Assume the reaction

A + 2 B → C + 2 D

is known to be ﬁrst order in [A] and second

order in [B].

When 0.20 moles of A and 0.30

moles of B are placed in a 2.00 liter container,

it is observed that 1 × 10−5 moles of C are

formed per second with no change in volume.

What is the rate constant at 25◦C?

Answer in units of L2/mol2· s

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A + 2 B → C + 2 D

The vessel is of 2 litres' capacity. Concentrations of A and B in moles per litre are 0.2/2=0.1 and 0.3/2=0.15. from the law of mass action, rate of a chemical reaction is proportional to the active mass of the reacting components. As the reaction is first order with respect to A and second order with respect to B, mathematically one can say,

Rate = K.[A]. [B]2 , where K is a proportionality constant, known as rate constant. Putting the values in the equation, one gets

1 × 10−5 = K. 0.1. (0.15)2

Or, K = 0.0044 L2/mol2· s.

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