Chapter 25
ENERGY FROM CHEMICAL
REACTIONS
Thermochemical Equations
• An explosion is an exothermic reaction in
which the chemical energy in the explosive is
transformed into thermal energy as chemical
bonds are broken and new ones are formed.
• We have seen that ∆H = Hproducts – Hreactants
• The heat of reaction, ∆H, is negative when
there is an overall release of energy
(exothermic) and positive when heat is
absorbed (endothermic).
Thermochemical Equations
 The enthalpy change obtained by burning
petrol molecules is directly proportional to
the number of mole of those molecules.
 In general for any reaction
 ∆H is directly proportional to the amount of
substance
 Turn to page 402
Thermochemical Equations
 Octane is a major component of petrol.
 Complete combustion of one mole of octane
molecules to form carbon dioxide and steam
releases 5054 kJ.
 We can write this as a thermochemical equation
:
C8H18(g) + 12½O2(g) → 8CO2(g) + 9H2O(g); ∆H = -5054 kJ mol-1
 If we were to burn twice as much octane, twice as
much energy would be released (10 108 kJ)
Thermochemical Equations
 When working with thermochemical equations you
should be aware that:
 The coefficients of the reactants indicate the amounts, in
mole, of each substance that react to give the specified
heat energy change.
 States of reactants and products must be specified, since
energy changes occur when states are converted. If we
combusted octane to form water rather than steam we
get a different ∆H value.
 If reactions occur in reverse it has the same ∆H but the
opposite sign.
Calculations Using
Thermochemical Equations
 Calculate the heat energy released when
50.00 mL of 0.200 M sodium hydroxide reacts
with excess dilute hydrochloric acid
H+(aq) + OH-(aq) → H2O(l); ∆H = 57.2 kJ mol-1
Calculations Using
Thermochemical Equations
 Calculate the energy released when 250.0 g
of petrol burns completely in a car engine.
Assume petrol is mainly octane and burns
according to the equation:
2C8H18(g) + 25O2(g) → 16CO2(g) + 18H2O(g);
∆H = -10 108 kJ mol-1
Calculations Using
Thermochemical Equations
 What volume of methane, measured at
standard laboratory conditions, is burnt to
form carbon and water in order to provide
4.00 x 104 kJ.
Your Turn
 Page 405
 Questions 1-3
The connection between
energy and temperature
change
 If I were to heat the exact same volume of
water and oil on a stove top would their
temperatures rise at the same rate?
 The increase in temperature when a given
amount of energy is absorbed depends upon
the material’s ability to store thermal energy
in its bonds.
The connection between
energy and temperature
change
 The amount of energy
required to raise the
temperature of one gram of
a substance by 1°C is called
the specific heat capacity.
 The higher the specific heat
capacity, the more
effectively a material will
store heat energy.
Calculations
 Calculate the energy required to heat 120 mL
of water for a cup of coffee to boiling point if
the initial water temperature is 20.0°C.
 A useful formula to use in calculations such as
this is:
Energy (J) = specific heat capacity x mass x
temperature rise
Measuring the heat released
during a reaction
 Enthalpy changes are measured directly using an




instrument called a calorimeter.
One such device is called a bomb calorimeter and is used
for reactions that involve gaseous reactants of products.
The reaction vessel within a bomb calorimeter is
designed to withstand the high pressures that may be
created during reactions
Figure 25.7 on page 406 compares a bomb calorimeter to
a solution calorimeter.
Both calorimeters are insulated to reduce loss or gain of
energy to or from the outside environment
Calorimeters
Measuring the heat released
during a reaction
 When a reaction takes place in a calorimeter,
the heat change causes a rise or fall in the
temperature of the contents of the
calorimeter.
 Before the use of a calorimeter we must first
determine how much energy is required to
change the temperature within a calorimeter
by 1°C.
 This is known as the calibration factor
Calibration Factor
 The calorimeter is calibrated by using an electric heater to
release a known quantity of thermal energy and measuring
the resultant rise in temperature.
 The calibration factor can be calculated using the formula
Calibration factor = VIt
∆T
 Where
 V is voltage (volts)
 I is current (amps)
 t is time (seconds)
 ∆T is temperature rise (°C)
 Look at the worked example on page 407
Heat of Combustion
 The heat of combustion of a substance is defined
as the energy released when a specified amount of
the substance burns completely in oxygen.
 Many substances, such as wood, coal and
kerosene are mixtures of chemicals and do not
have a specific chemical formula or molar mass.
 The energy released when they burn is therefore
measured per gram or per litre, rather than per
mole
Heat of Combustion
Your Turn
 Page 409
 Questions 4 -6
 Chapter Questions
 Page 410
 Q 7, 10, 11, 13
 Pg 411
 Q 16, 21, 22, 24, 26 and 27
 Pg 412
 Q 30, 33