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Balancing Equations & Ionic Equations

Chemistry: IB Diploma Higher Level, topic 1.7.4

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.
  • Remember atoms are rearranged in chemical reactions, so there needs to be an equal number of atoms of each element either side of the equation.
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.
Molecular equations

Molecular equations

  • When aqueous solutions of CaCl2 and AgNO3 are mixed, a reaction takes place producing aqueous Ca(NO3)2 and solid AgCl:
  • CaCl2(aq)+2AgNO3(aq)⟶Ca(NO3)2(aq) +2AgCl(s)
  • This balanced equation, derived in the usual fashion, is called a molecular equation
  • This is because it doesn’t explicitly represent the ionic species that are present in solution.

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?

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.

Ionic Equations

Ionic equations are used to describe reactions in solution.

Ionic solids

Ionic solids

  • When ionic compounds dissolve in water, they may dissociate into their constituent ions.
  • These ions are subsequently dispersed homogenously throughout the resulting solution.
  • Ionic compounds dissolved in water are, therefore, more realistically represented as dissociated ions, in this case:
  • E.g. NaCl(aq) ⇌ Na+(aq) + Cl−(aq)
Ionic equations

Ionic equations

  • Net 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.
  • Complete ionic equations show spectator ions.
Complete ionic equations

Complete ionic equations

  • Explicitly representing all dissolved ions results in a complete ionic equation.

  • The formulas for the dissolved ionic compounds are replaced by formulas for their dissociated ions:

  • Ca2+(aq) +2Cl-(aq)+ 2Ag+(aq)+2NO3−(aq)⟶Ca2+(aq) +2NO3−(aq)+2AgCl(s)

Spectator ions

Spectator ions

  • Examining the previous equation shows that two chemical species are present in identical form on both sides of the arrow:
  • Ca2+(aq) and NO3−(aq).
  • These are spectator ions.
  • Spectator ions presence is required to maintain charge neutrality.
  • Spectator ions are neither chemically nor physically changed by a reaction.
  • So, they may be eliminated from the equation to yield a more succinct representation called a net ionic equation.
Net ionic equations

Net ionic equations

  • The net ionic equation for the above reaction is:

  • Cl−(aq)+Ag+(aq)⟶AgCl(s)

  • This net ionic equation indicates that solid silver chloride may be produced from dissolved chloride and silver(I) ions.

  • This is regardless of the source of these ions.

  • Molecular and complete ionic equations provide additional information, namely, the ionic compounds used as sources of Cl- and Ag+.

Types of equation

Types of equation

Test yourself on Balancing Equations & Ionic Equations

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

  • 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?

  • Which of the following is the best definition of "net ionic equation"?

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