Free Rate of Reaction MCQs with Answers

1. The rate of a chemical reaction is defined as the change in

• A. concentration of a reactant or product per unit time
• B. temperature of the mixture per unit time
• C. mass of the catalyst per unit time
• D. total pressure of the system

Explanation: Rate measures how quickly reactants are consumed or products formed, so its units are usually moles per cubic decimetre per second. It is always positive, which is why a minus sign is written in front of the reactant term. Rate is greatest at the start, when reactant concentration is highest, and falls as the reaction proceeds.

2. According to collision theory, a reaction occurs only when colliding molecules

• A. are of the same size
• B. collide with energy equal to or greater than the activation energy and with the correct orientation
• C. collide at low temperature
• D. collide exactly head on, regardless of energy

Explanation: Most collisions are fruitless because the molecules either rebound with too little energy to break existing bonds or meet at the wrong angle for new bonds to form. Only collisions that satisfy both conditions are effective, and the fraction that do so is what determines the rate. Raising the temperature increases both the collision frequency and, far more importantly, the fraction with sufficient energy.

3. The rate determining step of a multi step reaction is

• A. the fastest step
• B. the slowest step, which controls the overall rate
• C. always the first step
• D. the step with the largest number of molecules

Explanation: An overall reaction can proceed no faster than its slowest stage, in the same way that a road is only as clear as its narrowest point, so the rate equation reflects the species involved up to and including that step. This is why the experimentally measured order often differs from the coefficients in the overall equation. The slow step is the one with the highest activation energy.

4. In the reaction between marble chips and hydrochloric acid, the rate can be followed by measuring

• A. the mass of the flask, which falls as carbon dioxide escapes
• B. the colour of the acid
• C. the temperature of the marble
• D. the mass of the flask, which rises as gas is produced

Explanation: Carbon dioxide leaves the open flask, so the total mass decreases steadily and the loss per unit time is a direct measure of the rate. The alternative method is to collect the gas in a syringe and record its volume at fixed intervals. Mass could never increase, since nothing is entering the flask from outside.

5. 2A + B gives 3C + D. Which of the following does not express the reaction rate?

• A. -d[B]/dt
• B. d[D]/dt
• C. -1/2 d[A]/dt
• D. -1/3 d[C]/dt

Explanation: Rate is expressed as the change in concentration divided by the coefficient, with a minus sign for reactants because they are being consumed and a plus sign for products because they are being formed. C is a product, so its term must be positive, and writing it with a minus sign makes the expression wrong. The other three all give the same positive rate.