Chapter 4: Q34P (page 233)

The following reaction is a common synthesis used in the organic chemistry laboratory course.
When we double the concentration of methoxide ion (CH₃O^-) , we find that the reaction rate doubles. When we triple the concentration of 1-bromopropane , we find the reaction rate triples.
(a) What is the order of this reaction with respect to 1-bromopropane? What is the order with respect to methoxide ion? Write the rate equation for this reaction. What is the overall order?
(b) One lab textbook recommends forming the sodium methoxide in methanol solvent, but before adding 1-bromopropane ,it first distills off enough methanol to reduce the mixture to half of its original volume. What difference in rate will we see when we run the reaction (using the same amounts of reagents) in half the volume of solvent?

Short Answer

Expert verified

(a) The reaction is first order with respect to 1-bromopropane and also the reaction is first order with respect to methoxide ion.

rate = k_r[C₃H₇Br][CH₃O^-]

The reaction is second order overall.

(b) If the volume of methanol is reduced by 50% , then it will increase the concentration of methoxide ion by 50% .

Step by step solution

Rate equation

It may be defined as the relationship between the concentrations of the reactants and the observed reaction rate. Each reaction has its own rate equation which may be determined experimentally by altering the concentration of the reactants and then measuring the change in rate.

Consider a general reaction of the type: aA + bB → products

The reaction rate is usually proportional to the concentrations of the reactants and which are raised to powers and respectively. The rate expression can, then be written as:

rate = k_r[A]^a [B]^b , where k_r= rate constant.

Order of a reaction.

The order of a reaction may be defined as the sum of the exponents to which the concentration terms in the rate law are raised to express the observed rate of reaction.

From general equation of the type: aA + bB → products , the rate expression can be

written as,rate = k_r[A]^a [B]^b .

and are called the orders of the reaction with respect to A and B . Depending on whether (a+b) is equal to 0,1,2, or,3 , the reactions are said to be of zero order, first order, second order, and third order respectively.

Explanation

(a) From the given equation, it can be seen that the reaction is first order with respect to and also the reaction is first order with respect to methoxide ion.

The rate equation can be written as: rate = k_r [C₃H₇Br] [CH₃O^- ]

Overall order =(1 + 1) = 2 (second order). The reaction is second order overall.

(b) If the volume of methanol is reduced by 50% , then it will increase the concentration of methoxide ion by 50% .