Rate of Reaction
The speed or rates of reaction vary greatly. You can get very fast, explosive reactions like the combustion of hydrogen, and very slow reactions that form no products. But changing the conditions of the reactions can make a difference.
Increasing the Reaction Speed
There are three main factors that affect the rate of reaction: temperature, concentration and surface area. They have the following effect on the rate of reaction:
Temperature Heat is just one form of energy, so if you add more heat to an reaction the atoms will have more energy and move about more, hence more collisions. (see collision theory below).
Concentration. When you have a substance that has a higher concentration, it means that you have more of it dissolved in solution. If there are more particles but in the same amount of space, they will be closer together, meaning less energy is required for a successful collision.
Surface Area Say you have a cube of an alkali metal. The water can react with all 6 sides (pressuming it is fully submerged, even though alkali metals float.. If you cut the cube in half there is still the same surface area, plus the extra that you have 'revealed' by cutting it. Therefore there are more atoms for it to react with.
Reaction speed can also be increased by adding a catalyst to the reaction. These increase reaction speed by lowering the activation energy required to start the reaction without themselves being used up. The most famous example of a catalytic usage is in the harber process.
Enzymes are biological catalysts, so speed up the reactions in biological systems. Examples of enzymes being used are in biological washing detergent, they help remove certain stains (proteases for protein).
Collision Theory
Collision thoery explains how and why chemical reactions occur, and is especially useful for explaining why the rate of reaction changes.
The first thing, is that chemical reactions are the resut of the reactants colliding with each other, with enough energy to form a chemical bond. These are known as successful collisions.
Secondly. If you put more energy into the reaction, the particles will move about more (more kinetic energy) and there is a higher chance of collisions and a higher chance that they will have enough energy to form a chemical bond.
If you have more of a particle (i.e. higher surface area or concentration) they will be more likely to collide and react.