Energy Transformations
Thermochemistry is the study of energy
changes that occur during chemical reactions
and changes in state.
The energy stored in the chemical bonds of a
substance is called chemical potential energy.
Energy Transformations
Heat, represented by q, is energy that
transfers from one object to another because
of a temperature difference between them.
Heat always flows from a warmer object to a
cooler object.
Exothermic and Endothermic
Processes
In studying energy changes, you can define a
system as the part of the universe on which
you focus your attention.
The surroundings include everything else in
the universe.
The law of conservation of energy states that
in any chemical or physical process, energy is
neither created nor destroyed.
Exothermic and Endothermic
Processes
An endothermic process is one that absorbs
heat from the surroundings.
In an endothermic process, the system gains
heat as the surroundings cool down.
Exothermic and Endothermic
Processes
An exothermic process is one that releases
heat to its surroundings.
In an exothermic process, the system loses
heat as the surroundings heat up.
Units for Measuring Heat Flow
A calorie is defined as the quantity of heat
needed to raise the temperature of 1 g of pure
water 1°C.
The energy in food is usually expressed in
Calories.
Units for Measuring Heat Flow
One joule of heat raises the temperature of 1
g of pure water 0.2390°C.
The joule is the SI unit of energy.
1 J = 0.2390 cal
4.184 J = 1 cal
Heat Capacity and Specific Heat
The amount of heat needed to increase the
temperature of an object exactly 1°C is the
heat capacity of that object.
Heat Capacity and Specific Heat
The specific heat capacity, or specific heat, of
a substance is the amount of heat it takes to
raise the temperature of 1 g of the substance
by 1°C.
Heat Capacity and Specific Heat
Calorimetry
Calorimetry is the precise measurement of
the heat flow into or out of a system for
chemical and physical processes.
In calorimetry, the heat released by the
system is equal to the heat absorbed by the
surroundings, and vice versa.
Calorimetry
The insulated device used to measure the
absorption or release of heat in chemical or
physical processes is called a calorimeter.
Calorimetry
The heat content of a system at constant
pressure is the same as a property called
enthalpy (H) of the system.
Thermochemical Equations
A chemical equation that includes the
enthalpy change is called a thermochemical
equation.
In a chemical reaction, the enthalpy change
for the reaction can be written as either a
reactant or a product.
Thermochemical Equations
The heat of reaction is the enthalpy change
for the chemical equation exactly as it is
written.
Thermochemical Equations
The heat of combustion is the heat of reaction
for the complete burning of one mole of a
substance.
Thermochemical Equations
Heats of Fusion and Solidification
The heat absorbed by one mole of a solid
substance as it melts to a liquid at a constant
temperature is the molar heat of fusion
(ΔHfus).
The molar heat of solidification (ΔHsolid) is the
heat lost when one mole of a liquid solidifies
at a constant temperature.
ΔHfus = -ΔHsolid
Heats of Vaporization and
Condensation
The amount of heat necessary to vaporize one
mole of a given liquid is called its molar heat
of vaporization (ΔHvap).
The amount of heat released when 1 mol of
vapor condenses at the normal boiling point is
called its molar heat of condensation (ΔHcond).
ΔHvap = -ΔHcond
Heats of Vaporization and
Condensation
Heat of Solution
During the formation of a solution, heat is
either released or absorbed.
The enthalpy change caused by dissolution of
one mole of a substance is the molar heat of
solution (ΔHsoln).
Hess’s Law
Hess’s law states that if you add two or more
thermochemical equations to give a final
equation, then you can also add the heats of
reaction to give a final heat of reaction.
Standard Heats of Formation
The standard heat of formation (ΔHf°) of a
compound is the change in enthalpy that
accompanies the formation of one mole of a
compound from its elements with all
substances in their standard states at 25°C.
Standard Heats of Formation