Sorting reactions by type

Now let us sort the five reactions of the exercise, one by one, applying the two questions from the previous page to each.

The problem

Separate the following reactions into chemical combination, decomposition, single displacement and double displacement.

I. 4Na + O₂ → 2Na₂O

II. 2Ag₂CO₃ → 4Ag + 2CO₂ + O₂

III. Zn + H₂SO₄ → ZnSO₄ + H₂

IV. NaCl + AgNO₃ → AgCl + NaNO₃

V. 2Na + 2H₂O → 2NaOH + H₂

The solution

I — combination. There is only one substance on the right (Na₂O), so this is a combination reaction.

II — decomposition. There is only one substance on the left (Ag₂CO₃), so this is a decomposition reaction.

III — single displacement. Zn stands alone on the left and H₂ alone on the right, so this is a single displacement reaction.

IV — double displacement. Both sides hold compounds only; Na and Ag have traded places. So this is a double displacement reaction.

V — single displacement. Na on the left and H₂ on the right are both lone elements. The sodium has displaced the hydrogen in the water, so this is a single displacement reaction.

Watch out for V

V has two reactants and two products, so it is tempting to call it a double displacement. But the deciding sign is not the count — it is whether a lone element is present, and here both Na and H₂ are.