Chemistry, Third edition

REVISION QUESTIONS

3.The binding of the ethene and hydrogen molecules to the metal surface weakens

the bonds within the CH 2 =CH 2 and H 2 molecules. This reduces the activation

energy for the redistribution of bonding electrons involved in the hydrogenation

reaction. This makes the hydrogenation much faster than the direct reaction of

ethene and hydrogen in the absenceof a catalyst. The product, ethane, is pro-

duced already adsorbed on the metal surface (Fig. 14.12(c)).

4.The ethane desorbs from the metal surface, and moves into the gas phase (Fig.

14.12(d)). The nickel is then able to adsorb fresh reactants.

263

Revision questions

14.1.Carefully explain the following key terms:

(i)rate of reaction (ii)transition state (iii)activation energy

(iv)order of reaction (v)elementary reaction (vi)reaction

mechanism(vii)reactant half-life.

14.2.Which of the following statements are true?

(i) Catalysts do not alter the enthalpy change (H) of a reac-

tion.

(ii)Reactions with high activation energies are usually very

fast.

(iii)Activation energies vary with temperature.

(iv)Catalysts do not take part in the reactions they speed up.

(v)Reaction intermediates cannot be isolated

14.3.Which of the following diagrams shows the variation of

reaction rate with temperature for (i)an enzyme-catalysed

reaction and (ii)an uncatalysed reaction?

14.4.The decomposition of SO 2 Cl 2 ,

SO 2 Cl 2 (g)SO 2 (g)Cl 2 (g)

follows the rate expression

rate of reaction k[SO 2 Cl 2 ]

withk2.00 10 ^5 s^1 at 300 °C. In an experiment, SO 2 Cl 2

was heated to 300 °C. The initial concentration of SO 2 Cl 2 (g)

was 0.100 mol dm^3.

(i) What is the order of reaction?

(ii)What is the initial rate of reaction?

(iii)After 9 h the SO 2 Cl 2 (g) was still decomposing, but its con-

centration has fallen to 0.0523 mol dm^3. What is the rate

of reaction at t9h?

(iv)What is the half-life of SO 2 Cl 2 (g) at 300 °C?

14.5.The decomposition of dinitrogen pentoxide at 65 °C:

2N 2 O 5 (g)4NO 2 (g)O 2 (g)

follows the rate expression

rate of reaction k[N 2 O 5 (g)]

Reaction rate

Temperature

III

Reaction rate

Temperature

II

Reaction rate Ri

Temperature

I

Fig. 14.12Catalytic hydrogenation of ethene (adsorption is shown by dotted lines).

—

—