A reaction rate is simply the speed at which a reaction occurs. Chemical kinetics, the study of reaction rates, analyzes the five main factors that influence the rate of a chemical reaction: temperature, concentration, particle size, the nature of the reaction, and the presence of catalysts.
A reaction rate increases as the temperature increases. Increasing the temperature of a reaction is the equivalent of increasing the energy into a reaction. More collisions will occur between the particles, speeding up the reaction, as explained by the collision theory. The Maxwell-Boltzmann distribution theory goes more in-depth, stating that when heat is added, more of the colliding particles will have the necessary activation energy to collide, react, and speed up the reaction.
It is the collision between these particles (with at least the activation energy for the reaction) that make the reaction go faster. For most reactions, the reaction doubles for every 10 degrees Celsius increase of temperature.
An easy way to increase temperature is by stirring. Although this will manually speed up a reaction, as particles move faster (as stirring will do), their temperature will increase in the form of thermal energy. Stirring can have a strong effect on reaction rate for heterogeneous reactions.
An increase in the concentration of a reactant will increase the rate of a reaction. A higher concentration means more particles are present, therefore increasing the frequency at which they collide because they are in closer contact at any given time. Think of people in a room - contact between people is going to increase as more people enter.
This also relates to pressure. In gaseous reactions, as pressure increases, the molecules will become higher-concentrated and packed together more closely, thus increasing the number of collisions and the total reaction rate.
The concentration gradient of a reaction can also influence the reaction's speed, as is especially true of a substance entering or leaving a cell membrane. A substance going down its concentration gradient (from a higher to lower concentration; passive transport) is faster and requires less energy than a substance going against its concentration gradient (from a lower to higher concentration; active transport).
Reaction rate increases with smaller particles. This is common sense, as smaller particles are naturally easier to dissolve than larger particles. Such is especially true with reactions like diffusion and osmosis, where the chemical reaction involves the dissolving of particles. If the particles are large, the reaction will take longer to complete. The smaller the size of the particles, the greater the area that the reaction can take place in.
NATURE OF THE REACTION
The rate of a reaction will vary depending on what substances are reacting. Some reactions are naturally faster than others, while bonding between atoms and molecules will make any reaction take longer to complete. The bonds will need to break in order for the reaction to occur, which takes time and energy. The number of reactants and the complexity of the reaction are bound to influence its rate as well.
Meanwhile, the physical state of the reactants will also be an influence. If the reactant particles are in the same phase (solid, liquid, or gas), it will be very easy for them to mix with each other, giving the particles maximum opportunity to collide. If they are in different phases, however, the reaction can only occur at the area of contact. For example, if one of the reactants is a solid, the reaction can only occur on the surface of the solid. Stirring or other manual labor may be needed for the reaction to fully complete.
A catalyst is something that speeds up a reaction by lowering the amount of activation energy needed for it to begin. In most reactions, an enzyme acts as the catalyst; it alters the rate without being consumed in the process, and can be reused for numerous reactions. One provides an alternate mechanism by which a reaction can occur. By lowering activation energy, it increases the frequency of collisions and reduces intermolecular bonding between the molecules.