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Energy Changes in Chemical Reactions
Bond length :
The distance between nuclei of two bonded atoms
Reactant:
A starting material in a chemical reaction
Products:
A substance formed in a chemical reaction
Exothermic reaction:
Energy (in the form of heat and light) is released during a chemical reaction
Endothermic reaction:
Energy is taken if from its surroundings
Activation Energy:
The energy needed to get a reaction to start.
Activated Complex:
The transition state between the reactant and product.
Enthalpy change ΔH:
The amount of heat energy gained or lost by a reaction.

Enthalpy change or Heat of the reaction ΔH = energy of products – energy of reactants

For endothermic reactions, energy of the products is greater than the energy of the
reactants ΔH > 0 (positive value)

For exothermic reactions, energy of the products is less than the energy of the reactants
ΔH < 0 (negative value)
Bond strength:
The greater the number of bonding electron pairs between a pair of atoms, the
shorter the bond length. (and stronger the bond)
 atoms held together more tightly when there are multiple bonds e.g. N ≡ N
greater bond energy for molecules with multiple bonds
Bond Energy
 The energy that must be supplied to a gaseous molecule to separate two of its atoms is called the
bond dissociation energy or bond energy for short.

Bond energy represents energy supplied to the molecule from its surroundings.

The process of breaking bonds in a molecule is endothermic.

During a chemical reaction, the molecules of the reactants break up and the atoms rearrange
themselves to form new molecules (the products).
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
The initial breaking of chemical bonds takes in energy. When new bonds are formed in the products,
energy is released.

Bond energies tell us how much energy will be taken in (or given out) when a mole of particular bonds
are broken (or formed).

Table of average bond energies: (These values might differ from one source to another)
Bond
H-H
C-H
O–H
C-C
C=O
O=O
N≡N
Energy (kJ/mol)
436
414
460
347
799
499
946
The values in this table are defined in
terms of gaseous atoms.
If a
reactant or product is in the solid or
liquid state, you must first provide
energy to convert it to a gas before
using these values.
Examples
1.
How much energy will be given out when one mole of O = O bonds are formed? ……………………..
2.
How much energy will be taken in when 4 moles of O – H bonds are broken? ……………………………
3.
How may C – H bonds occur in one methane molecule? ………………………
4.
How much energy is required to break up one methane molecule? …………………………
5.
Consider the formation of water molecules when hydrogen burns in oxygen.
2H2 + O2  2H2O
5.1
How many H – H bonds must be broken? …………………………
5.2
How many O = O double bonds must be broken? ………………………………..
5.3
How many O – H bonds are formed? ………………………………………………
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2H2 + O2  2H2O
5.4
How many kilojoules of energy will be needed to break up one mole of oxygen molecules?
………………………………….
5.5
How many kilojoules of energy will be needed to break up two moles of hydrogen molecules?
………………………………….
5.6
How many kilojoules of energy will be released when two moles of water molecules are
formed? ………………………………..
5.7
Will the overall reaction be exothermic and endothermic? Why? …………………………………..
…………………………………………………………………………………………………………………………………………………………..
…………………………………………………………………………………………………………………………………………………………..
6.
Consider the reaction between propane and oxygen (combustion of propane).
When propane burns in oxygen or air, the products are carbon dioxide and water.
6.1
Balance the equation for this reaction:
……..C3H8
+
…….O2

……..CO2
+
……H2O
6.2
How many C – H bonds in propane need to be broken? ……………….
6.3
How much energy is needed to break the C – H bonds? …………………….
6.4
How many C – C bonds in propane need to be broken? ………………………………..
6.5
How much energy is needed to break the C – C bonds? …………………….
6.6
How many O = O bonds need to be broken in total? …………………………………
6.7
How much energy is needed to break the O = O bonds? …………………….
6.8
How many carbon dioxide C = O bonds are formed in total? …………………………………………………
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6.9
How much energy will be released when the C = O bonds form? ……………………………………..
6.10
How many water O – H bonds are formed in total? …………………………………………………
6.11
How much energy will be released when the O – H bonds form? ……………………………………..
6.12
How much energy in total is used in breaking bonds? ……………………………
6.13
How much energy is released when the new bonds are formed? …………………………..
6.14
Will the overall reaction be exothermic and endothermic? Why? …………………………………..
…………………………………………………………………………………………………………………………………………………………..
…………………………………………………………………………………………………………………………………………………………..
Energy profile of an exothermic reaction
activated
complex
activation
energy
ΔH < 0 (negative value)
reactants
ΔH
products
Energy profile of an endothermic reaction
ΔH > 0 (positive value)
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