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4.3Chemical Reactions

Representations of Reactions — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 7 parts Lesson 6 of 12 1 quiz

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Lesson summary

4.3 Representations of Reactions — Notes — AP CHEMISTRY notes: Translate balanced equations into particulate diagrams and balance equations correctly.Symbolically by balanced equations.Particulately by particle diagrams.Macroscopically by observations.A balanced equation obeys co…

  • Coefficient
  • Limiting reactant
  • Excess reactant
  • A balanced equation obeys conservation of mass, with the same number of each atom on both sides
  • Subscripts are never changed when balancing
  • Write correct formulae for reactants and products

Section 1 of 7Ways to represent a reaction

Ways to represent a reaction
Symbolically by balanced equations.
Particulately by particle diagrams.
Macroscopically by observations.

Section 2 of 7Balancing rules

Balancing rules
A balanced equation obeys conservation of mass, with the same number of each atom on both sides.
Coefficients are the lowest whole-number ratio and give ratios of particles and moles, not masses.
Subscripts are never changed when balancing.
1
Write correct formulae for reactants and products.
2
Balance atoms in the most complex substances first.
3
Balance free elements next by adjusting their coefficients.
4
Clear fractions by multiplying all coefficients by the denominator.
5
Check that every element balances.

Section 3 of 7Particle diagrams

Particle diagrams
The number of each type of atom in the product diagram must equal that in the reactant diagram, including leftover excess reactant.
Phases must be shown correctly: gas molecules widely spaced, solids closely packed, and aqueous ions separated among water.
Diatomic elements H2, N2, O2, F2, Cl2, Br2 and I2 are drawn as pairs of atoms.

Section 4 of 7Limiting reactant in diagrams

Limiting reactant in diagrams
The limiting reactant is completely used up and determines the amount of product.
The other reactant remains in excess and is shown unreacted in the product diagram.
Before (2H2 + O2 → 2H2O)H2 afterO2 afterH2O formedLimiting reactant
4 H2, 2 O2004Neither
4 H2, 3 O2014H2
6 H2, 2 O2204O2

Section 5 of 7More from the specification

More from the specification
Covalent bonds in aqueous solution
  • Covalent bonds in aqueous solution molecular substances held by shared electron covalent bonds dissolve in solution without undergoing electrolytic dissociation.
Composition
  • What a substance or mixture is made of.
Ionic equation
  • An equation showing only the ions that react.
Physical change
  • A change in which no new substance is formed.
Covalent bond
  • A shared pair of electrons between two atoms, giving noble gas configurations.
Hydrogen
  • A gas that gives a squeaky pop with a lighted splint.
Oxygen
  • A gas that relights a glowing splint.
Mixture
  • Two or more substances mixed but not chemically combined.
Matter
  • Everything you can see and feel; it exists as solid, liquid or gas.

Section 6 of 7Key terms

TermMeaning
Balanced equationEquation with equal numbers of each atom on both sides.
CoefficientNumber in front of a formula showing the particle or mole ratio.
SubscriptNumber within a formula showing atoms in one unit.
Particle diagramDrawing representing individual atoms, molecules or ions.
Limiting reactantReactant used up first.
Excess reactantReactant left over after the reaction.

Section 7 of 7Exam technique

1
Changing subscripts instead of coefficients when balancing
2
Drawing diatomic elements as single atoms
3
Losing atoms between the reactant and product diagrams
4
Reading coefficients as mass ratios

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