GCSE Combined Science (AQA)

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Rate Calculations using Moles

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The rate and extent of chemical change Rate of reaction

Rate Calculations using Moles

4:26 Calculating Reaction Rates using Quantity and Time
Spec 4.6.1.1
Previous Tests for Gases Next Rate Changes & Graphs
  • Mean rate of reaction can be calculated using the quantity of reactants in moles.
  • Equation: Mean rate of reaction (moles/second) = Quantity of reactant used (moles) / Time taken for reaction (seconds).
  • Example: Reaction of 0.1 moles of sodium thiosulfate with 0.1 moles of hydrochloric acid.
  • Time for reaction to complete: 4 seconds.
  • Calculation: 0.1 moles / 4 seconds = 0.025 moles per second.
  • Units for rate of reaction when using moles: moles per second.
  • Rate of reaction changes over time and can be tracked using graphs.
  • Important to be comfortable with rate calculations using moles for exams.
  • Worked example: 0.28 moles of hydrogen peroxide decomposing in 6.2 hours.
  • Convert time from hours to seconds: 6.2 hours = 21,600 seconds.
  • Calculation: 0.28 moles / 21,600 seconds = 0.00001296 moles per second.
  • Round to two significant figures: 0.000013 moles per second.

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