Chemistry 6.1 Rate of reaction

6.1.1 Calculating rates of reactions

The rate of a chemical reaction can be found by measuring the quantity of a reactant used or the quantity of product formed over time:

Rate of reaction equations

- The quantity of reactant or product can be measured by the mass in grams, moles, or by a volume in cm3.
- The units of rate of reaction maybe given as g/s, mol/s, or cm3/s

Example Calculations

Example 1: Calculate the rate of reaction for the decomposition of hydrogen peroxide (H₂O₂) into water (H₂O) and oxygen (O₂).

Chemical equation (balanced): 2 H₂O₂ → 2 H₂O + O₂

  1. What is being measured? The quantity of reactant used: 0.5 mol of H₂O₂ decomposes.
  2. Time interval: 10 s.
  3. Formula: rate = (quantity of reactant used or product formed) / (time taken).
  4. Substitute the values:
    rate = 0.5 mol / 10 s
  5. Do the division:
    rate = 0.05 mol/s
  6. Units and interpretation: The rate is 0.05 mol s⁻¹. This means on average 0.05 moles of H₂O₂ are consumed each second over the measured period.

Example 2: Calculate the rate of reaction for the formation of ammonia (NH₃) from nitrogen (N₂) and hydrogen (H₂). Give the rate in g/s and also show the equivalent in mol/s.

Chemical equation (balanced): N₂ + 3 H₂ → 2 NH₃

  1. What is being measured? The quantity of product formed: 34.0 g of NH₃ (measured mass).
  2. Time interval: 20 s.
  3. Rate in g/s (direct):
    rate = mass of product / time = 34.0 g / 20 s = 1.7 g/s.
  4. Units and interpretation: The reaction produces 1.7 grams of NH₃ each second on average during the measured period.
  5. Optional — convert to mol/s:
    Molar mass of NH₃ = 14.01 (N) + 3×1.008 (H) ≈ 17.03 g/mol.
    Moles of NH₃ formed = 34.0 g ÷ 17.03 g/mol ≈ 1.997 mol ≈ 2.00 mol (to three significant figures).
    rate in mol/s = 2.00 mol / 20 s = 0.100 mol/s.

6.1.2 Factors which affect the rates of chemical reactions

Factor Increases frequency of collision Increases energy of particles
Temperature Yes Yes
Surface area to volume ratio Yes No
Increase concentration (solutions) Yes No
Increase pressure (gases) Yes No
Add a catalyst No No

6.1.3 Collision theory and activation energy

- Collision theory states that chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.
- The minimum amount of energy that particles must have to react is called the activation energy.


6.1.4 Catalysts

- Catalysts, while increasing the rate of a reaction, but are not used up during the reaction.
- This means that the catalyst is still there, unchanged, after the reaction.
- Different reactions need different catalysts.
- An example of a catalyst is enzymes in biological systems.
- Catalysts increase the rate of reaction by providing a different pathway for the reaction that has a lower activation energy.

Energy profile diagram

(stolen from the spec) Catalysis energy profile diagram

- According to collision theory, a catalyst lowers the activation energy so a greater proportion of collisions result in a successful reaction.
- The frequency of collisions remains the same.

Catalyst Examples

Catalyst Use Why is this an important use of catalysts?
Enzymes (as biological catalysts). Washing detergent Enzymes are used to break down food that may be on clothes or in the environment. This makes it a lot easier for the washing machine to remove it from the clothes.
Platinum, Palladium, and Rhodium Catalytic converters Platinum and Palladium: Help oxidise carbon monoxide (CO) and unburned hydrocarbons into carbon dioxide (CO₂) and water (H₂O).
Rhodium: Focuses on reducing nitrogen oxides (NOₓ) into nitrogen (N₂) and oxygen (O₂).
A finely divided iron catalyst, often enhanced with promoters like potassium oxide (K₂O) and alumina (Al₂O₃). Haber process The catalyst reduces the energy, and therefore heat, required for the reaction to occur, making it a lot cheaper to produce. The Haber-Bosch process makes ammonia, one of the world's most important chemicals, as it is used as fertiliser.
Platinum, palladium, rhodium, or nickel. Hydrogenation - making spreads from vegetable oils Hydrogenation can turn liquid vegetable oils (unsaturated fats) into solid or semi-solid fats, often used in food production.
Calcium Carbonate Biodiesel Calcium Carbonate is used to make biodiesel, which is a renewable fuel made from vegetable oils. This is important as use of biodiesel can reduce carbon emissions and the amount of new oil that needs to be drilled for.
Zeolites Petroleum refining In the chemical and petrochemical industries, zeolites are used as catalysts in processes like oil refining and the production of fuels, making production cheaper and more efficient.