Question

Which one of the following statements are TRUE or FALSE:

1.1. The bond strength of both reactants and products affect the reaction rate [ 1]

1.2. In most cases where reactions occur in a solution, the more the concentration of reactants decreases, the more the reaction rate increases. [1 ]

1.3. In general, in irreversible gas phase reactions, the total pressure of the mixture affects the reaction rate more than the partial pressures of the reactants. [1 ]

1.4. If the forward reaction is endothermic, the reverse reaction is exothermic. [1 ]

1.5. The reaction rate of endothermic reaction is faster at high temperatures while the rate of exothermic reactions is slower at high temperatures and vice versa [ 1]

1.6. The larger the activation energy, the more sensitive is the rate of reaction to temperature. [1 ]

1.7. The rate of combustion of bigger particles of coal is faster because they contain more energy than smaller particles of coal. [ 1]

1.8. During a catalytic reaction, the catalyst can react but it is not consumed [1 ]

1.9. The reaction rate constant k is not truly constant but it is independent of the concentration of the reactants. [1 ]

1.10. A catalyst lowers the activation energy of reactants so that the reaction rate is relatively high at lower temperatures than the reaction rates without the catalyst.

[1 ]

Answer 1

1.1 TRUE

Reason: Reactions involving the breaking of weaker bonds proceed faster than reaction involving the breaking of stronger bonds. Thus, the bond strength of reactants affects reaction rates. Once the bonds of the reactants are broken, new bonds have to be formed (for products). Weaker bonds are easier (or faster) to form than stronger bonds, thus the bond strength of products also affects the reaction rates.

1.2 FALSE

Reason: Consider a general reaction:

aA + bB ---> cC

For this reaction, the rate law is given by, - r = kCAaCBb (assuming elementary reactions)

where CA  and CB are the concentration of A and B respectively. As clear from the rate law, if the concentration of the reactants increases the rate will increase.

1.3 TRUE

Reason: If the total pressure is increased, the gas molecules will be forced more closer to each other ( Volume decreases on increasing pressure). As the molecules are more close to each other, they collide more frequently, thus increasing the rate constant and hence reaction rate. Changing the partial pressure of reactants (total pressure is not changed) doesn't change the rate constant but only the reaction rates. Thus, the change in total pressure of the mixture affects the reaction rate more.

1.4 TRUE

Reason: Heat of reaction = total energy of products - total energy of reactants

In the forward reaction is exothermic, Heat of reaction is negative which means the total energy of products is less than total energy of reactants.

For the backward reaction, the products will be reactants and reactants will be products, then the heat of reaction will be negative. Thus, backward reaction is endothermic.