Chapter 17
17.1

The temperature of lava from a volcano
ranges from 550°C to 1400°C. As lava flows,
it loses heat and begins to cool. You will learn
about heat flow and why some substances
cool down or heat up more quickly than
others.
17.1

Thermochemistry is the study of energy
changes that occur during chemical reactions
and changes in state.
 Every substance has a certain amount of energy stored
in chemical bonds, and this is called chemical potential
energy.
◦ During chemical reactions bonds break and form,
so there is a change in chemical potential energy.
This results in energy being absorbed or released
as heat.
◦ During changes of state heat is absorbed or
released causing molecules to move closer together
or farther apart. This results in a change in
potential energy.
 Heat, represented by q, is the 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.
17.1
◦ The law of conservation of energy states that in any
chemical or physical process, energy is neither
created nor destroyed.
 Therefore heat is not created or destroyed, it merely
transfers from one substance to another.
In an endothermic process, heat is absorbed.
In an exothermic process heat is released.
17.1
 An endothermic process is one that absorbs heat from
the surroundings.
17.1
 An exothermic process is one that releases heat to its
surroundings.
17.1

Heat flow is measured in two common units,
the calorie and the joule.
◦ The energy in food is usually expressed in Calories.
◦ In this course we will use the Joule (J)
 Ref. table D
17.1

The amount of heat needed to increase the
temperature of an object exactly 1°C is the
heat capacity of that object.
◦ The heat capacity of an object depends on both its
mass and its chemical composition.
17.1
 The specific heat capacity (C), or simply the specific
heat, of a substance is the amount of heat it takes to
raise the temperature of 1 g of the substance 1°C.
 Specific heat of water = 4.18 J/g-°C
see ref. table B
17.1
 Water releases a lot of heat as it cools. During freezing
weather, farmers protect citrus crops by spraying them
with water.
17.1
17.1
 Because it is mostly water, the filling of a hot apple pie
is much more likely to burn your tongue than the
crust.
17.2

A burning match releases
heat to its surroundings in
all directions. How much
heat does this exothermic
reaction release? You will
learn to measure heat flow
in chemical and physical
processes by applying the
concept of specific heat.
17.2
◦ Calorimetry is the precise
measurement of the heat
flow into or out of a
system for chemical and
physical processes.
◦ The insulated device used
to measure the
absorption or release of
heat in chemical or
physical processes is
called a calorimeter.
17.2
◦ In calorimetry, the assumption is made that there is
a total heat transfer between the system and the
surroundings. No heat is lost.
 Ref. table T – heat transfer equation:
q = m x C x ΔT




q = heat absorbed or released (J)
M = mass of water
C= specific heat
ΔT = temperature change
17.2
◦ The heat content of a system at constant pressure
is the same as a property called the enthalpy (H) of
the system.
 Most ordinary reactions take place at constant
pressure, so the terms enthalpy and heat are
interchangeable.
17.2
 Nutritionists use a bomb calorimeter to measure the
heat (energy) content of foods.
17.2
 A chemical equation that includes the enthalpy change
is called a thermochemical equation.
17.2
 In exothermic reactions, heat is given off to the
surroundings, and shown as a product.
 In endothermic reactions, heat is taken in from the
surroundings, and is shown as a reactant.
17.2
 The heat of reaction is the enthalpy change (∆H) for
the chemical equation.
 Exothermic reactions have a negative heat of reaction,
and endothermic reactions have a positive heat of
reaction.
See reference table I
17.2
Exothermic Reaction
17.2
Endothermic Reaction
17.2
 The heat of combustion is the heat of reaction
for the complete burning of one mole of a
substance.
17.3

During a race, an athlete
can burn a lot of calories
that either do work or are
released as heat. This
section will help you to
understand how the
evaporation of sweat
from your skin helps to
rid your body of excess
heat.
 The heat of fusion (Hf) is the heat absorbed by one
gram of a solid substance as it melts to a liquid at a
constant temperature.
 The heat of solidification (Hs) is the heat lost when one
mole of a liquid solidifies at a constant temperature.
For water:
Hf = 334 J/g
17.3
◦ The quantity of heat absorbed by a melting solid is
exactly the same as the quantity of heat released
when the liquid solidifies; that is, Hf = -Hs
◦ To calculate heat gained during melting:
q = mHf
Reference table T
17.3
 The amount of heat necessary to vaporize one gram of
a given liquid is called its heat of vaporization (Hv).
 The amount of heat released when 1 gram of vapor
condenses at the normal boiling point is called its heat
of condensation (Hc).
For water:
Hv= 2260 J/g
17.3
◦ The quantity of heat absorbed by a vaporizing
liquid is exactly the same as the quantity of heat
released when the vapor condenses.
◦ Hv = –Hc
◦ To calculate heat gained during boiling:
 q = mHv
 Reference table T
17.3
 Enthalpy changes accompany changes in state.
17.3
17.3
◦ During the formation of a solution, heat is either
released or absorbed.
 The enthalpy change caused by dissolution of one
mole of substance is the molar heat of solution
(∆Hsoln).
17.3
 When ammonium nitrate crystals and water mix inside
the cold pack, heat is absorbed as the crystals
dissolve.