- The molecules must approach each other in the correct orientation (steric factor)
- The molecules must have a minimum amount of energy (activation energy)
If the reactants are gases, when the pressure is increased there will be more moles of each gas per unit area than at lower pressures: -
This shows that as the molecules move around they will be far more likely to collide with another molecule at high pressure than at low pressure. Collision theory says that if more collisions occur then the proportion of successful collisions resulting in a reaction will also increase, increasing the rate of the reaction. This is the same for concentrations of reactants when in solution, when the concentrations are higher; the rate of the reaction is increased.
When the temperature of a reaction is increased, the molecules will gain more kinetic energy and move more. This increased energy means more molecules will have the required activation energy for a reaction to occur so more reactions do occur. Also they will be moving more rapidly so the frequency of collisions will be greater resulting in more reactions and thus a higher rate of reaction.
The molecules in a gas or liquid do not all have the same kinetic energies since they are moving with different speeds. The distribution of their speeds is called the Maxwell-Boltzmann distribution. The graphs that can be drawn to display this show, why at higher temperatures the rate of a reaction increases.
The graph shows that at any temperature, only a few molecules will have very high or very low energies, with most having a value around a mean peak value. As the temperature increases the curve broadens out and the peak value drops moving towards a higher energy level. The area under each curve represents the number of molecules and is constant for each curve.
The graph shows that to the right of the E act line, the molecules have enough energy for a reaction to occur. At the higher temperature (T3) a high proportion of the molecules have enough energy to react so the rate of the reaction is higher than at low temperatures (T1) where few molecules have enough energy for a reaction to occur.
Where solids are involved in a reaction, the surface area of the solid available to the other reactants is a very important factor that affects the rate of the reaction. Take the example of hydrochloric acid reacting with limestone; if two samples of equal mass are taken and one is left as a solid chip, and the other ground into a powder, then the powdered limestone will have a far greater surface area than the chip. When the samples are placed in acid the reaction will be far quicker with powdered limestone. This is all due to the increased surface area, collision theory states that for a reaction to occur there has to be a collision between the reactant molecules on the right plane and with enough energy (activation energy). If more limestone molecules are at the surface to the acid then the chance of a successful collision occurring is far greater, thus products are created faster and thus the rate is faster.
Catalysts are a chemical phenomenon that allow certain reactions to proceed at a far greater rate, without the catalyst being used up at all during the reaction. They are generally only required in small amounts. They usually work by providing an alternate route from reactants to products, which has lower activation energy. The simplified reaction profile for such reactions is: -
The graph shows simply that with the aid of a catalyst the activation energy required for the reaction is lowered so more of the molecules will have the energy required so the reaction will progress at a faster rate.
An example of a catalysed reaction is that between peroxodisulphate ions and iodide ions:
S2O82- (AQ) + 2I-(AQ) 2SO42-(AQ) + I2 (AQ)
This reaction can be catalysed by Fe2+ ions, two reactions occur:
S2O82- (AQ) + 2Fe2+(AQ) 2SO42-(AQ) + 2Fe3+(AQ)
Followed by:
2Fe3+(AQ) + 2I-(AQ) 2Fe2+(AQ) + I2 (AQ)
The catalyst is changed in the intermediate stage but at the end of the reaction is back in its original state. A reaction profile can be drawn for this reaction although it only shows a theory as to how the process works: -
The graph shows the reactants and products along with the two-step intermediate stage where two reactions occur. Overall this reaction is exothermic.