Outgoing Chemistry Unit 3 (Energy)

Notes on bond energies and calculations

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  • Created on: 23-05-12 09:15
Preview of Outgoing Chemistry Unit 3 (Energy)

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Energy Calculations
Comparing Energy Produced By Fuels
When a fuel is burnt, we use an exothermic reaction as a source of energy. Not all fuels
produce the same amount of energy when they burn; some reactions are more exothermic
than others.
It is important to know how much energy a fuel produces when it burns. Measurements like
this are carried out under carefully controlled conditions, in an instrument called a bomb
The amount of energy in food is measured in the same way. Data tables are drawn up by
burning the foods in a bomb calorimeter and measuring the amount of energy produced.
These measurements show how different foods produce different amounts of energy when
they react with oxygen.
Energy Changes in Reactions
Because energy has to be supplied to break down chemical bonds, breaking bonds is an
endothermic process (energy is taken in from the surroundings)
When new bonds are formed, energy is released (so making bonds is an exothermic
In some reactions, the energy released when new bonds are formed is more than the energy
needed to break the bonds in the reactants (they are exothermic as they transfer energy to
their surroundings).
In other reactions, the energy needed to break the bonds (in the reactants) is more than the
energy released when new bonds are formed in the products (they are endothermic as they
transfer energy from the surroundings to the reacting chemicals).
Energy level diagrams show us the relative amounts of energy contained in the reactants and
the products of a reaction. This energy is measured in kJ/mol. The energy change is
represented by the symbol H (which means the difference in energy between the products
and reactants)
In an exothermic reaction, H is always negative, because the products are at a lower
energy level then the reactants. The temperature of the surroundings increase.
In an endothermic reaction, H is always positive, as the products are at a higher energy
level than the reactants. The temperature of the surroundings decrease.
The activation energy is the minimum amount of energy needed to start a reaction. A catalyst
lowers the activation energy so that a higher proportion of reactant particles have sufficient
energy to react.

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Bond Energy Calculations
The energy needed to break the bond between two atoms is known as the bond energy for
that bond. Bond energies are measured in kJ/mol. They can be used to work out H for many
chemical reactions.…read more


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