Energy Changes
You will learn about:
 Exothermic & Endothermic Reactions
 Energy level diagrams
 Bond Making and Breaking
 Calculating enthalpy change
 Activation energy
 Energy profile diagrams
 Photography
 Photosynthesis
Exothermic &
Endothermic Reactions
 Exo:out, Thermic: Heat energy
 Exothermic reaction: one in which heat
energy is released and there is an
increase in temperature of the mixture.
 Endo:in
 Endothermic reaction: one in which heat
energy is absorbed and there is a
decrease in the temperature of the
mixture.
Endothermic reaction
 Dissolution of potassium nitrate in water
- eg. of endothermic reaction
 - heat energy is absorbed
 => decrease in the temperature of the
mixture
Exothermic reaction
 Dissolution of calcium oxide in water
eg. of exothermic reaction
 - heat energy is released
 => increase in temperature of the mixture
Enthalpy Change
 Heat energy given out or taken in, during a
reaction is measured in units called Joules (J).
 Large amounts are measured in kiloJoules
(kJ).
 The symbol for the heat of reaction (heat
energy change or enthalpy change) is H.
 Numerically,
H = Change in heat energy content = Total
energy content of products – total energy
content of reactants
Enthalpy Change
 If
Total energy content of products > Total energy content
of reactants
H > 0
Reaction is endothermic
 If
Total energy content of products < Total energy content
of reactants
H < 0
Reaction is exothermic
 The sign of H will tell whether the reaction is
exothermic or endothermic!
Examples
 Example 1:
When 1 mole of hydrochloric acid reacts
with one mole of sodium hydroxide, 57.3
kJ of heat is produced. Draw an energy
level diagram to represent this reaction.
Examples
 Example 2:
When 1 mole of hydrogen reacts with 1
mole of iodine to give 2 moles of
hydrogen iodide, 52 kJ of heat is
absorbed. Draw an energy level diagram
to represent this reaction.
Bond Making and Breaking
 When 2 atoms joined together by
chemical bond, heat energy is given out.
Bond making is a exothermic change.
(H = -ve)
 Energy is needed to break chemical
bond. Bond breaking is a endothermic
change. (H=+ve).
 Energy for making and breaking a
covalent bond is the same. This is the
bond energy.
Enthalpy Change in a
Reaction
 Old bonds are broken in a reaction
(endothermic) while new bonds are formed
(exothermic).
 The Enthalpy Change, H, (overall heat
change) of the reaction = [heat absorbed to
break old bonds in reactants] - [heat given
out to form new bonds in products]
 The sign of H will tell whether reaction is
exothermic or endothermic.
Enthalpy Change
 If the energy absorbed for bond breaking is
less than that released during bond forming,
the reaction is exothermic.
 If the energy absorbed for bond breaking is
more than that released during bond
forming, the reaction is endothermic.
QUIZZES
1. Classify each of the following changes
as either endothermic, exothermic or no
heat exchange.
Change
Type of heat change
Forming a covalent bond
Exothermic
No heat change
Exothermic
Mixing hydrogen and oxygen
Reacting hydrogen and oxygen together
Forming nitrogen atoms from nitrogen
molecules
Endothermic
Forming bromine vapour from liquid bromine
Endothermic
2) Fe + 2HCl  FeCl2 + H2; H=-156kJ
(i) How do you know from the equation
above that the reaction is exothermic?
The negative sign in front of H.
(ii) How much heat is produced in the
reaction of 1 mole of HCl?
Amount of heat produced by 1 mole HCl = 156/2=78kJ
Worked example
Method 1:
 E.g. 2H2 + O2  2H2O
 Heat released when new bonds are formed
= bond energies of 4 O-H bonds
= 4x 463kJ = 1852kJ
 Heat absorbed when old bonds are broken
= bond energies of 2 H-H bonds + 1 O=O bond
= (2x436) + (1x496) = 1368kJ
 Enthalpy Change, H of reaction
= 1368 -1852 = -484kJ (the -ve =>exothermic)
Method 2:
E.g. 2H2 + O2  2H2O
 Heat involved in bond making of 4 O-H bonds
= 4x (-463)kJ = -1852kJ
 Heat involved in bond breaking of
2 H-H bonds + 1 O=O bond
= 2x(+436) + 1x(+496) = +1368kJ
 Enthalpy Change, H of reaction
= +1368 -1852 = -484kJ (-ve =>exothermic)
Activation Energy
 For a chemical reaction to occur, the reactant
particles must collide
 Most particles do not have enough energy
when they collide to start a reaction. They just
bounce apart and do not react.
Activation Energy
 Some particles collide with enough
energy to start a reaction.
Activation Energy
 The minimum energy that molecules must
possess in order for a chemical reaction to
occur is called the activation energy, Ea.
 A reaction that takes place easily at room
temperature has a fairly low activation
energy.
 A reaction that requires a higher
temperature to occur has a higher
activation energy.
Activation Energy
 Activation energy is the energy barrier that the
colliding molecules must overcome in order for
a reaction to occur
Activation Energy
 Consider the reaction of hydrogen with
oxygen: 2H2(g) + O2(g) → 2H2O (l)
 The reaction takes place in two steps:
1. A burning match provides the necessary
activation energy. This energy gives the
reactant molecules enough energy to break
their covalent bonds when they collide,
forming hydrogen and oxygen atoms.
