Rates of Reaction

Practical: effect of surface area on the rate of reaction

• Aim: show that powdered marble reacts faster with hydrochloric acid than the same mass of large lumps
• Apparatus: conical flask, delivery tube with bung, water trough, 100 cm³ inverted measuring cylinder, stopwatch, balance, 1.0 mol dm⁻³ hydrochloric acid, three samples of calcium carbonate (large chips, small chips, powder), each weighed to the same fixed mass (e.g. 4.0 g)
• Method:
  • Set up the gas-collection apparatus over the water trough with the inverted measuring cylinder filled with water
  • Pour 50 cm³ of HCl into the conical flask
  • Tip in the 4.0 g of large chips, immediately push in the bung, and start the stopwatch
  • Read off the gas volume in the measuring cylinder every 15 seconds for 2 minutes
  • Empty and dry the apparatus; repeat with 4.0 g of small chips and again with 4.0 g of powdered calcium carbonate
• Sample results (volume of CO₂ collected, cm³):

• Plot volume of CO₂ (y) against time (x) for all three runs on the same axes
• The powder curve is the steepest at the start and levels off first, but every curve plateaus at the same final volume because the same total mass of CaCO₃ was used in each run

Practical: effect of a catalyst on the rate of reaction

• Aim: compare the rate of hydrogen peroxide decomposition with several different solids to see which one is the best catalyst
• Apparatus: as above (gas collection over water), plus 20-vol H₂O₂ solution and a set of solid candidate catalysts measured out to the same mass (e.g. 0.5 g): manganese(IV) oxide, lead(II) oxide, iron(III) oxide, copper(II) oxide
• Method:
  • Add 25 cm³ of H₂O₂(aq) to the conical flask
  • Sprinkle in 0.5 g of the first solid, close the bung, start the stopwatch
  • Read off the volume of O₂ collected every 15 seconds for 90 seconds
  • Repeat with each of the other solids in turn (fresh H₂O₂ solution each time)
• Plot volume of O₂ (y) against time (x) on a single set of axes — the steepest curve identifies the most effective catalyst
• Manganese(IV) oxide gives by far the steepest line and is the standard school catalyst for this decomposition