Concentration time graphs

In this section, we would look at how the concentration of reactants vary with time. Using a hypothetical A + B --> C reaction as an example, lets assume that the order of reaction with respect to A is zero order, i.e. the rate of reaction is independent of the [A]. The concentration time graph of A can be represented by a straight line. In other words, [A] decreases at a constant rate with time. This is because the rate of reaction is independent of the [A]. So although the concentration of A decreases with time, the rate of reaction remains the same. If the rate of reaction remains the same, it also means that reactant A is used up at a constant rate, hence the [A] decreases at a constant rate.


Now lets consider reactant B. Lets assume that the order of reaction w.r.t B is first order. Similarly the concentration of B will decrease with time. However the rate of reaction is dependent on the concentration of B, hence as the concentration of B decreases, the rate of reaction also decreases . As a result the concentration of B decreases at a decreasing rate. This can be illustrated by drawing tangents to the curve at different time points. You can see that the gradient of the tangent becomes gentler with time, indicating that the rate of decrease is decreasing.