Rate of chemical reaction
Rate of chemical reaction is described how quickly reactants are used up or how quickly products are formed during a chemical reaction. It is measured in moles per second (mols/second or mol.s−1).
In a chemical reaction, we have the reactants which are the substances that are undergoing the reaction, and also we have the products which are the substances that form as a result of the reaction.
The rate of chemical reaction describes how quickly or slowly the reaction takes place. Now the question is, how do we know whether a reaction is slow or fast? The answer lies on how quickly or slowly a product is formed.
Rate of chemical reaction is expressed as moles of the reactants used up per time taken or the moles of the product formed per time taken
Using the magnesium reaction as an example
Average reaction rate = moles Mg used reaction / time (s)
Or Average reaction rate = moles O2 used reaction / time (s)
Or Average reaction rate =moles MgO produced reaction / time (s)
Without much delay let us look at the factors that affect the rates of chemical reaction and how they affects them.
Factors affecting rate of chemical reaction
Several factors aﬀect the rate of a reaction. It is important to know these factors so that reaction rates can be controlled. This is particularly important when it comes to industrial reactions, so that productivity can be maximized. The following are some of the factors that aﬀect the rate of a reaction.
- Nature of reactants: Substances have diﬀerent chemical properties and therefore react diﬀerently and at diﬀerent rates. For example, metals in group 1 will react faster in water than metals in group 3 because of their chemical properties
- Concentration (or pressure in the case of gases) As the concentration of the reactants increases, so does the reaction rate. Some reaction occurs faster at a higher concentration.
- Temperature If the temperature of the reaction increases, so does the rate of the reaction.
- Catalyst Adding a catalyst increases the reaction rate.
- Surface area of solid reactants Increasing the surface area of the reactants (e.g. if a solid reactant is ﬁnely broken up) will increase the reaction rate.
We are going to use collision theory to explain to explain why and how this factors affects rate of chemical reaction. It should be clear now that the rate of a reaction varies depending on a number of factors. But how can we explain why reactions take place at diﬀerent speeds under diﬀerent conditions? Why, for example, does an increase in the surface area of the reactants also increase the rate of the reaction? One way to explain this is to use collision theory.
According to collision theory, for a reaction to occur, there must be an effective collision between the particles that are colliding with one another. Only a fraction of all the collisions that take place actually cause a chemical change. These are called ’successful’ collisions. When there is an increase in the concentration of reactants, the chance that reactant particles will collide with each other also increases because there are more particles in that space. In other words, the collision frequency of the reactants increases. The number of successful collisions will therefore also increase, and so will the rate of the reaction. In the same way, if the surface area of the reactants increases, there is also a greater chance that successful collisions will occur.
Deﬁnition: Collision theory Collision theory is a theory that explains how chemical reactions occur and why reaction rates diﬀer for diﬀerent reactions. The theory assumes that for a reaction to occur the reactant particles must collide, but that only a certain fraction of the total collisions, the eﬀective collisions, actually cause the reactant molecules to change into products. This is because only a small number of the molecules have enough energy and the right orientation at the moment of impact to break the existing bonds and form new bonds.
When the temperature of the reaction increases, the average kinetic energy of the reactant particles increases and they will move around much more actively. They are therefore more likely to collide with one another. Increasing the temperature also increases the number of particles whose energy will be greater than the activation energy for the reaction.