6.1 - Rates of reaction
Rate of reaction is concerned with how quickly a reaction reaches a certain point. It can be defined as the decrease in concentration of the reactants per unit time or the increase in concentration of the products per unit time.
A graph may be plotted of concentration against time, with time on the x-axis and some measure of how far the reaction has gone (ie concentration, volume, mass loss etc) on the y-axis. This will produce a curve and the rate at any given point is the gradient of the tangent to this curve.
Collision theory -- reactions take place as a result of particles (atoms or molecules) colliding and then undergoing a reaction. Not all collisions cause reaction, however, even in a system where the reaction is spontaneous. The particles must have sufficient kinetic energy, and the correct orientation with respect to each other for them to react.
This is the minimum energy that particles colliding must have in order to produce successful reaction. It is given the symbol Ea (Energy of Activation). The energy of particles is expressed by their speed.
Changing the conditions
Increasing the temperature of a substance increases the average speed (Energy) of the particles and consequently the number of particles colliding with sufficient energy (Ea) to react. At higher temperatures there are more successful collisions and therefore a faster reaction.
At higher concentrations there are more collisions and consequently a faster reaction.
Catalysts lower the activation energy by providing an alternative mechanism for the reaction/ greater probability of proper orientation. This results in a faster reaction.
In hetrogeneous reactions (where the reactants are in different states) the size of the particles of a solid may change reaction rate, since the surface is where the reaction takes place, and the surface area is increased when the particles are more finely divided (therefore smaller solid particles in a hetrogeneous reaction tend to produce a faster reaction).
Most reactions involve several steps, which can be individually slow of fast, and which, all together, make up the complete reaction. The slowest of these steps is called the rate determining step, as is determines how fast the reaction will go. It is also not necessary that all the reactants are involved in ever step, and so the rate determining step may not involve all the reactants. As a result, increasing the concentration (for example) of a reactant which is not involved in the rate determining step will not change the overall reaction rate.