1) For a reaction to take place between two reagents, two conditions are necessary: (A) The two particles (atoms, ions, radicals, or molecules) must come into physical contact (collide) with each other and (B) The reactants must collide with sufficient kinetic energy to bring about the reaction.
2) There are also some factors that affect the rate of the chemical reaction: (A) Increasing the concentration of the reactants or the pressure of a gaseous reaction. As the quantity of the particles increases, the number of collisions between the reactant particles will increase. (B) Increasing the temperature of the reaction. At a higher temperature, the reactant particles are moving faster, so there will be more collisions, which will make a relatively small contribution towards increasing the rate. (C) Decreasing the particle size of solid reactants. When a solid reacts with a liquid, only the surface particles of the solid can come into direct contact with the liquid reactant particles. If the solid is increasing in surface area (meaning breaking into smaller pieces), more solid particles would react.
3) The minimum amount of energy required to bring about the reaction is known as the activation energy. This is the minimum amount of kinetic energy that must be given to reactants before they will actually react. If two reactants molecules collide without this minimum amount of kinetic energy, they will simply bounce off each other without reacting. In a chemical reaction, if the activation energy is lower, then the reaction is exothermic because the reaction gives off more heat. However, if the activation energy is higher, the reaction is endothermic, because the reaction absorbs more heat.