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