New
New
Year 10
AQA
Higher

# Limiting reactants

I can identify a reactant as being in excess or the limiting reactant and use that information to calculate a theoretical yield.

New
New
Year 10
AQA
Higher

# Limiting reactants

I can identify a reactant as being in excess or the limiting reactant and use that information to calculate a theoretical yield.

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## Lesson details

### Key learning points

1. In a chemical reaction, the atoms in reactants are rearranged and are the same atoms that are in the products.
2. A theoretical yield is calculated using the stoichiometry of a balanced equation & relevant mathematical relationships.
3. The number of moles of limiting reactant available is used to calculate the theoretical yield of a reaction.
4. If a reactant is added in excess, not all of it will react and some will be found in the final product mixture.
5. A limiting reactant is one that restricts the amount of product that can form from the available particles.

### Common misconception

Pupils sometimes fail to understand that excess reactant particles remain in the reaction mixture after the reaction.

Provide a balanced equation and challenge pupils to draw diagrams of a final mixture, stressing that excess particles would also be present. Challenge pupils to consider how the product could be extracted from the excess particles.

### Keywords

• Limiting reactant - The reactant that completely reacts, thereby determining the maximum amount of product that can be formed in a chemical reaction. All other reactants will be present in excess.

• Excess reactant - Any reactant present in a greater amount than is necessary to completely react with the limiting reactant.

• Theoretical yield - The maximum calculated mass of product expected to form from a given amount of reactants.

• Mole - A mole of a substance contains 6.02 × 10²³ particles of it. The mass of one mole of a substance is its relative mass in grams.

• Stoichiometry - The molar ratio of the reactants to the products in a chemical reaction.

### Equipment

2 cm strip of Mg crucible + lid balance tongs heatproof mat, Bunsen burner, tripod clay triangle

### Content guidance

• Equipment requiring safe usage.

### Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

## Starter quiz

### 6 Questions

Q1.
Match the following terms to the correct definition.
Correct Answer:relative atomic mass,the average mass of the atoms of an element compared to carbon-12

the average mass of the atoms of an element compared to carbon-12

Correct Answer:relative formula mass,the sum of the relative atomic masses of all the atoms in a formula

the sum of the relative atomic masses of all the atoms in a formula

Correct Answer:Avogadro's constant,the number of particles in one mole of a substance

the number of particles in one mole of a substance

Q2.
The formula for ammonium nitrate is NH₄NO₃. Which of the following ratios are correct?
Correct answer: The ratio of nitrogen to oxygen is 2 : 3.
Correct answer: The ratio of nitrogen to hydrogen is 1 : 2.
Correct answer: The ratio of oxygen to hydrogen is 3 : 4.
The ratio of oxygen to nitrogen is 2 : 3.
Q3.
Chemical equations represent how the atoms in the rearrange to form the products.
Q4.
Which of the following is the equation which links mass, relative formula mass and amount of substance?
Correct answer: mass = amount of substance × relative formula mass
mass = amount of substance ÷ relative formula mass
amount of substance = relative formula mass × mass
amount of substance = relative formula mass ÷ mass
Q5.
True of false? The following equation is balanced: C₂H₄ + 2O₂ → 2CO₂ + 2H₂O.
true
Q6.
Calculate the relative formula mass for calcium carbonate, CaCO₃. Relative atomic masses: Ca = 40; C = 12; O = 16.
Correct Answer: 100, one hundred, a hundred

## Exit quiz

### 6 Questions

Q1.
Match the following key terms to the correct definitions.
Correct Answer:limiting reactant,the reactant that completely reacts during a reaction

the reactant that completely reacts during a reaction

Correct Answer:excess reactant,a reactant that is left over at the end of the reaction

a reactant that is left over at the end of the reaction

Correct Answer:theoretical yield ,the maximum calculated mass of product expected from a reaction

the maximum calculated mass of product expected from a reaction

Correct Answer:mole,the unit for amount of substance

the unit for amount of substance

Q2.
Which of these statements about the equation are true? 2H₂ + O₂ → 2H₂O.
One mole of hydrogen reacts with one mole of oxygen.
Correct answer: One mole of oxygen reacts with two moles of hydrogen.
Correct answer: Two moles of water are formed for every two moles of hydrogen reacted.
Two moles of oxygen are needed to form two moles of water.
Q3.
True or false? In a chemical reaction there are the same number of atoms in the products as there were in the reactants.
false
Q4.
A student oxidises magnesium in a Bunsen burner flame. The oxygen is in excess. 2Mg + O₂ → 2MgO. How many moles of magnesium oxide would form from eight moles of magnesium?
Q5.
Which is the limiting reactant if 6 moles of methane reacts with 4 moles of oxygen? CH₄ + 2O₂ → CO₂ + 2H₂O.
methane
carbon dioxide
water
Q6.
A student oxidises 2.0 g of magnesium in a Bunsen burner flame. The oxygen is in excess. 2Mg + O₂ → 2MgO. What mass of magnesium oxide would form from 2.0 g of magnesium? Give your answer to 2 s.f.
Correct Answer: 3.3, 3.3 g, 3.3g