Energy changes are an important topic in GCSE AQA Chemistry. This topic covers various aspects of energy changes that occur during chemical reactions, including exothermic and endothermic reactions, energy level diagrams, bond energies, and Hess's Law. Students learn about how to calculate and inte...
1. Exothermic reactions: In an exothermic reaction, energy is given out to the
surroundings. This results in a temperature rise in the surroundings. The energy
change in an exothermic reaction is negative. Examples of exothermic reactions
include burning, respiration, and neutralization.
2. Endothermic reactions: In an endothermic reaction, energy is taken in from the
surroundings. This results in a temperature drop in the surroundings. The energy
change in an endothermic reaction is positive. Examples of endothermic reactions
include thermal decomposition and photosynthesis.
3. Enthalpy change: The enthalpy change is the heat energy transferred in a reaction at
constant pressure. It is represented by the symbol ΔH.
4. Bond energy: Bond energy is the energy required to break a bond. It is also the energy
released when a bond is formed. The bond energy of a bond is usually expressed in
kJ/mol.
5. Hess's Law: Hess's Law states that the enthalpy change of a reaction is independent
of the route taken. This means that the enthalpy change of a reaction can be
calculated from the enthalpy changes of the reactants and products.
6. Standard enthalpy change: The standard enthalpy change is the enthalpy change
when one mole of a substance is formed from its constituent elements under
standard conditions. Standard conditions are defined as 298K and 1 atm pressure.
7. Calorimetry: Calorimetry is the measurement of heat transfer in a chemical reaction.
A calorimeter is used to measure the heat change in a reaction. The heat absorbed or
released can be calculated from the temperature change of the surrounding water.
8. Activation energy: The activation energy is the minimum amount of energy required
for a reaction to take place. It is the energy needed to break the bonds of the
reactants and start the reaction.
9. Catalyst: A catalyst is a substance that increases the rate of a reaction by lowering
the activation energy. Catalysts are not used up in the reaction.
10. Reaction profile: A reaction profile is a diagram that shows the energy changes in a
reaction. It shows the energy of the reactants, the activation energy, and the energy of
the products.
These are some of the key concepts in energy changes in GCSE AQA Chemistry.
Understanding these concepts will help you to solve problems related to energy changes in
chemical reactions.
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