Reactions with Oxygen, Acids and Bases: Mechanism

Mechanism

Follow the mechanism step by step and distinguish what causes the change from what is merely observed.

This extension applies that lens specifically to Reactions with Oxygen, Acids and Bases.

Four families of reaction cover most of what you meet in Grade 9.

1. Metal $+$ oxygen $\rightarrow$ metal oxide. Magnesium burns with a blinding white flame: $2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$. Iron does the same thing slowly, and we call it rust. Metal oxides that dissolve in water make the solution basic (pH above 7).

2. Non-metal $+$ oxygen $\rightarrow$ non-metal oxide. Carbon burns to carbon dioxide, $\text{C} + \text{O}_2 \rightarrow \text{CO}_2$; sulfur burns to sulfur dioxide, $\text{S} + \text{O}_2 \rightarrow \text{SO}_2$. Non-metal oxides dissolve in water to make the solution acidic (pH below 7). This is exactly how acid rain forms downwind of coal-fired power stations on the Mpumalanga Highveld: sulfur in the coal burns to $\text{SO}_2$, which dissolves in cloud droplets.

3. Metal $+$ acid $\rightarrow$ salt $+$ hydrogen gas. Drop zinc into hydrochloric acid and it fizzes: $\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2$. Hold a burning splint at the mouth of the test tube and the escaping hydrogen makes a squeaky "pop" — the standard test for hydrogen.

4. Acid $+$ base $\rightarrow$ salt $+$ water. This is neutralisation: $\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}$. An antacid tablet neutralises excess stomach acid; a farmer spreads lime on acidic soil to bring the pH back towards 7.

Worked example — write the word equation for magnesium and hydrochloric acid.

This is family 3: metal $+$ acid. The salt is named from the metal and what remains of the acid (hydrochloric acid gives chlorides):

The pH scale runs from 0 to 14: below 7 is acidic, exactly 7 is neutral, above 7 is basic.