CBSE 2025, Set 3

Question

A plot between concentration of reactant [R] and time 't' is shown below. Which of the given order of reaction is indicated by the graph?

![Zero order reaction: [R] vs time linear graph with negative slope k = -slope](https://www.edvaya.com/images/chemistry/cbse/2025/set3/q7-zero-order-r-vs-time-graph.webp) A). Third order B). Second order C). First order D). Zero order

Edvaya · Accepted Answer

The correct answer is Zero order. For a **zero order reaction**, the integrated rate law is: $$[	ext{R}]_t = [	ext{R}]_0 - kt$$ This is a **linear equation** in the form $y = c - mx$. A plot of [R] vs time gives a **straight line** with: - Y-intercept = $[	ext{R}]_0$ (initial concentration) - Slope = $-k$ (negative of rate constant) The graph shows exactly this — a straight line with negative slope where $k = -	ext{slope}$. **Answer: Zero order reaction** For comparison: - First order: $\ln[	ext{R}]$ vs $t$ is linear - Second order: $1/[	ext{R}]$ vs $t$ is linear

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