In many chemical reactions, the amounts of reactants are not perfectly matched to the balanced equation. The reactant that runs out first is the limiting reactant, and it controls how much product can form. The other reactant is in excess, meaning some of it remains unused after the reaction stops.
Understanding this idea is essential for predicting product amounts in labs, industry, and everyday chemical processes.
Stoichiometry connects balanced chemical equations to mole ratios, which lets you compare reactants fairly. Once the limiting reactant is found, it is used to calculate the theoretical yield, the maximum possible amount of product. In a real experiment, the actual yield is often lower because of side reactions, incomplete reactions, spills, or purification losses.
Percent yield compares the actual yield to the theoretical yield and measures how efficient the reaction was.
Key Facts
- The limiting reactant is the reactant that produces the smaller amount of product.
- The excess reactant is left over after the limiting reactant is completely consumed.
- Always use the balanced chemical equation before doing stoichiometry calculations.
- Moles = mass / molar mass
- Theoretical yield is the maximum product predicted from the limiting reactant.
- Percent yield = actual yield / theoretical yield x 100%
Vocabulary
- Limiting reactant
- The reactant that is used up first and determines the maximum amount of product that can form.
- Excess reactant
- A reactant that remains after the limiting reactant has been completely consumed.
- Theoretical yield
- The maximum amount of product that can be made based on stoichiometry and the limiting reactant.
- Actual yield
- The amount of product actually collected from an experiment.
- Percent yield
- A comparison of actual yield to theoretical yield expressed as a percentage.
Common Mistakes to Avoid
- Using grams directly in mole ratios is wrong because balanced equations compare particles or moles, not masses.
- Choosing the reactant with the smaller mass as the limiting reactant is wrong because different substances have different molar masses and coefficients.
- Forgetting to balance the equation first is wrong because all mole ratios come from the balanced coefficients.
- Calculating percent yield with actual yield in the denominator is wrong because percent yield compares what was obtained to the maximum possible amount.
Practice Questions
- 1 For the reaction 2H2 + O2 -> 2H2O, if 6.0 mol H2 reacts with 2.0 mol O2, identify the limiting reactant and calculate the theoretical yield of H2O in moles.
- 2 For the reaction N2 + 3H2 -> 2NH3, 14.0 g N2 reacts with 3.0 g H2. Identify the limiting reactant and calculate the theoretical yield of NH3 in grams.
- 3 A student calculates a theoretical yield of 12.0 g but collects only 9.0 g of product. Explain what the percent yield tells you about the experiment and give two realistic reasons it might be less than 100%.