2. Once the reaction starts, the hydrogen
and oxygen atoms combine to form water.
As new bonds are formed, energy is given
out.
Activation energy –
exothermic rxn
 If a reaction is exothermic, enough energy is
given out in the reaction of a few particles to
provide the activation energy for the remaining
particles. So the reaction keeps going.
 This is the case for the reaction between
hydrogen and oxygen; when the first hydrogen
molecule reacts, the heat give out enables
other molecules to react.
Activation energyendothermic rxn
 If the reaction is endothermic, insufficient
energy is given out when bonds are
made to provide the activation energy
needed for the reaction to continue.
 Thus, heat energy must be continually
added to provide sufficient activation
energy.
Energy profile diagrams
 An energy profile diagram is a way of
representing the energy changes that
occur during a chemical change. The
energy difference between the products
and reactants represents the heat of
reaction.
 It includes the energy barrier, the
activation energy.
Energy Profile for exothermic rxn
 Exothermic reaction
Energy profile for endothermic rxn
 Endothermic reactions
Examples of exothermic
changes
 Changes of state (condensation and freezing)
 Dissolving of some salts in water
eg. Na2CO3(s) → Na2CO3(aq) + heat
 Combustion reactions
All combustion reactions are exothermic.
eg. 2H2(g) + O2(g) → 2H2O(l) + heat
 Neutralisation reactions
H+(aq) + OH-(aq) → H2O(l) + heat
 Metal displacement reactions
Zn(s) + Cu2+ (aq) → Zn2+ (aq) + Cu(s) + heat
Examples of endothermic
changes
 Changes of state (melting & boiling)
 Dissolving of some salts in water
eg. NH4Cl(s) + heat → NH4Cl(aq)
 Decomposition of compounds
eg. CuCO3(s) + heat → CuO(s) + CO2(g)
 Reactions between acids and hydrogencarbonates
eg. when some headache tablets (mixture of solid
acids such as citric acid and sodium
hydrogencarbonate) are added to water, heat
energy is absorbed.
 Formation of nitrogen oxide in a car engine
N2(g) + O2(g) → 2NO(g)
Combustion of fuels
 What are fuels?
 Fuels are substances that can burn
easily in air to give out energy.
 The most commonly used fuels are fossil
fuels such as coal, petroleum and natural
gas. They are formed from decayed plant
and animals that lived millions of years
ago.
Combustion of fuels
 What happens when fuels burn?
 Coal is mainly carbon.
C(s) + O2(g) → CO2(g) + heat energy
 Most fuels contain carbon and hydrogen. When these
fuels burn, carbon dioxide, water and heat energy are
produced. The equation for the combustion of natural
gas, which contains mainly methane is
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + heat
energy
 All combustion reactions are exothermic processes.
Combustion of fuels
• We can calculate the heat of combustion of fuels
given the bond energy values.
Covalent bond
Bond energy kJ/mol of bonds
H-H
436
C-C
348
C=C
612
C-H
412
C-O
323
C=O
743
O-O
138
O=O
496
O-H
463
Table: Bond energy values
Combustion of fuels
 Calculate the enthalpy change for the
complete combustion of butene with
the following structure.
H
H
H
H
H
C
C
C
C
H
H
H
Calculation of heat of combustion
C4H8 + 6O2 →4CO2 + 4H2O
Heat involved in breaking 8C-H, 2C-C,
1C=C & 6O=O bonds
= 8(+412) + 2(+348) +612 + 6(+496)
= +7580 kJ
Calculation of heat of combustion
 Heat involved in making 8C=O & 8O-H
bonds
= 8(-743) + 8(-463)
= -9648 kJ
 Enthalpy change = +7580 – 9648
= -2068 kJ
Group work –
Alternative Fuels
Research on the following, highlighting one of the
fuels and do a powerpoint presentation, including
the chemistry behind the fuel, the social-economic
issues involved, and show examples where this
fuel is being used. Do a comparative study on the
alternative energy that is being employed in
Singapore and another country.
 1. Hydrogen fuel & other types of fuel cells
 2. Biofuels
 3. Nuclear energy & solar energy
Light and Chemical
Reactions
 Reactions in which light energy is
absorbed are endothermic.
 Photography
 Photosynthesis
Photography
 Photographic film: coated with crystals of
silver bromide or silver chloride.
 Light hit film electrons transferred from
the chloride or bromide ions to the silver
ions.
 Silver metals formed the dark areas on
film.
 This is an example of redox reaction.
 2AgBr 2Ag + Br2
H = +199kJ
Photosynthesis
 Chlorophyll absorbs light energy used to
make sugars from carbon dioxide and
water.
 6CO2 + 6H2O  C6H12O6 + 6O2
H = +2816kJ
Group work –
Alternative Fuels
Research on the following, highlighting one of the
fuels and do a powerpoint presentation, including
the chemistry behind the fuel, the socialeconomic, environmental issues involved, and
show examples where this fuel is being used. The
pros and cons as well. Do a comparative study on
the alternative energy that is being employed in
Singapore and China.
 1. Hydrogen fuel & other types of fuel cells
 2. Solar energy
 3. Biofuels
 4. Nuclear energy