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Energy changes in reactions

Exothermic reactions:

In an exothermic reaction, heat energy is given out.

An energy level

diagram to show

an exothermic


Reaction progress

This is because the products are lower in energy than the reactants, so the excess
energy from the reactants are given out as…

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This is because the products are higher in energy than the reactants, so the extra
energy needs to be taken from the surroundings.

Examples: thermal decomposition of metal carbonates (products CO2 + metal

Experiments to measure energy changes

1. Reactions involving solids

e.g. For neutralisation reactions, displacement reactions, dissolving,…

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Conical flask



Known volume of water

Spirit burner


The alcohol is allowed to burn for a given time. The mass of alcohol is recorded
before and after burning, so that the mass of alcohol burnt is known. The change
in temperature of the water is measured.


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1. Heat energy (kJ) = (mass (g) x specific heat x temperature change (oC)) ÷

2. Work out per mole

3. Give sign (+ H or ­ H)

Why do reactions give in or take in energy?

Break bonds --> energy taken in (endothermic).

New bonds formed --> exothermic…

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Bonds that are broken: 4 x H ­ C

1 x Cl ­ Cl

Bonds that are made: 3 x C ­ H

1 X C ­ Cl

1 X H ­ Cl

Bond energies:

C ­ H = +413 kJ mol-1

Cl ­ Cl = + 243 kJ mol-1…

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= -432 kJ mol-1

Total energy given out = -2017 kJ mol-1

Add together the values for total energy taken in and total energy given out to find
the overall energy change:

Overall energy change H = +1895 ­ 2017

= -122 kJ mol-1

Because H is negative, we can…


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