Measuring rates of reactions

1. Definition of Reaction Rate

• The rate of a reaction refers to how fast the products form or how fast the reactants are consumed.
• It is measured as the change in concentration, mass, or volume of reactants/products per unit time.

2. Experimental Techniques to Measure Reaction Rate

Different methods can be used depending on the type of reaction:

2.1 Measuring Turbidity (Cloudiness)

• Used when a precipitate forms, making the solution opaque.

• Example reaction: $\text{Na}_2\text{S}_2\text{O}_3(aq) + 2\text{HCl}(aq) → 2\text{NaCl}(aq) + \text{H}_2\text{O}(l) + \text{SO}_2(g) + \text{S}(s)$

• Method:

  1. Draw a black cross on paper and place it under a conical flask.
  2. Add sodium thiosulphate solution $(\text{Na}_2\text{S}_2\text{O}_3)$ to the flask.
  3. Quickly add hydrochloric acid $(HCl)$ and start a stopwatch.
  4. Swirl the flask and observe until the black cross is no longer visible.
  5. Record the time taken for the solution to turn opaque.

• Key Factors:

  • Can change concentration of reactants or temperature to study its effect on reaction rate.
  • The faster the cross disappears, the higher the reaction rate.

2.2 Measuring the Volume of a Gas Produced

• Used when a reaction produces a gaseous product.

• Example reactions:

  • $\text{Mg}(s) + 2\text{HCl}(aq) → \text{MgCl}_2(aq) + \text{H}_2(g)$
  • $\text{CaCO}_3(s) + 2\text{HCl}(aq) → \text{CaCl}_2(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g)$
  • Decomposition of Potassium Chlorate $\text{KClO}_3(s) → 2 \text{KCl}(s) + 3\text{O}_2(g)$

• Method: 6. Collect gas using a gas syringe or downward displacement of water in a burette. 7. Record the volume of gas produced at regular time intervals. 8. Stop timing when no further gas is produced. 9. Plot a graph of gas volume vs. time.

• Key Factors:

  • Reaction rate is higher if the gradient (slope) of the graph is steeper.

2.3 Measuring the Mass of Reactants and Products

• Used when a gas escapes from the reaction vessel.
• Example reactions:
  • $\text{CaCO}_3(s) + 2\text{HCl}(aq) → \text{CaCl}_2(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g)$
• Method: 10. Record the initial mass of the reaction flask and contents. 11. Start a stopwatch when reactants are mixed. 12. Measure the mass decrease at regular intervals. 13. Stop the stopwatch when the mass remains constant. 14. Plot a graph of mass vs. time.
• Key Factors:
  • The steeper the slope, the faster the reaction rate.
  • A horizontal line on the graph means the reaction has stopped.

3. Interpretation of Graphs

Graphs of reaction rates typically have three phases:

Stage	Graph Shape	Explanation
1: Fastest Rate	Steepest slope	Reactant concentration is highest, leading to more collisions and faster product formation.
2: Slowing Down	Gentle slope	Reactants are used up, leading to fewer collisions and slower reaction.
3: Reaction Complete	Flat line	Limiting reactant is used up; no further reaction occurs.

4. Key Definitions

• Turbidity: The cloudiness of a liquid due to suspended particles blocking light.
• Opaque: Not transparent; does not allow light to pass through.
• Limiting Reactant: The reactant that is completely used up, stopping the reaction.
• Excess Reactant: The reactant that remains after the reaction is complete.