4.1 - Acids, Bases and Neutralisation
- 1The definitions of acids and bases
- 2The difference between strong and weak acids and bases
- 3Neutralisation reactions between acids and bases
- 4Reactions of acids with metals, metal compounds and ammonia
Acids are proton donors, bases are proton acceptors
An acid is defined as a substance that donates protons (H^+^) when dissolved in water. These protons combine with water molecules to form hydronium ions (H3O^+^), indicating the acidic nature of the solution.
Examples of common acids include:
- Hydrochloric acid (HCl)
- Sulfuric acid (H_2_SO_4_)
- Nitric acid (HNO_3_)
- Ethanoic acid (CH_3_COOH)
A base, in contrast, is a substance that accepts protons. When a base dissolves in water, it can produce hydroxide ions (OH^-^), making the solution alkaline. Bases that dissolve in water are specifically called alkalis.
Examples of common alkalis include:
- Sodium hydroxide (NaOH)
- Potassium hydroxide (KOH)
- Aqueous ammonia (NH_3_ in H_2_O)
Aqueous ammonia is a special case; it doesn’t produce hydroxide ions directly but accepts protons from water to form ammonium (NH_4_^+^) and hydroxide ions (OH^-^):
NH_3_ + H_2_O ➔ NH_4_^+^ + OH^-^
Strength depends on extent of dissociation
The strength of an acid or a base is determined by its ability to dissociate in water. This dissociation process can be reversible, as shown in the equations below:
- Acid dissociation: HA + H_2_O ⇌ H_3_O^+^ + A^-^
- Base dissociation: B + H_2_O ⇌ BH^+^ + OH^-^
Strong acids and bases:
Strong acids, such as hydrochloric acid (HCl), and strong bases, like sodium hydroxide (NaOH), dissociate completely in water. This results in a significant release of H^+^ and OH^-^ ions, respectively, with the forward reaction being predominantly favoured:
- HCl_(aq)_ ➔ H^+^(aq) + Cl^-^(aq)
- NaOH_(aq)_ ➔ Na^+^(aq) + OH^-^(aq)
Weak acids and bases:
Conversely, weak acids and bases, like ethanoic acid (CH_3_COOH) and ammonia (NH_3_), only partially dissociate in water, releasing fewer H^+^ and OH^-^ ions. In these cases, the reverse reaction is favoured:
- CH_3_COOH_(aq)_ ⇌ H^+^(aq) + CH_3_COO^-^(aq)
- NH_3(aq)_ + H_2_O_(l)_ ⇌ NH_4_^+^(aq) + OH^-^(aq)
Neutralisation produces salt and water
Neutralisation is the reaction between an acid and an alkali, resulting in the formation of water and a salt. This reaction specifically involves the combination of H^+^ ions from the acid with OH^-^ ions from the alkali to form water (H_2_O).
The general ionic equation for this process is:
H^+^(aq) + OH^-^(aq) ➔ H_2_O_(l)_
A salt is an ionic compound formed when the H^+^ ions in an acid are replaced by metal ions or other positive ions, such as ammonium ions (NH_4_^+^). In a neutralisation reaction, the salt is comprised of the cation from the base, which can be a metal or ammonium ion, and the anion from the acid.
The type of salt produced depends on the specific acid used in the reaction:
| Acid | Anion in salt |
|---|---|
| Sulfuric acid (H_2_SO_4_) | Sulfate (SO_4_^2-^) |
| Hydrochloric acid (HCl) | Chloride (Cl^-^) |
| Nitric acid (HNO_3_) | Nitrate (NO_3_^-^) |
For instance, the reaction between hydrochloric acid and sodium hydroxide yields sodium chloride and water:
HCl_(aq)_ + NaOH_(aq)_ ➔ NaCl_(aq)_ + H_2_O_(l)_
Acids react with metals and metal compounds
- Acids and reactive metals - Acids react with certain metals, producing a salt and hydrogen gas: Metal + acid ➔ salt + hydrogen
For example, calcium reacting with sulfuric acid: Ca_(s)_ + H_2_SO_4(aq)_ ➔ CaSO_4(aq)_ + H_2(g)_
- Acids and metal oxides - These reactions yield a salt and water: Metal oxide + acid ➔ salt + water
An example is zinc oxide with hydrochloric acid: ZnO_(s)_ + 2HCl_(aq)_ ➔ ZnCl_2(aq)_ + H_2_O_(l)_
- Acids and metal hydroxides - Similar to oxides, these reactions produce a salt and water: Metal hydroxide + acid ➔ salt + water
For example, potassium hydroxide reacting with nitric acid: KOH_(aq)_ + HNO_3(aq)_ ➔ KNO_3(aq)_ + H_2_O_(l)_
- Acids and metal carbonates - These reactions produce a salt, water, and carbon dioxide: Metal carbonate + acid ➔ salt + water + carbon dioxide
An example is sodium carbonate with hydrochloric acid: Na_2_CO_3(s)_ + 2HCl_(aq)_ ➔ 2NaCl_(aq)_ + H_2_O_(l)_ + CO_2(g)_
Ammonia reacts with acids to form ammonium salts
Ammonia interacts with acids to form ammonium salts, exemplified by a proton transfer from the acid to the ammonia:
Ammonia + acid ➔ ammonium salt
For instance, ammonia reacting with sulfuric acid produces ammonium sulfate:
2NH_3(aq)_ + H_2_SO_4(aq)_ ➔ (NH_4_)2_SO_4(aq)
The ionic equation highlights the proton transfer process:
NH_3(aq)_ + H^+^(aq) ➔ NH_4_^+^(aq)