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Gas Forming Reactions

Sometimes a gas will be involved as one of the reactants or products in a solution reaction. For example,

2 HCl(aq) + Na2S(aq) H2S(gas) + 2 NaCl(aq)

In this example the complete ionic equation would be:

2 H+(aq) + 2Cl-(aq) + 2Na+(aq) + S2-(aq) H2S(g) + 2Na+(aq) + 2Cl-(aq)

Removing the spectator ions we obtain the net ionic equation:

2 H+(aq) + S2-(aq) H2S(g)

H2S is just one example of a gaseous substance that can form in a solution reaction. Another way gases can form in solution is through the decomposition of weak electrolytes. For example, H2CO3 readily decomposes into H2O and CO2 gas,

H2CO3(aq) H2O(l) + CO2(g)

So, any solution reaction that leads to the production of H2CO3, such as

HCl(aq) + NaHCO3(aq) NaCl(aq) + H2CO3(aq)

will ultimately lead to the production of H2O and CO2 gas as the H2CO3 decomposes, giving us a new equation:

HCl(aq) + NaHCO3(aq) NaCl(aq) + H2O(l) + CO2(g)

If we eliminate all the spectator ions we would write the net ionic equation as

H+(aq) + HCO3-(aq) H2O(l) + CO2(g)

Two other substances that will decompose and form gases are H2SO3 and NH4OH:

H2SO3(aq) H2O(l) + SO2(g)

NH4OH(aq) H2O(l) + NH3(g)

Homework from Chemisty, The Central Science, 10th Ed.

4.19, 4.21, 4.23, 4.25, 4.27, 4.39, 4.41, 4.43, 4.45, 4.47, 4.49, 4.51, 4.53, 4.55, 4.57