Thermochemistry
Thermochemistry
 Heat of a reaction is the heat change when the number of
moles of reactants indicated in a balanced equation for the
reaction react completely
 - ΔH is exothermic ( heat given out to surroundings)
 +ΔH is endothermic ( heat taken in)
 i.e Heat taken in to (break bonds) – heat given out in (bond
formation)
If the numbers of moles in a balanced equation is changed the heat
of reaction also changes
Understanding Equations
H2(g) + 1/2O2(g)
2H2(g) +O2(g)
H2O(g) ΔH = -242 kJ/mol
2 H2O(g) ΔH = -484 kJ/mol
You can see when you double the number of moles of reactants you
double the energy released
Equations like these are called Thermochemical equations , they must be
balanced, ΔH must be given and the physical states of the reactants
must be given
 The Term “Heat of Reaction” is used to describe any type of
chemical reaction
 In some cases a different term is used to indicate the
particular type of chemical reaction
 For example Combustion reaction when something is
burned in excess oxygen
 A neutralisation reaction when an acid reacts with a base
Heat of Combustion
 The heat of combustion is the heat change (in kilojoules)
when one mole of a substance is completely burned in
excess oxygen
 C (s) + O2(g)
CO2(g) ΔH = -393 kJ/mol-1
 It is important to say completely burned in excess oxygen as
some elements have more than one oxide
C (s) + ½O2(g)
CO(g) ΔH = -111 kJ/mol-1
 This value of ΔH is not the heat of combustion since
complete combustion of carbon in excess oxygen forms
carbon dioxide
 Another important phrase in the definition is the term “One
Mole”
 Consider this reaction describing butane being burned from a
gas cylinder
 2C 4H10(g) + 13 O2(g)
8CO2(g) + 10H2O
ΔH = -5720 kJ/mol-1
 5720 is not the heat of combustion of butane as heat of
combustion involves 1 mole of a substance, because there are
2 moles in this equation the true heat of reaction is found by
dividing by 2
 C 4H10(g) + 6½ O2(g)
4CO2(g) + 5H2O
ΔH = -2860 kJ/mol-1
Measuring Heats of Combustion
 Heats of combustion are accurately measured using a bomb
calorimeter
 A calorimeter is any container used to measure heat changes
 A bomb calorimeter consists of a small metal container
(known as “The Bomb”) with a screw on cap
 The sample whose heat of combustion is to be measured is
placed in a crucible in the bomb
 The bomb is placed in a container of water (the calorimeter)
 Oxygen is pumped into the bomb and it is ignited with
electric wires
 By measuring the rise in temperature of the water and
applying mathematic formulas based on the ability of water
to absorb heat it is possible to calculate the heat of
combustion of the fuel
 The heats of combustion of some common substances are
given on page 327 of your book
http://www.wwnorton.com/
chemistry/tutorials/ch3.htm
A calorimeter is
an instrument
used to
measure heat
changes which
happen during
chemical
reactions
When you examine the side of a
cornflakes box what do you see?
 This information is
The Kilogram calorific
value of a fuel is the heat
energy produced when 1kg
of the fuel is completely
burned in oxygen
obtained using a bomb
calorimeter
 You may notice that the
values are given in mass (g)
rather than in moles
 For this purpose chemists
define a term called
kilogram calorific
value
Bond Energy
 So why do different reactions have different ΔH values?
 Consider the combustion of methane
CO2(g) + 2H2O ΔH = -890 kJ/mol-1
 When chemical reactions occur bonds must be broken and bonds
must be formed
 Energy is required to break bonds and energy is released
when bonds are formed
 In the above reaction 4 C-H bonds and 2 O=O bonds must be
broken on the left hand side while 2 C=O bonds and 4 O-H bonds
are formed on the right hand side (see page 328)
 C H4(g) + 2O2(g)
 The energy required to break bonds is called bond energy
and is defined as follows
 Bond Energy is the energy required to break one
mole of covalent bonds and to separate the neutral
atoms completely from eachother
 Thus the value of ΔH for a particular reaction is
dependent on the amount of energy needed to break the
bonds and the amount of energy released when new
bonds are formed
 If more energy is released than absorbed the reaction is
exothermic and vice versa
 NB table p328
Heat of Neutralisation
 Neutralisation is the reaction between an acid and a base to
form salt and water
 Eg
HCl + NaOH
NaCl + H2O
 We have also learned that the essential reaction taking
place here is H+ + OHH2O
 Heat of neutralisation is the heat change that occurs
when one mole of H+ ions from an acid reacts with one
mole of OH- ions from a base
 NB examples of heats of neutralisation on p328
 When a weak acid or base is involved the ΔH is numerically les
than 57kj mol-1 as weak acids/bases do not dissociate fully
Experiment to determine the heat
of Neutralisation
 Read through the experiment and the following slides
explain some of the key terms
THERMOCHEMISTRY
 Heat of neutralisation
→ NaCl + H2O
 ΔH = - mass (kg) × c (specific heat capacity of water) ×ΔT
(heat change in degrees or kelvin)
 The specific heat capacity of water is 4.2 kilojoules/Kelvin or
2400 joules
 HCl + NaOH
Heat of Formation
Hess’ Law
 Hess’s law states if a chemical reaction takes place in a
number of stages, the sum of the heat changes in the
separate stages is equal to the heat change if the
reaction is carried out in one stage
