Correct Option: (D) Zero order
Step 1 (Governing Law & Definition): [1/3 Mark]
For a zero-order reaction, the rate of reaction is independent of the concentration of the reactant. The integrated rate equation is given by:
$$[R] = -kt + [R]_0$$Where:
- $[R]$ = Concentration of reactant at time $t$
- $[R]_0$ = Initial concentration of reactant
- $k$ = Rate constant
- $t$ = Time
Step 2 (Mathematical Working & Graphical Analysis): [1/3 Mark]
The equation $[R] = -kt + [R]_0$ follows the linear equation format $y = mx + c$:
- $y = [R]$ (Concentration on y-axis)
- $x = t$ (Time on x-axis)
- $m = -k$ (Slope of the line)
- $c = [R]_0$ (y-intercept)
Since the plot of $[R]$ versus $t$ is a straight line with a negative slope ($-k$), it mathematically confirms that the rate of change of concentration is constant, which is the defining characteristic of a zero-order reaction.
Step 3 (Final Concluded Answer): [1/3 Mark]
The graph represents a linear decrease in concentration over time, which is unique to Zero order reactions. Therefore, the correct option is (D).
💡 EL10 Concept Insight:
Think of a zero-order reaction like a tap dripping at a constant speed regardless of how much water is left in the tank; because the speed doesn't change as the amount decreases, the graph of "amount left" vs "time" is just a perfectly straight line going down.