14
Thermochemistry
Thermochemistry is the study of the thermal (heat) changes that are associated
with physical and chemical changes. Thermodynamics is much broader in
scope because it includes the study of all forms of energy, including work.
Some of these aspects are considered in later chapters.
CHANGES IN TEMPERATURE
If heat is applied to a substance, the temperature is raised; if heat is withdrawn,
the temperature is lowered. The unit of heat is the calorie (cal), which is defined
as the quantity of heat required to raise the temperature of 1 g of water 1 degree
Celsius. We shall not deal with problems so accurately as to be concerned about
the very small difference between a "15° calorie" (the heat needed to raise 1 g
of water from 14.5°C to 15.5°C), or the "mean calorie" (1 one-hundredth of the
heat needed to raise 1 g of water from 0°C to 100°C).
The number of calories required to raise the temperature of an object 1°C is
called the heat capacity of the object. The molar heat capacity of a substance is
the number of calories needed to raise the temperature of a mole of the substance rc.
The specific heat of any substance is the number of calories required to raise
one gram of it 1°C. From our definition of the calorie, it follows that the specific
heat of water is 1 cal/g °C.
Thermochemistry
Thermochemistry is the study of the thermal (heat) changes that are associated
with physical and chemical changes. Thermodynamics is much broader in
scope because it includes the study of all forms of energy, including work.
Some of these aspects are considered in later chapters.
CHANGES IN TEMPERATURE
If heat is applied to a substance, the temperature is raised; if heat is withdrawn,
the temperature is lowered. The unit of heat is the calorie (cal), which is defined
as the quantity of heat required to raise the temperature of 1 g of water 1 degree
Celsius. We shall not deal with problems so accurately as to be concerned about
the very small difference between a "15° calorie" (the heat needed to raise 1 g
of water from 14.5°C to 15.5°C), or the "mean calorie" (1 one-hundredth of the
heat needed to raise 1 g of water from 0°C to 100°C).
The number of calories required to raise the temperature of an object 1°C is
called the heat capacity of the object. The molar heat capacity of a substance is
the number of calories needed to raise the temperature of a mole of the substance rc.
The specific heat of any substance is the number of calories required to raise
one gram of it 1°C. From our definition of the calorie, it follows that the specific
heat of water is 1 cal/g °C.