1.1 Short Notes: Rate of Reaction

  1. Rate of reaction definition = Speed in which a chemical reaction occurs.
  2. Rate of reaction formula
  3. Example: If 10cm3 of Hydrogen (H2) is produced in 10 seconds when Zinc (Zn) is added into Sulphuric acid (H2SO4), then the rate of reaction is calculated as follows:
    • 10cm3 / 10s = 10cm3s-1
  4. Factors affecting the rate of reaction:
    • Particle size of a solid reactant (smaller particle size = higher reaction rate) *to study this, one of the reactants must be in solid form.
  • In Experiment 1, 30cm3 of Hydrogen gas is released in 30 seconds. This gives us a reaction rate of 1cm3s-1
  • In Experiment 2, 30cm3 of Hydrogen gas is released in 15 seconds. This gives us a reaction rate of 2cm3s-1
  • The chart below compares Experiment 1 and 2
Observation: Experiment 2 is 2x faster than Experiment 1. Smaller particle sizes increase total surface area, resulting in higher reaction rates.
  • Concentration of reactants (higher concentration of solution = higher reaction rate). Only applicable to aqueous solutions.
    • Particles in concentrated solutions are more closely packed resulting in more frequent particle collisions.
  • Temperature of reaction (higher temperature of reaction = higher reaction rate). Applicable for solids, liquids and gasses.
    • Higher temperature increases kinetic energy of reactants resulting in more frequent particle collisions.
  • Pressure (higher pressure = higher reaction rate). Only applicable reactions involving gasses.
    • Higher pressure forces gaseous particles closer together resulting in more frequent particle collisions)
  • Presence of a catalyst. A catalyst is any chemical capable of activating the rate of energy resulting in higher reaction rates.

Questions

  1. How can we measure the rate of reaction for the reaction between zinc and hydrochloric acid
  2. Meat cut into smaller pieces cook faster than meat cut into large pieces. Explain
  3. In the reaction of sodium thiosulphate and sulphuric acid, how do we measure the rate of reaction? Explain.
  4. In any reaction, why does the rate always decrease with time?

Answers:

  1. Measure the mass of zinc used up over time or the volume of hydrogen gas released over time.
  2. Meat cut into smaller pieces have a larger surface area exposed and will cook faster.
  3. We measure the time taken for a yellow precipitate to form to cover the ‘x’ mark at the bottom of the flask. We cannot measure the volume of gas released as SO2 gas is soluble in water and cannot be collected.
  4. The concentration of the solution decreases with time as more reactant particles are used up with time.

Discussion

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