This set of Chemistry Exam Questions for Schools focuses on “Chemical Kinetics – Factors Influencing Rate of a Reaction”.
1. Rate of a reaction depends on the ΔH of the reaction.
Explanation: ΔH refers to the enthalpy of the reaction. It tells us how much energy was absorbed or released as a result of the reaction. It can only tell us if a reaction is exothermic or endothermic and as a result, the rate of a reaction does not depend on it.
2. Which of the following is not a direct factor affecting the rate of a reaction?
b) Presence of catalyst
c) Order of reaction
Explanation: Molecularity of a reaction is the number of atoms, ions or molecules that must collide with one another simultaneously so as to result into that chemical reaction. It does not affect the rate of the reaction directly.
3. Which of the following will lead to an increase in the rate of the reaction?
a) Decrease in temperature
b) Decreasing concentration of reactants
c) Addition of catalyst
d) Addition of inhibitor
Explanation: The presence of a catalyst in a reaction lowers the activation energy barrier and hence the reaction becomes faster. Hence, the addition of catalysts helps to increase the rate of a reaction.
4. A catalyst only increases the rate of the reaction in the forward direction.
Explanation: A catalyst increases the rate of the chemical reaction in both the forward and reverse direction. It provides an alternate pathway for the reaction to increase its speed in both the forward and reverse direction.
5. What happens to the rate of the reaction on increasing its temperature?
a) Rate of reaction increases
b) Rate of reaction decreases
c) Rate of reaction fluctuates between its maxima and minima
d) Rate of reaction is independent of temperature
Explanation: Increasing the temperature of a reaction results in an increase in the average kinetic energy of the particles taking part in the reaction. As the kinetic energy increases, the particles move faster and collide with each other more frequently and with a higher energy. This makes it easier for chemical reactions to take place. Hence, increasing the temperature increases the rate of the reaction.
6. Which of the following laws state that the rate of a chemical reaction is directly proportional to the concentration of reactants?
a) Henry’s law
b) Law of conservation of mass
c) Law of mass action
d) Dalton’s law
Explanation: The law of mass action is the proposition which states that the rate of a chemical reaction is directly proportional to the concentrations of the reactants. On the other hand, the other laws mentioned in the options are unrelated to the rate of a reaction.
7. What happens to the value of ΔG of a reaction when a catalyst is added to it?
a) It increases
b) It decreases
c) It remains unchanged
d) It is undefined on adding a catalyst
Explanation: Gibbs free energy(ΔG) is a state function, that is, it does not depend on the path taken by the reaction to reach the end product. It only depends on the enthalpy and the entropy of the reaction which in turn depend on the products and reactants of the reaction. Since a catalyst only changes the pathway taken by the reaction, it does not alter the value of ΔG.
8. What is the change in the rate of a second order reaction when the concentration of the reactant is increased by 2 times its initial value?
a) It doubles
b) No change
c) It quadruples
d) It triples
Explanation: Let the initial concentration of reactant = [A]
Given concentration of reactant = 2 x [A]
We know that the rate of a second order reaction = k x [A]2
Therefore, rate for 2 x [A] = k x 4 x [A]2
Therefore, the rate quadruples when the concentration of the reactant is doubled in a second order kinetic reaction.
9. When the two reactants are in the same phase, then which common phase will ensure that the products are formed at the fastest pace?
d) The rate of a reaction is independent of the phase of the reactants
Explanation: When two reactants are in the same phase, it is better for the speed if the reaction if they are in a gaseous phase as the particles are free to move and can collide more frequently with each other, resulting in a chemical reaction.
10. Which of the following is a factor that automobile engines use to increase the rate of its internal reactions?
a) Concentration of fuel
b) Viscosity of the fuel
c) Surface area of the reactants
d) Nature of the fuel
Explanation: Automobile engines cannot change the concentration, viscosity or the nature of the fuel. In the engines, fuel is injected in the form of microscopic droplets so that it has a larger surface area than when fed into the engine as a stream. This allows the fuel to burn rapidly.
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