Adding or removing energy from matter causes a physical change as matter moves from one state to another. For example, adding thermal energy (heat) to liquid water causes it to become steam or vapor (a gas). When heat is applied to a solid, its particles begin to vibrate faster and move farther apart.

Note that melting and vaporization are endothermic processes in that they absorb or require energy, while freezing and condensation are exothermic process as they release energy.

Changes of state are physical changes in matter. They are reversible changes that do not involve changes in matter’s chemical makeup or chemical properties. Common changes of state include melting, freezing, sublimation, deposition, condensation, and vaporization. These changes are shown in Figure below.

Yes, energy can be released when matter changes. In physical changes, such as phase changes, energy is released when changing from a higher energy…

The energy change in a chemical reaction is due to the difference in the amounts of stored chemical energy between the products and the reactants. This stored chemical energy, or heat content, of the system is known as its enthalpy.

To calculate an energy change for a reaction:

1. add together the bond energies for all the bonds in the reactants – this is the ‘energy in’
2. add together the bond energies for all the bonds in the products – this is the ‘energy out’
3. energy change = energy in – energy out.

All chemical reactions involve energy. Energy is used to break bonds in reactants, and energy is released when new bonds form in products. Endothermic reactions absorb energy, and exothermic reactions release energy. The law of conservation of energy states that matter cannot be created or destroyed.

Transition state

Explanation: Increasing the concentration of reactants usually increases the frequency of effective collisions between reactant molecule. This is because as the concentration of the reactant is increased then more of the reacting molecules or ions are present to form the reaction products.

Catabolic reactions give out energy. They are exergonic. In a catabolic reaction large molecules are broken down into smaller ones. For example, the reverse of the condensation reactions described above, i.e. hydrolysis reactions, are catabolic.

Enzymes are biological catalysts. Catalysts lower the activation energy for reactions. The lower the activation energy for a reaction, the faster the rate. Thus enzymes speed up reactions by lowering activation energy.