C2bL7 Energy Changes

Key Words

Anhydrous - No water in the crystal structure.

Combustion - burning.

Hydrated - water present in the crystal structure

Neutralisation - chemical reaction in which the products form a solution with pH 7.

Oxidation - reaction with oxygen.

Reversible reaction - reaction that can go backwards as well as forwards.

Thermal decomposition - compounds break up into simpler substances when heated.

Grade E

All combustion reactions give out heat.  They are exothermic.

Other exothermic reactions include:

• oxidation reactions, like the reaction between magnesium and oxygen;

• neutralisation reactions like

hydrochloric acid + sodium hydroxide  ®  sodium chloride  + water

HCl _(aq) + NaOH _(aq) ® NaCl _(aq) + H₂O _(l)

Endothermic reactions need heat to be taken in before the reaction happens.  They include:

• Photosynthesis in plants, that requires energy from the sun;
• Thermal decomposition reactions, for example:

calcium carbonate  ® calcium oxide + carbon dioxide

CaCO_{3 (s)} ® CaO _(s) + CO_{2 (g)}

Grade C

If you heat copper sulphate crystals, they change from blue to white.  This is because water is driven out of the crystal structure.  The reaction is reversible.  If you let the white copper sulphate cool, then add water, it gets very hot, and changes back to blue.

Making anhydrous copper sulphate from hydrated copper sulphate is endothermic.

Making hydrated copper sulphate from anhydrous copper sulphate is exothermic.

Grade A

We can work out how exothermic a reaction is by measuring the energy.

You may do an experiment where you measure how much heat is given out by different fuels:

• Heat 100 g water using 1 g ethanol;
• Measure the temperature change;
• Now do the same with 1 g propanol.
• You then use the equation:

energy = mass of water × specific heat × temperature change

The specific heat of water is the amount of energy needed to heat 1 gram of water by 1 degree Celsius.  The value is 4.2 J g^{-1 o}C^-1.