Reaction Rates

 

The rate of a reaction is defined as a positive quantity that tells how the concentration of a reactant or a product changes with time. The rate of a reaction can be expressed either as the rate of appearance of products(s) or as the rate of disappearance of reactant(s).

To measure the rate of a reaction, it is necessary to know how the concentration changes with time. Consider the hypothetical reaction, A(g) B(g), shown below. Because the coefficients of the reaction are both 1, the numerical values of these rates are equal. When comparing the two graphs below, it is evident that the rate of disappearance is highest when the concentration of reactant(s) is the highest and the rate of appearance is highest when the concentration of the products(s) is the lowest.

 

 

The average rate of the disappearance of A over the 500 s interval from the beginning is given by:

Ave Rate_A = -Δ[A]/Δt = -(0.053 M - 1.7 M)/(500 s - 0 s) = 0.0033 M/s

 

 

The average rate of the appearance of B over the same 500 s interval from the beginning is given by:

Ave Rate_B = Δ[B]/Δt = (1.6 M - 0 M)/(500 s - 0 s) = 0.0033 M/s

 

 

Sometimes we are interested in the reaction rate at a particular instant of time, called the instantaneous rate. The instantaneous rate is determined by calculating the slope of the tangent at a particular time as shown below.

The initial rate of the reaction (the instantaneous rate at t = 0) is given by:

Inst Rate_{t = 0 s} = -Δ[A]/Δt = -(0.24 M - 1.7 M)/(120 s - 0 s) = 0.012 M/s

The instantaneous rate at t = 200 s, is given by:

Inst Rate_{t =200 s} = -Δ[A]/Δt = -(0.40 M - 1.0 M)/(200 s - 0 s) = 0.0032 M/s

 

 

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