Excess Reactants - Higher Chemistry Revision - BBC Bitesize - BBC

In this guide

1. Revise
2. Test

Pages

1. The design stage
2. Molar ratio
3. Moles and solutions
4. Molar volume
5. Percentage yield
6. Atom economy
7. Excess reactants

Excess reactants

A good way to ensure that one reactant fully reacts is to use an excess of the other reactant. This is financially efficient when one of the reactants is very cheap.

When one reactant is in excess, there will always be some left over. The other reactant becomes a limiting factor and controls how much of each product is produced.

While using excess reactants can help to increase percentage yields, this is at the expense of atom economy.

A balance between the economic and environmental value of the use of excess reactants must be established.

Example one

What volume of CO2 would be produced when 5 g of calcium carbonate (CaCO3) reacts with excess hydrochloric acid (HCl)? (molar volume = 22.4 litres mol-1)

Start with a balanced equation and look at the molar ratio. We know the acid is in excess, so the number of moles of calcium carbonate that react will control how many moles of product are formed.

\(2HCl + CaCO_{3}\,\,\,\rightarrow \,\,\,\,CaCl_{2} + H_{2}O + CO_{2}\)

\(\,\,\,\,\,\,\,\,\,\,\,\,1\,mole\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,1\,mole\)

\(1\,\,mole\,\,CaCO_{3}\rightarrow 1\,\,mole\,\,CO_{2}\)

\(0.05\,\,moles\,\,CaCO_{3}\rightarrow 0.05\,\,moles\,\,CO_{2}\)

\(1\,\,mole\,\,CO_{2}=22.4\,\,litres\)

\(0.05\,\,moles\,\,CO_{2}=\frac{0.05}{1}\times 22.4\)

\(=1.12\,\,litres\)

Example two

Which reactant is in excess when 0.25 g of magnesium is added to 100 cm3 of 0.1 mol l-1 hydrochloric acid (HCl)?

Start with a balanced equation.

\(Mg + 2HCl \rightarrow MgCl_{2} + H_{2}\)

The equation shows that one mole of magnesium will react with two moles of acid. Use the information in the question to calculate how many moles of each reactant are present.

For all of the magnesium to be used up, we would need twice the number of moles of acid and this is not the case.

All of the acid could react with half the number of moles of magnesium. From the calculation, it is clear that magnesium is in excess.

All of the acid will react with 0.005 moles of magnesium, leaving 0.005 moles of magnesium unreacted.

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More guides on this topic

• Revise: Controlling the rate
• Revise: Chemical energy
• Revise: Equilibria
• Revise: Chemical analysis

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