Mean Rate of Reaction

1. Graphs of Reactions

• We can measure either:

  • Quantity of product formed (g or cm³ if gas).

  • Quantity of reactant used (g).

Graph: Product formed vs. Time

• Steep slope = fast reaction (many reactant molecules available).

• Slope decreases = reaction slowing (reactants being used up).

• Flat line (slope = 0) = reaction has stopped (all reactants used).

Graph: Reactant used vs. Time

• Rapid decrease at first = fast reaction.

• Decrease slows = fewer reactant molecules left.

• Flat line = all reactants used → reaction stops.

2. Mean Rate of Reaction

Formula:

• Units:

  • If product measured in g → g/s.

  • If product is a gas measured in cm³ → cm³/s.

3. Worked Examples

Example 1 (Product formed):

• 60 g of product in 15 s.

\text{Rate}=\frac{60}{15}=4\,\text{g/s }

Practice Q:

• 20 g of product in 40 s.

\text{Rate}=\frac{20}{40}=0.5g/s

Example 2 (Reactant used):

• 30 g of reactant in 10 s.

\text{Rate}=\frac{30}{10}=3g/s

Practice Q:

• 150 g reactant used in 75 s.

R\text{Rate}=\frac{150}{75}=2g/s

✅ Exam Tip:

• Always check if the question gives reactant used or product formed.

• Watch the units (grams vs cm³).

• On graphs: steep = fast, flat = stopped.