Chemical Equations

Chemical reaction (chemical change) is a process by which one or more substances change into new substance.

Substances which take part in a chemical reaction are called reactants.

The new substances produced as a result of chemical reaction are called products.

Chemical equation is the short way of describing a chemical reaction by means of symbols and formulae.


In a chemical equation
A + B →C + D
A and B are reactants whereas C and D are products.

Types of Chemical Equations

There are three types of chemical equations which are

Word equations

Word equation is an equation which states the substances involved in a chemical reaction in words.
Example Copper + oxygen → copper (II) oxide.
Lead (II) oxide + hydrogen → Lead + water

Molecular or Formula equations

Molecular equations are the chemical equations which use chemical symbols and formulae of the substances involved.
Example
Cu + O2 → CuO
PbO + H2 →Pb + H2O

Balancing Chemical Equations

Balancing chemical equation is the process of making the number of different types of atoms equal on both side of an equation.

A balanced equation contains the same number of each kind of atom involved in the chemical reaction on each side of the equation, that is, before and after the arrow in an equation.

Rules for writing a balanced Chemical equation

  1. Write the formula or chemical symbol of the reactants on the left hand side and products on the right hand side.
    Zn + HCl →ZnCl2 + H2
  2. Balance the equation by writing small whole numbers in front of one or more formulae, if necessary.
    Zn + 2HCl → ZnCl2 + H2
  3. Check the number of the atoms of each element (or compounds) on both sides of the equation if they are equal.
Worked Examples Balance the following chemical equations
2Na + 2HCl→2NaCl + H2
2NaOH + H2SO4 →Na2SO4 +2H2O
CaCO3 + 2HCl → CaCl2 + H2O +CO2

Indicating the physical states of reactants and products in the chemical equation.

Reactants and products may be solids, liquids, gases or solution. These states must be indicated in the chemical equation. These states are usually abbreviated as follows: NOTE
Some substances can be produced in the chemical reaction and they are insoluble in water. The substances are said to be precipitates and denoted by the same symbol (s). The soluble and insoluble substance can be determined by Solubility rule. The states always are indicated after the symbol or formula of the named substance and are bracketed.

Solubility Rule

Is the simple rule which shows what substances are soluble in water and which one are not soluble in water.
Soluble Insoluble
All compounds of sodium, potassium and ammonium.None
All nitrates (NO3-) None
All chlorates (ClO3-) None
All hydrogencarbonates None
All acetates (ethanoates) (CH3COO⁻)None
All chlorides, bromides and iodides EXCEPT→ Those of silver and lead (II)
All sulphates EXCEPT→ Those of lead, barium and calcium (sparingly soluble)
Carbonates of sodium, potassium and ammonium EXCEPT→All other carbonates
Hydroxide of sodium, ammonium, barium and potassium EXCEPT→ All other hydroxides
Oxides of calcium, barium, sodium, potassium EXCEPT→ All other oxides.
Sulphites and phosphates of ammonium, sodium and potassium All other sulphites and phosphates

Examples
Indicate the physical state of the following equation
Mg + H2SO4 →MgSO4 + H2
Solution

Mg(s) + H2SO4 (aq) →MgSO4(aq) + H2(g)

Ionic Equations

Ionic equation is an equation which shows only the actual ions taking part in a chemical reaction.

Spectator ion is ion that exists in the same form on both the reactants and products sides of a chemical equation.

Rules for Writing Ionic Equation.

  1. Write the molecular equation of the compounds involved in the reaction.
  2. Balance the chemical equation by indicating their physical states
  3. Separate the compounds reacting or produced into their component ions. Liquids, solids and gaseous compounds are not separated into ions.
  4. Cancel the spectator ions.

Example Write the ionic equation for the following chemical equations
H2SO4 + Na2CO3 →Na2SO4 + H2O + CO2
soln
  1. H2SO4 (aq) + Na2CO3(aq) →Na2SO4 (aq)+ H2O(l) + CO2(g)
  2. 2H+(aq) + SO 2-4 (aq) + 2Na+(aq) + CO2+3(aq) → 2Na+(aq) + SO 42- (aq) + H2O(l) + CO2(g)
  3. 2H+(aq) + SO 2-4 (aq) + 2Na+(aq) + CO2+3(aq)→ 2Na+(aq) + SO 2-4 (aq) + H2O(l) + CO2(g)
  4. 2H+(aq) + CO2+3(aq) → H2O(l) + CO2(g)
Types of chemical reactions
  1. Combination reaction (synthesis reaction)
    2. This is the reaction in which two or more reactants combine to give a single product.
    A + B → AB
    The substances A and B could be either elements or compounds.
    eg. Fe + S →FeS
    CO2 + H2O → H2CO3
  2. Decomposition reaction
    3. This is the reaction whereby a single substance breaks down into two or more simple substances. Sometimes this reaction takes place under the presence of heat.
    AB → A + B
    AB must be a compound while products A and B can be either elements or compounds.
    2HgO → 2Hg + O2
    2KClO3 → 2KCl + 3O2
  3. Displacement reaction
    4. It is the reaction in which one reactive element replaces less reactive one.
    A + AB → AC + B
    A is more reactive than B.
    Mg + CuSO4 →MgSO4 + Cu
  4. Double decomposition reaction
    5. Two aqueous compounds exchnage their constituents or ions to produce two distinct compounds.
    AB + CD → AD + CB
    HNO3 + NaOH → NaNO3 + H2O
  5. Precipitation reaction
    6. In this reaction an insoluble compound is formed when the solutions are mixed. This means that one of the product is solid.
    Precipitate is the solid that forms in a solution during a chemical reaction.
    KCl(aq) + AgNO3(aq)→AgCl(s) + KNO3(aq)
  6. Redox reaction
    7. Where one substance is oxidized and one is reduced. Example of redox reaction are
  7. Combustion reaction.
    8. When a substance combines with oxygen to form a product.
    CH4 + 2O2 → CO2 + 2H2O

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