the system to the surroundings or from the surroundings to the system. No energy is created or destroyed as the result of these heat transfers.

The relationship of heat and temperature. Heat is energy transferred between substances. The transfer of heat to a substance increases the temperature of the substance by causing faster molecular motion, an increase in the kinetic energy of the particles of the substance. A change in the temperature of a substance as measured with a thermometer or temperature probe is explained theoretically as a change in the average kinetic energy ∆Ek of the particles of the substance.

Temperature Change and Heat Capacity, pg. 630-634 MHR, pg 341 Nelson ∆t = change in temperature If heat flows out of a substance, and there are no phase changes, then the temperature of the substance decreases. If heat flows into a substance, and there are no phase changes, then the temperature of the substance increases. ∆t depends on the quantity of heat, q flowing into or out of a substance. ∆t also depends on the heat capacity of the substance.

Heat Capacity Heat capacity is the heat required to change the temperature of a unit quantity of the substance. Two types: Specific heat capacity: the quantity of heat required to raise the temperature of a unit mass (one gram) of a substance one degree Celsius. water = 4.19 J/(g .°C) Volumetric heat capacity: the quantity of heat required to raise the temperature of a unit volume (one cubic metre or one litre) of a substance one degree Celsius. Useful with gases and liquids. air = 1.2 kJ/(m3 .°C)

Heat Capacity Continued Density of pure water = 1g/ml or 1kg/L, therefore, the values of the specific and volumetric heat capacities of water are numerically the same: cH2O = 4.19 J/g °C = 4.19 kJ/L°C = 4.19 MJ/L °C The quantity of heat q that flows varies directly with the quantity of substane (mass m or volume v), the specific heat capacity c, and the temperature change ∆t. q = mc∆t or q = vc∆t Refer to inside back cover tables of specific and volumetric heat capacities. Try problems 1-4, pg 634, MHR and 6-13, page 342 and 343, Nelson.