📘 IB Understanding
Sketch, identify and analyse graphical representations of zero, first and second order reactions from concentration-time graphs.
[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 |
📋 Exam Tip
To prove a reaction is first order from a concentration-time graph, measure consecutive half-lives. If t½ is constant (e.g. 1.0 M → 0.5 M in 20 s, and 0.5 M → 0.25 M in 20 s), the reaction is first order.