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Chemistry topic 5: energy changes, Screenshot 2023-05-13 15.41.23,…
Chemistry topic 5: energy changes
Exothermic reactions
An exothermic reaction is one that transfers energy to the surroundings
This causes the temperature of the surroundings to increase
The products have less energy than the reactants
Examples of exothermic reactions include: combustion, neutralisation and oxidation reactions, or in everyday use: self-heating cans/hand warmers
Endothermic reactions
An endothermic reaction takes in energy from the surroundings
This causes the temperature of the surroundings to decrease
The products have more energy than the reactants
Examples of endothermic reactions include: thermal decomposition, sports injury ice packs
Energy transfer
Energy is conserved: the amount of energy at the beginning of a reaction is the same as at the end of a reaction
Chemical reactions can only occur when the reacting particles collide with eachother, with sufficient energy
The activation energy is the minimum amount of energy that particles must have to react
The energy change of reactions
During a chemical reaction, energy must be supplied to break the bonds in the reactants
Energy is released when bonds in the products are formed
EXOTHERMIC REACTION:
energy taken to break bonds < energy released when bonds are formed
ENDOTHERMIC REACTION:
energy taken to break bonds > energy released when bonds are formed
Cells and batteries
Cells contain chemicals which react to produce electricity
The voltage produced by a cell is dependent on many factors, such as the type of electrode and electrolyte
A simple cell can be made by connecting 2 different metals in contact with an electrolyte
Batteries are 2 or more cells connected in series to provide a greater voltage
Non-rechargeable cells/batteries:
The chemical reaction stops once all of the reactants are used up
Eg alkaline batteries
Rechargeable cells/batteries:
Can be recharged because the chemical reactions are reversed when an external electrical current is supplied
