Balanced equations and reacting massesEdexcel International A Level Chemistry: Revision notes
Section 1
Writing balanced full equations
A balanced equation has the same number of atoms of each element on both sides. Balance by changing the large numbers in front of formulae, never the subscripts. State symbols show the physical state: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution.
Example: Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g). A full equation lists every substance as a complete formula. Check the balance by counting atoms (and charge, in ionic equations) on each side.
Never change a subscript to balance an equation: that changes the substance. Only change the coefficients.
Section 2
Ionic equations
An ionic equation shows only the species that change. To write one: (1) write the full equation with state symbols; (2) split soluble ionic compounds, (aq), into ions; (3) cancel spectator ions that appear unchanged on both sides; (4) check atoms and charge balance.
Example: Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq) becomes Pb2+(aq) + 2I-(aq) → PbI2(s). Solids, liquids and gases (including insoluble carbonates, metals, water and CO2) are never split into ions.
Both mass and charge must balance in an ionic equation: Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s) has 2+ on each side.
Section 3
Reacting masses
To find reacting masses: (1) write the balanced equation; (2) convert the known mass to amount using mass / molar mass; (3) use the equation ratio to find the amount of the other substance; (4) convert to mass using amount x molar mass.
Example: 80.0 g of Fe2O3 is 0.501 mol; Fe2O3 + 2Al → 2Fe + Al2O3 needs 1.00 mol of Al, which is 27.1 g. For solutions, amount = concentration x volume (dm3). The limiting reagent is the reactant that is used up first; the other is in excess, and the limiting reagent decides how much product forms.
Section 4
Displacement and acid reactions
In a displacement reaction a more reactive metal displaces a less reactive metal from solution: Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s). Observations: the blue colour fades and a brown (pink) solid forms on the zinc. The same idea applies to halogens, such as Cl2(aq) + 2Br-(aq) → 2Cl-(aq) + Br2(aq), where the solution turns orange.
Typical acid reactions: metal + acid gives a salt + H2 (effervescence, metal dissolves); carbonate + acid gives a salt + water + CO2 (effervescence): MgCO3(s) + 2H+(aq) → Mg2+(aq) + H2O(l) + CO2(g); base or alkali + acid gives a salt + water: H+(aq) + OH-(aq) → H2O(l).
Section 5
Precipitation reactions and observations
A precipitation reaction forms an insoluble solid when two solutions are mixed. The ionic equation shows only the ions that combine: Ag+(aq) + Cl-(aq) → AgCl(s) (white precipitate); Pb2+(aq) + 2I-(aq) → PbI2(s) (yellow precipitate); Ba2+(aq) + SO4 2-(aq) → BaSO4(s) (white precipitate).
Link equations to what you see. Say what is observed, not what is made: 'a yellow precipitate forms', not 'lead iodide forms'. Describe colour changes of the solution and any effervescence, using terms such as dissolves, fades, precipitate and bubbles.
Observations describe what you can see (a colour, a solid, bubbles). Do not name the products or use words like 'forms lead iodide'.
Must Know
- Balance by coefficients; always include state symbols
- Ionic equation: split only (aq) ionic compounds, remove spectator ions, check charge
- Reacting masses: balanced equation, mass to moles, ratio, moles to mass
- Limiting reagent: find which reactant is used up first
- Link equations to observations: displacement, acid reactions and precipitation
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Balanced equations and reacting masses
- A student places granules of zinc in blue copper(II) sulfate solution, CuSO₄(aq), and watches the reaction over several minutes.Explain why this reaction occurs, and identify the spectator ion.2 marks
- A student adds magnesium carbonate in small portions to 50.0 cm³ of hydrochloric acid of concentration 1.00 mol dm⁻³ until no more solid dissolves. Relative atomic masses: C = 12.0, O = 16.0, Mg = 24.3.Calculate the maximum mass of magnesium carbonate that can react with the acid.2 marks
- A teacher demonstrates a precipitation reaction by mixing 25.0 cm³ of 0.100 mol dm⁻³ lead(II) nitrate solution, Pb(NO₃)₂(aq), with an excess of potassium iodide solution, KI(aq). Relative atomic masses: I = 126.9, Pb = 207.2.Write the full equation, with state symbols, and the ionic equation, with state symbols, for the reaction, and state what is observed.3 marks
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).