Chemical Change: Bond Breaking and Forming

In a chemical reaction, new substances are formed as a result of the breaking and forming of chemical bonds.
The breaking of chemical bonds requires energy, while the formation of new bonds releases energy.
Energy is supplied to break bonds during the reactant stage of a chemical reaction, often in the form of heat. This is called the endothermic process.
When new bonds are formed in the products of the reaction, energy is released. This is known as the exothermic process.
The difference between the energy needed to break the bonds in the reactants and the energy released when new bonds are formed in the products determines if the overall reaction is exothermic or endothermic.
An exothermic reaction will have a net release of energy to the surroundings, meaning the energy needed to break bonds is less than the energy released when new bonds are formed.
Conversely, an endothermic reaction will absorb energy from the surroundings, meaning the energy needed to break bonds is more than the energy released when new bonds are formed.
The energy changes in reactions can be modelled using energy level diagrams. In these, the energy level of the reactants and products are displayed, with the difference between them representing the overall energy change.
Activation energy is the minimum amount of energy required to start a reaction by breaking the initial bonds. This is often supplied as heat.
Catalysts can be used to reduce the activation energy needed for a reaction, allowing the reaction to occur more efficiently.
In every chemical reaction, the total number of atoms is conserved. This is known as the conservation of mass and means that the quantity of each type of atom in the reactants equals the quantity in the products.
Balancing a chemical equation involves ensuring that the number of each type of atom is the same on both sides of the equation, reflecting the conservation of mass.