2.4 - Chemical reactions and energy

Exothermic reactions transfer energy to the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to become hotter. The temperature increase can be detected using a thermometer. When a flame burns it transfers heat to ists surroundings

Exothermic reactions can be used for everyday purposes. For example, hand warmes and self-heating cans for drinks (such as coffee) use exothermic reactions. Some examples reactions. Some examples of exothermic reactions are:

.Combustion

.Neutralisation reactions between acids and alkalis  

.

These are reactions that take in energy from the surroundings. The energy is usually transferred as heat energy, causing the reaction mixure and its surroundings to get colder. The temperature decrease can also be dectected using a thermometer. Some exampless of endothermic reactions are:

.Electrolysis

.The reaction between ethanoic acid and sodium carbonate 

.The thermal decomposition of calcium carbonate in a blast furnace 

Endothermic reactions can be used for everyday purposes. For example, certain sports injury cold packs use endothermic reactions 

1. stand the polystyrene cup in the beaker 

2. Use the measuring cylinder to measure out 10cm3 of sodium hydroxide solution and pour it into the polystyrene cup. Rinse the measuring cylinder with water so that it can be used for experiment 2.

3. Measure the initial temperature of the sodium hydroxide solution and record it in a suitable table 

4. Measure out 10cm3 of hydrochloric acid and carefully add this to the sodium hydroxide solution in the polystyrene cup. Stir with the thermometer and record the maximum or minimum temperature reached 

5. work out the temperature change and decide if the reaction is exothermic and endothermic

6. Discard the mixture. Rinse out and dry the polystyrene cup.

1. Repeat steps 1-3 of Reaction 1, using sodium hydrogencarbonate solution in place of sodium hydroxide solution. Rinse the measuring cylinder with water so that it can be used for Reaction 3

2. Add 4 small spatula measures of citric acid, stir with the thermometer and record the maximum or minimum temperature reached.

3. Work out the temperature change and decide if the reaction is exothermic or endothermic 

4. Discard the mixture. Rinse out and dry the polystyrene cup.

1. repeat steps 1-3 of reaction 1, using sulfuric acid in place of sodium hydroxide solution. rinse the measuring cylinder with water so that it can be used for reaction 4.

2. add one 3cm piece of magnesium ribbon. Stir with the thermometer and record the maximum or minimum temperature reached.

3.work out the temperature change and decide if the reaction is exothermic or endothermic.

4.Discard the mixture. Rinse out and dry the polystyrene cup.

1. Repeat steps 1-3 of Reaction 1, using copper (II) sulfate solution in the place of sodium hydroxide solution.

2. Add 1 small spatula measure of magnesium powder. Stir with the thermometer and record the maximum or minimum temperature reached 

3. Work out the temperature change and decide if the reaction is exothermic or endothermic

Exo - products are lower than reactants (Endo is the other way round) 

Exo - products are lower than reactants (Endo is the other way round)