254 14 · SPEED OF CHEMICAL REACTIONS
Example 14.3
The formation of nitrosyl chloride (NOCl) at 400 K
NO(g)Cl 2 (g)NOCl(g)Cl(g)follows the rate expression
rate of reaction k[NO(g)][Cl 2 (g)]The initial rate of reaction (with both reactants at an initial
concentration of 0.010 mol dm^3 ) was found to be
3.2 10 ^6 mol dm^3 s^1.(a) What is the order of reaction?
(b) What is the value of kat 400 K?
Answer
(a)The reaction is first order with respect to nitric oxide, first order with respect to
chlorine, and second order overall.
(b)kinitial rate of reaction
[NO(g)] 0 [Cl 2 (g)] 0Therefore,k(3.2 10 ^6 )/(0.010 0.010)3.2 10 ^2 mol^1 dm^3 s^1.Calculating the rate of reaction
The reaction between nitric oxide and oxygen gas,
2NO(g) O 2 (g)2NO 2 (g)
is found to follow the rate expression
rate of reaction k[NO]^2 [O 2 ]
(i)What is the overall order of reaction?
(ii)At 300 °C the rate constant kis 7000 mol^2 dm^6 s^1. What is the rate of reaction at the
instant when [NO] and [O 2 ] are both 0.060 mol dm^3?Exercise 14H
Calculation of rate constant
The hydrolysis of sucrose in the presence of acid catalyst is often represented as
[H(aq)]
C 12 H 22 O 11 H 2 O ——2C 6 H 12 O 6
The rate expression is found to be
rate of reaction k[C 12 H 22 O 11 ][H(aq)]
(Notice that in this example, the concentration of catalyst appears in the rate expression.)
A reaction mixture at 298 K initially contained 0.20 mol dm^3 sucrose and 0.10 mol dm^3
H. The initial rate of reaction was found to be 4.0 10 ^6 mol dm^3 s^1.
(i)What is the overall order of the reaction?
(ii)Calculatekat 298 K and state its units.Exercise 14I