Let's first start with a complete chemical equation and see how the net ionic equation is derived. Take for example the reaction of lead(II) nitrate with sodium chloride to form lead(II) chloride and sodium nitrate, shown below:

Pb(NO3)2(aq) + 2 NaCl(aq) --> PbCl2(s) + 2 NaNO3(aq)

This complete equation may be rewritten in ionic form by using the solubility rules. Lead(II) nitrate is soluble and therefore dissociated. The same for NaCl. As products, sodium nitrate is predicted to be soluble and will be dissociated. The lead(II) chloride, however, is insoluble. The above equation written in dissociated form is:

Pb2+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq) --> PbCl2(s) + Na+(aq) + NO3-(aq)

At this point, one may cancel out those ions which have not participated in the reaction. Notice how the nitrate ions and sodium ions remain unchanged on both sides of the reaction.

Pb2+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq) --> PbCl2(s) + Na+(aq) + NO3-(aq)

What remains is the net ionic equation, showing only those chemical species participating in a chemical process:

Pb2+(aq) + 2 Cl-(aq) --> PbCl2(s)

Practice Problems and additional help/information

Diatomic Molecules must always be written as 2 atoms of the element:

H2, Cl2, F2, I2, N2, Br2, O2

There are five basic types of reactions:

Helpful Hints:

  • Remember, a correctly written equation must have all the correct formulas and it must be balanced.

  • Always make sure that you have the oxidation numbers of the elements within a compound balanced to zero.

  • If an element has more than one oxidation number, the state in which it exsts as a reactant is the same state it will exist as a reactant unless otherwise stated.

  • Reactants ----> Products

  • 1. Decomposition Reaction

    compound ---> 2 or more substances
    H2CO3 ---> CO2 + H2O

    Ba(OH)2 ---> BaO + H2O

    MgCO3 ---> MgO + CO2

    NaClO3 ---> NaCl + O2

    MgO ---> Mg + O2

    NaCl ---> Na + Cl2

    Sample Problems

    2. Synthesis Reaction

    element/compound + element/compound ---> new compound

    Na + Cl2 ---> NaCl

    SO2 + H2O ---> H2SO3

    BaO + H2O ---> Ba(OH)2

    Sample Problems

    3. Single Replacement Reaction

    Na + Zn(NO3)2 ---> NaNO3 + Zn

    Al + Pb(NO3)2 ---> Al(NO3)3 + Pb

    Al + H2SO4 ---> ZnSO4 + H2

    Cl2 + NaBr ---> NaCl + Br2
    Sample Problems

    4. Double Replacement Reaction

    compound + compound ---> new compound + new compound

    KOH + H2SO4 ---> K2SO4 + H2O
      AgNO3 + NaCl ---> AgCl + NaNO3
      Ca(OH)2 + HCl ---> CaCl2 + H2O
    Sample Problems

    5. Combustion Reaction

    CH2 + O2(exc) ---> CO2 + H2O
    C6H6 + O2(lim) ---> CO + H2O

    (NH4)2CO3 + HCl --> NH4Cl + H2CO3

    carbonic acid will continue to decompose to form:

    (NH4)2CO3 + HCl ---> NH4Cl + H2O + CO2
    (NH4)2CO3 + NaOH ---> Na2CO3 + NH4OH

    ammonium hydroxide will continue to decompose:

    (NH4)2CO3 + NaOH ---> Na2CO3 + NH3 + H2O

    Extra Problems

    Write the correct formulas, equation, and balance the equation: