AQA GCSE Chemistry: Energy Changes
Conservation of Energy
Energy cannot be created or destroyed. It can only be
transferred into or out of the system.
The system is the chemical reaction.
The surroundings are everything else.
Exothermic and Endothermic Reactions
Exothermic Reactions
Definition: Reactions where heat energy is transferred
from the system to the surroundings.
Effect on Surroundings: Increased temperature.
Energy of System: Lower energy products than
reactants.
Examples:
Combustion (burning fuels)
Many oxidation reactions
Neutralization reactions
Everyday Uses:
Self-heating cans
Hand warmers
Endothermic Reactions
Definition: Reactions where heat energy is transferred
, from the surroundings into the system.
Effect on Surroundings: Decreased temperature.
Energy of System: Higher energy products than
reactants.
Examples:
1. Thermal decomposition (e.g. calcium carbonate)
2. Citric acid and sodium hydrogen carbonate reaction.
Everyday Uses:
1. Cold packs for sports injuries
Bond Breaking and Bond Making
Bond Breaking: Energy must be added into the system.
Bond Making: Energy is released from the system.
Reaction Profiles
Reaction profiles are diagrams that show the energy
changes that take place during a chemical reaction.
Exothermic Reaction Profile
The reaction profile begins at the energy level of the
reactants and ends at a lower energy level for the
products.
Activation Energy (Ea): This represents the minimum
amount of energy required to start the reaction. It is
Conservation of Energy
Energy cannot be created or destroyed. It can only be
transferred into or out of the system.
The system is the chemical reaction.
The surroundings are everything else.
Exothermic and Endothermic Reactions
Exothermic Reactions
Definition: Reactions where heat energy is transferred
from the system to the surroundings.
Effect on Surroundings: Increased temperature.
Energy of System: Lower energy products than
reactants.
Examples:
Combustion (burning fuels)
Many oxidation reactions
Neutralization reactions
Everyday Uses:
Self-heating cans
Hand warmers
Endothermic Reactions
Definition: Reactions where heat energy is transferred
, from the surroundings into the system.
Effect on Surroundings: Decreased temperature.
Energy of System: Higher energy products than
reactants.
Examples:
1. Thermal decomposition (e.g. calcium carbonate)
2. Citric acid and sodium hydrogen carbonate reaction.
Everyday Uses:
1. Cold packs for sports injuries
Bond Breaking and Bond Making
Bond Breaking: Energy must be added into the system.
Bond Making: Energy is released from the system.
Reaction Profiles
Reaction profiles are diagrams that show the energy
changes that take place during a chemical reaction.
Exothermic Reaction Profile
The reaction profile begins at the energy level of the
reactants and ends at a lower energy level for the
products.
Activation Energy (Ea): This represents the minimum
amount of energy required to start the reaction. It is
