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Balanced Equations

Chemistry: AQA A Level, topic 1.2.5

Balanced Equations

Balanced equations are absolutely crucial to any kind of quantitative chemistry.

What are they?

What are they?

  • There are a few key features of balanced equations:
  • They must have an equal number of atoms on either side.
  • You add more atoms by adding reactants or products to either side.
    • There's no method to do this apart from trial and error - just find what works.
Why are they useful?

Why are they useful?

  • Balanced equations can be used to calculate masses of reactants and volumes of gases.
  • We'll demonstrate these via some worked examples in this session.
Titration

Titration

  • Titration is an experimental technique for finding the concentrations of solutions.
  • Titrations use balanced equations to calculate the concentration of an unknown solution from a volume of a known solution.
  • The experimental technique involves the use of a burette, and often an indicator.
  • A small volume of solution is dropped from the burette into the reaction flask. This is repeated until the indicator shows no further reaction happening.

Ionic Equations

Ionic equations are used to describe reactions in solution.

Ionic solids

Ionic solids

  • Ionic solids will dissolve in solution and will split into their constituent ions.
  • E.g. NaCl(aq) ⇌ Na+(aq) + Cl−(aq)
Reacting in solution

Reacting in solution

  • Some of the ions dissolved in the solution will react.
  • But, some of the ions dissolved will not be involved in any reaction.
  • These ions are called spectator ions.
Ionic equations

Ionic equations

  • Ionic equations are equations that have excluded all the non-reacting spectator ions.
  • The charge on either side of the ionic equation should be equal.

State Symbols

State symbols are letters that tell us what state of matter a substance is in. There are four different state symbols:

(s)

(s)

  • (s) is used for solids.
  • E.g. Mg(s)
  • E.g. Na(s)
(l)

(l)

  • (l) is used for pure liquids.
  • E.g. H2O(l)
  • E.g. Hg(l) (mercury is liquid at room temperature).
(aq)

(aq)

  • (aq) is used for aqueous solutions (substances dissolved in water), and therefore it is used more often than (l).
  • E.g. NaOH(aq)
  • E.g. H2SO4(aq)
(g)

(g)

  • (g) is used for gases.
  • E.g. H2(g)
  • E.g. CO2(g)

Why are they useful?

  • State symbols are useful to chemists because the state a reactant is produced in can affect experimental techniques.
  • For example, you wouldn't want to carry out a reaction that generates 30 moles of CO2(g) in a 10cm3 sealed vessel - it would explode.
  • If you left your state symbols out, you might try this.

Test yourself on Balanced Equations

  • What volume of hydrogen gas will react with 2 litres of chlorine gas given that the reactant ratio is 1:1?

  • Which of the following do ionic equations NOT contain?

  • What state symbol is correct for each of these substances at room temperature?

  • What do we call the letters that tell us the state of matter of substances in a chemical equation?

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