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chemistry igcse acapprinted 19 · PDF 19
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19
• Limiting and Excess Reagents
• Often, questions may ask you to identify the limiting or excess reagent in a given
equation.
• Limiting Reagent: The limiting reagent in a chemical reaction is the reactant that will be
consumed completely. Once there is no more of that reactant, the reaction cannot proceed.
Therefore it limits the reaction from continuing.
• Excess Reagent: The excess reagent is the reactant that could keep reacting if the other
had not been consumed.
• Questions will often give you the mass (and maybe formulae) of the reactants, or will give
you the concentration and volume of the reactants. You must then calculate their moles
and then compare them with each other via their ratios in the balanced equation. If the
number of moles you get from the ratio method is more than the number of moles of that
specific reactant that you actually have, then that reactant is the limiting reagent.
Q. Use the following BALANCED equation.
2C2H6 + 7O2 + 4CO2 + 6H2O
If 15g of C2H6 reacts with 45g of O2, how many grams of water will be produced?
Step 1: Calculate the moles of the reactants.
Step 2: Compare the reactants using the ratios from the balanced equation
=
2
: 7