Acid–Base Reactions (Grade 11 NSC Matric Physical Sciences): Revision Notes

Acid–Base Reactions

What are acid-base reactions?

Acid-base reactions occur when an acid reacts with a base. These reactions are also called neutralisation reactions because the acid and base neutralise each other's properties. When an acid and base react together, they typically form a salt and water.

Conjugate acid-base pairs

When acids and bases react, they form conjugate acid-base pairs. These are pairs of compounds that are related by the transfer of a proton (H⁺ ion). In any acid-base reaction, you can identify these pairs by looking at which species donate and accept protons.

For example, in the reaction: $\text{H}_2\text{SO}_4(aq) + \text{H}_2\text{O}(l) \rightarrow \text{H}_3\text{O}^+(aq) + \text{HSO}_4^-(aq)$

• $\text{H}_2\text{SO}_4$ and $\text{HSO}_4^-$ form one conjugate pair
• $\text{H}_2\text{O}$ and $\text{H}_3\text{O}^+$ form another conjugate pair

Types of acid-base reactions

There are three main types of acid-base reactions, depending on what type of base is used. In each case, the products formed follow predictable patterns.

Acid + metal hydroxide

When an acid reacts with a metal hydroxide, the products are always a salt and water.

General equation: $n\text{H}^+(aq) + \text{M}(\text{OH})_n(aq) \rightarrow n\text{H}_2\text{O}(l) + \text{M}^{n+}(aq)$

Where $n$ is the group number of the metal and $\text{M}$ represents the metal.

Acid + metal oxide

When an acid reacts with a metal oxide, the products are also a salt and water.

General equation: $2y\text{H}^+(aq) + \text{M}_x\text{O}_y(aq) \rightarrow y\text{H}_2\text{O}(l) + x\text{M}^{n+}(aq)$

Where $n$ is the group number of the metal, and $x$ and $y$ represent the ratio in which the metal combines with oxygen based on the metal's valency.

Acid + metal carbonate

When an acid reacts with a metal carbonate, the products are a salt, carbon dioxide gas, and water.

Practical applications of acid-base reactions

Acid-base reactions have numerous practical applications in our daily lives, from agriculture to medicine to environmental protection.

Domestic uses

• Soil treatment: Calcium oxide (lime) is spread on acidic soil to neutralise excess acidity, making soil more suitable for plant growth
• Limestone in agriculture: Used in pit latrines and farming to neutralise acidic waste and improve soil quality

Biological uses

• Stomach problems: When there's too much hydrochloric acid in the stomach, antacids (which are bases) are taken to neutralise the excess acid
• Common antacids: Aluminium hydroxide, magnesium hydroxide ("milk of magnesia"), and sodium bicarbonate ("bicarbonate of soda")
• Bee stings: These are acidic and can be soothed using basic substances like bicarbonate of soda

Industrial uses

• Pollution control: Calcium hydroxide (limewater) is used to absorb harmful acidic $\text{SO}_2$ gas released from power stations and fossil fuel burning

Laboratory investigations

General experiment: acid-base reactions

Using natural indicators

Many household substances can act as indicators - chemicals that change colour depending on whether they're in acidic or basic conditions.

Natural indicators include:

• Red cabbage
• Beetroot
• Berries
• Curry powder
• Tea
• Red grapes
• Onions (by smell)
• Vanilla essence (by smell)
• Baking powder

Testing for carbon dioxide

When acids react with carbonates, carbon dioxide gas is produced. This can be tested using limewater:

Method:

1. Set up apparatus with delivery tube connecting the reaction vessel to limewater
2. Add acid to metal carbonate
3. Observe the limewater - it turns milky when $\text{CO}_2$ is present

Exam tips and common mistakes