IB Understanding
Graphical Signatures of Reaction Orders ([A] vs t)
Sketch, identify, and analyse graphical representations of zero, first, and second order reactions from concentration-time plots. Differentiate constant rate (linear slope) from exponential decay and successive half-life behaviour.
[A] vs Time for Different Orders
Key Features
| Order | Graph Shape | Gradient | Half-life |
|---|---|---|---|
| Zero | Straight line (negative slope) | Constant = −k | Increases as [A] drops |
| First | Exponential decay curve | Decreases as [A] drops | Constant (unique to 1st order) |
| Second | Steeper curve, flattens faster | Decreases more sharply | Increases as [A] drops |
Examiner Tip
Proving First Order Kinetics via Consecutive Half-Lives
To definitively prove a reaction is first order from a concentration-time graph: measure and annotate at least two consecutive half-lives (e.g. \(t_1\) from \(1.0 ightarrow 0.5 ext{ M}\), and \(t_2\) from \(0.5 ightarrow 0.25 ext{ M}\)). Demonstrating \(t_1 pprox t_2\) confirms first order.
AQA GCSE & IB Chemistry
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