Collisions in Solution

Collision theory applies to reactions in solution just as it does to gases. Dissolved reactant particles (molecules or ions) move through the solvent, and a reaction happens only when two reactant particles collide with energy greater than or equal to the activation energy and in the correct orientation.In solution, a reactant particle is surrounded by solvent molecules. Most of its collisions are with the solvent, not with the other reactant. The rate therefore depends on the concentration of the dissolved reactants in mol dm⁻³, which controls how often reactant particles meet. Changing the pressure has almost no effect, because liquids are almost incompressible.The nature of the particles also matters:• Reactions between oppositely charged ions are usually very fast. The ions attract each other and no covalent bonds need to be broken, so the activation energy is very low. For example, a precipitate forms instantly when silver nitrate solution is added to a solution of chloride ions:Ag⁺(aq) + Cl⁻(aq) → AgCl(s)• Reactions in which covalent bonds must be broken are slower, because the activation energy is higher. The hydrolysis of a halogenoalkane by water takes minutes or hours, because the C–X bond must break.When a solid reacts with a solution, only the surface of the solid is in contact with the solution. Stirring brings fresh solution to the surface and carries products away, so it increases the rate. For a reaction between two dissolved substances, stirring only mixes them; once mixed, the rate depends on concentration and temperature.