Rates of Reaction

Chemical reactions only occur when reacting particles collide with each other w sufficient energy. the minimum amount of energy required to cause a reaction is called the activation energy. 

there are four important factors that affect the rate of reaction:

• temperature- in a cold reaction mixture the particles move quite slowly. they collide less often, w less energy so fewer collisions are successful
• concentration- in a low concentration reaction, the particles are spread out. they collide less often, so fewer successful collisions. in a high concentration reaction, particles are crowded close together. collide more often, more successful collisions. increasing the pressure of reacting gases also increases frequency of collisions
• surface area- large pieces of a solid reactant have a small surface area in relation to their volume. fewer particles are exposed & available for collisions, meaning fewer collisions & slower reaction. small pieces of a solid reactant have a large surface area in relation to their volume, so more particles exposed & available for collisions, meaning more collisions & faster reaction
• use of a catalyst - reduces amount of enery needed for a successful collision, makes more collisions successful, speeds up r eaction, provides a surface for molecules to attact to, increasesing chances of bumpring into each other. 

Exothermic reactions are accompanied by a temperaure rise. they transfer heat to the surroundings. examples of exothermic reactions include:

• neutralising alkalis w acids
• oxidation
• combustion
• self-heating can
• handwarmers

Endothermic reactions accompanied by a fall in temperature. heat energy is transferred from the surroundings. 

thermal decomposition & dissolving ammonium nitrate crystals in water are examples of endothermic reactions