UPSC Prelims 2001 Q116: Science & Technology – Chemistry
Assertion (A): A chemical reaction becomes faster at higher temperatures.
Reason (R): At higher temperatures, molecular motion becomes more rapid.
Which of the following is correct?
- (a) Both A and R are individually true and R is the correct explanation of A
- (b) Both A and R are individually true but R is NOT a correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer: (a) Both A and R are individually true and R is the correct explanation of A
Explanation
Assertion-Reason: reactions are faster at higher temperatures
| Statement | Verdict |
|---|---|
| A. A chemical reaction becomes faster at higher temperature | True — rate roughly doubles for every 10 °C rise |
| R. At higher temperature, molecular motion becomes more rapid | True |
| Link | Faster molecules collide more often and more of them cross the activation energy |
| Law | Arrhenius equation: k = A e^(−Ea/RT) |
Heating makes molecules move faster. They collide more often and, more importantly, a larger share of collisions have enough energy to react. So R explains A.
Answer: (a) Both A and R are individually true and R is the correct explanation of A.
Why each option is right or wrong
- (a) CORRECT. Faster molecular motion explains faster reactions at higher temperature.
- (b) Incorrect. R is the reason for A.
- (c) Incorrect. R is true; molecules move faster when heated.
- (d) Incorrect. A is true; reactions speed up with temperature.
Key point
Other factors raising reaction rate: concentration, surface area, catalysts (which lower activation energy).
Remember
Higher temperature → faster molecules → faster reaction.
Common mistakes
- Thinking temperature affects only the number of collisions, not their energy.
- Marking (b) out of excess caution about the link.
- Mixing up catalysts and temperature.
Exam tip
When R gives the textbook mechanism for A, choose (a).
Sources
- NCERT Class XII — Chemistry Part I, chapter 'Chemical Kinetics' (effect of temperature)
- NCERT Class XI — Chemistry Part I, chapter 'States of Matter' (kinetic theory)