Energetics

IGCSE chemistry Energetics revision notes :)

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Energetics
Energy changes in reactions
Exothermic reactions:
In an exothermic reaction, heat energy is given out.
An energy level
diagram to show
an exothermic
reaction.
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 heat energy.
Examples: combustion reactions
metals with an acid
neutralisation
(metals in water)
Endothermic reactions:
In an endothermic reaction, heat energy is taken in.
An energy level
diagram to show
an endothermic
reaction.
Reaction progress

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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
oxide)
Experiments to measure energy changes
1. Reactions involving solids
e.g. For neutralisation reactions, displacement reactions, dissolving, metal and
acid etc.
Polystyrene cup
Reaction mixture
Thermometer
The change in temperature of the reaction is measured.…read more

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Conical flask
Clamp
Thermometer
Known volume of water
Spirit burner
Alcohol
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.…read more

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Heat energy (kJ) = (mass (g) x specific heat x temperature change (oC)) ÷
1000
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
If more energy needed to break bonds than given out making more bonds ­
reaction is endothermic.
If less energy needed to break bonds than given out making more bonds ­ reaction
is exothermic.…read more

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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
C ­ Cl = + 346 kJ mol-1
H ­ Cl = + 432 kJ mol-1
Bonds that are broken:
4xH­C
= 4 x +413
= +1652 kJ mol-1
1 x Cl ­ Cl
=…read more

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J 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 see that the reaction is exothermic.…read more

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