What is a spectator ion?

An ion that is unchanged and appears on both sides of the equation.

Ionic equation for the precipitation of lead(II) iodide.

Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s)

Ionic equation for calcium carbonate and acid.

CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + CO₂(g) + H₂O(l)

Three steps for a reacting-mass question.

Mass to moles, use the equation ratio, moles to mass.

What is the limiting reactant?

The reactant that is used up first; it decides the amount of product.

Ionic equation for magnesium displacing copper.

Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s)

Observations when magnesium reacts with copper(II) sulfate solution.

Blue colour fades and red-brown copper is deposited.

Ionic equation for neutralisation.

H⁺(aq) + OH⁻(aq) → H₂O(l)

Balanced equations and reacting massesEdexcel A-Level Chemistry: Flashcards

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What makes an equation balanced?

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What makes an equation balanced?: The same number of atoms of each element, and the same total charge, on both sides.
What does (aq) mean?: Aqueous: dissolved in water.
What is a spectator ion?: An ion that is unchanged and appears on both sides of the equation.
Ionic equation for the precipitation of lead(II) iodide.: Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s)
Ionic equation for calcium carbonate and acid.: CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + CO₂(g) + H₂O(l)
How do you find amount from a solution?: n = c × V, with V in dm³.
How do you find the amount of a gas at RTP?: n = V ÷ 24.0, with V in dm³.
What is the molar volume at RTP?: 24.0 dm³ mol⁻¹.
Three steps for a reacting-mass question.: Mass to moles, use the equation ratio, moles to mass.
What is the limiting reactant?: The reactant that is used up first; it decides the amount of product.
Ionic equation for magnesium displacing copper.: Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s)
Observations when magnesium reacts with copper(II) sulfate solution.: Blue colour fades and red-brown copper is deposited.
Ionic equation for neutralisation.: H⁺(aq) + OH⁻(aq) → H₂O(l)
Convert 25.0 cm³ to dm³.: 0.0250 dm³.

Exam questions on Balanced equations and reacting masses

A student reacts 2.50 g of marble chips (calcium carbonate) with excess dilute hydrochloric acid in a conical flask.
Calculate the volume of carbon dioxide produced at room temperature and pressure. M(CaCO₃) = 100.1 g mol⁻¹ and the molar volume of a gas is 24.0 dm³ mol⁻¹.2 marks
A student mixes 20.0 cm³ of 0.150 mol dm⁻³ aqueous lead(II) nitrate with excess aqueous potassium iodide. A precipitate forms.
Calculate the maximum mass of lead(II) iodide formed. M(PbI₂) = 461.0 g mol⁻¹.2 marks
A student adds 1.20 g of magnesium ribbon to 50.0 cm³ of 0.500 mol dm⁻³ aqueous copper(II) sulfate and stirs. The mixture warms up.
Write an ionic equation, with state symbols, for the reaction and explain the observations in terms of this equation.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).