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Limiting & Excess Reactants (HL IB Chemistry)

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Limiting & Excess Reactants

Excess & limiting reactants

  • Sometimes, there is an excess of one or more of the reactants (excess reactant)
  • The reactant which is not in excess is called the limiting reactant
  • To determine which reactant is limiting:
    • The number of moles of the reactants should be calculated
    • The ratio of the reactants shown in the equation should be taken into account eg:

    C  + 2H2      →     CH4

  • What is limiting when 10 mol of carbon are reacted with 3 mol of hydrogen?
    • Hydrogen is the limiting reactant and since the ratio of C : H2 is 1:2 only 1.5 mol of C will react with 3 mol of H2

Worked example

9.2 g of sodium metal is reacted with 8.0 g of sulfur to produce sodium sulfide, Na2S.Which reactant is in excess and which is limiting?

Answer:

Step 1: Calculate the moles of each reactant

number space of space moles space left parenthesis Na right parenthesis space equals space fraction numerator 9.2 space straight g over denominator 22.99 space straight g space mol to the power of negative 1 end exponent end fraction equals space 0.40 space mol

number space of space moles space left parenthesis straight S right parenthesis space equals space fraction numerator 8.0 space straight g over denominator 32.07 space straight g space mol to the power of negative 1 end exponent end fraction space equals space 0.25 space mol

Step 2: Write the balanced equation and determine the coefficients

2Na + S → Na2S

Step 3: Divide the moles by the coefficient and determine the limiting reagent

    • divide 0.40 moles of Na by 2, giving 0.20 - lowest
    • divide 0.25 moles of S by 1, giving 0.25

Therefore, sodium is limiting and sulfur is in excess

Exam Tip

An easy way to determine the limiting reactant is to find the moles of each substance and divide the moles by the coefficient in the equation. The lowest number resulting is the limiting reactant

  • In the example above:
    • divide 10 moles of C by 1, giving 10
    • divide 3 moles of H by 2, giving 1.5, so hydrogen is limiting

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Philippa

Author: Philippa

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.