Energy Conservation
Chemistry: AQA GCSE Higher, topic 5.1.1
Energy Transfer/Conservation
In chemical reactions, energy is not created or destroyed. Energy is conserved (saved) and transferred between objects. Chemical reactions are exothermic or endothermic, depending on the energy transfers that happen:
Exothermic reactions
- Energy from the reacting chemicals is transferred to the surroundings, which often increase in temperature as a result. Examples include:
- Combustion.
- Neutralisation.
- Oxidation.
Endothermic reactions
- Energy from the surroundings is transferred to the reacting chemicals, causing the temperature of the surroundings to decrease. Examples include:
- Thermal decomposition.
- The reaction that happens when citric acid is combined with sodium hydrogen carbonate (sodium bicarbonate).
Test yourself on Energy Conservation
Why does the temperature of the surroundings often increase during an exothermic reaction?
Select the statement that is true in each pair
A student mixes an acid with an alkali in a neutralisation reaction and measures the temperature of the mixture. What happens to the temperature, and why?
A student claims that an exothermic reaction used up all the energy in the reacting chemicals. According to the law of conservation of energy, what actually happened to that energy?
A thermometer records a decrease in temperature as two chemicals react in a beaker. What can you conclude about the direction of energy transfer during this reaction?