Question

The rate of a chemical reaction can be expressed in terms of (a) rate of consumption of catalyst (b) rate of consumption of reactants only (c) rate of consumption of reaction and formation of products both (d) rate of formation of products only.

Answer 1

Chemical reactions vary greatly in the speed at which they occur. Some are essentially instantaneous, while others may take years to reach equilibrium. The Reaction Rate for a given chemical reaction is the measure of the change in concentration of the reactants or the change in concentration of the products per unit time.

Definition of reaction rate

The speed of a chemical reaction may be defined as the change in concentration of a substance divided by the time interval during which this change is observed:

\[ \text{rate}=\dfrac{\Delta \text{concentration}}{\Delta \text{time}} \tag{2-1}\]

For a reaction of the form \(A + B \rightarrow C\), the rate can be expressed in terms of the change in concentration of any of its components

\[\text{rate}=-\dfrac{\Delta [A]}{\Delta t}\]\[\text{rate}=-\dfrac{\Delta [B]}{\Delta t}\]\[\text{rate}=\dfrac{\Delta [C]}{\Delta t}\]

in which Δ[A] is the difference between the concentration of A over the time interval t2 – t1:

\[ \Delta [A] = [A]_2 – [A]_1 \tag{2-2}\]

Notice the minus signs in the first two examples above. The concentration of a reactant always decreases with time, so \(\Delta [A]\) and \(\Delta [A]\) are both negative. Since negative rates do not make much sense, rates expressed in terms of a reactant concentration are always preceded by a minus sign to make the rate come out positive.

Consider now a reaction in which the coefficients are different:

\[A + 3B \rightarrow 2D\]

It is clear that \([B]\) decreases three times as rapidly as \([A]\), so in order to avoid ambiguity when expressing the rate in terms of different components, it is customary to divide each change in concentration by the appropriate coefficient:

\[\text{rate}= -\dfrac{\Delta [A]}{\Delta t} = -\dfrac{\Delta [B]}{3\Delta t} = \dfrac{\Delta [D]}{2\Delta t} \tag{2-3}\]