FACTORS AFFECTING RATE OF REACTION:
The rate of a chemical reaction measures how quickly reactants are consumed or products are formed. It depends on the conditions under which the reaction is carried out.
- Concentration: Generally, an increase in reactant concentration leads to an increase in the reaction rate. This is because a higher concentration provides more reactant particles, leading to a higher collision frequency.
- Temperature: The rate of reaction increases with the increase in temperature. In most cases, the rate of reaction becomes nearly double for 10° rise in temperature.
- Catalysts: Catalysts are substances that accelerate a reaction by providing an alternative reaction pathway with lower activation energy. They increase the rate of reaction without being consumed in the process.
For the reaction \( \ce{2NO(g) + O2(g) -> 2NO2(g)} \), the rate law found by experiment is Rate \( = k[\ce{NO}]^2[\ce{O2}] \). At a fixed temperature, the concentration of NO is doubled and the concentration of O2 is halved. How does the rate of the reaction change?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → It becomes two times
The rate law shows how the rate depends on the concentration of each reactant. Each concentration acts through its own power: here [NO] is squared and [O2] has the power one.
\( \displaystyle r_1 = k[\ce{NO}]^2[\ce{O2}] \)
\( \displaystyle r_2 = k\left(2[\ce{NO}]\right)^2\left(\frac{1}{2}[\ce{O2}]\right) \)
\( \displaystyle r_2 = 4 \times \frac{1}{2} \times k[\ce{NO}]^2[\ce{O2}] = 2r_1 \)
Doubling [NO] alone makes the rate four times, because [NO] is squared. Halving [O2] then halves that, so the new rate is two times the old rate.
NCERT (2026-27 edition):
3.2.2 Rate Expression and Rate Constant – “It is obvious, after looking at the results, that when the concentration of NO is doubled and that of O2 is kept constant then the initial rate increases by a factor of four from 0.096 to 0.384 mol L–1s–1. This indicates that the rate depends upon the square of the concentration of NO. When concentration of NO is kept constant and concentration of O2 is doubled the rate also gets doubled indicating that rate depends on concentration of O2 to the first power.”