Acids, Bases and Neutralisation Revision Notes for OCR A-Level Chemistry A

Acids, Bases and Neutralisation

Understanding acids

What are acids?

Hydrogen is present in every acid molecule. When an acid dissolves in water, it releases hydrogen ions (also called protons, with the symbol H⁺) into the solution. This release of hydrogen ions is what gives acids their characteristic properties.

For example, when hydrogen chloride gas dissolves in water, it releases hydrogen ions:

$\text{HCl(g)} + \text{aq} \rightarrow \text{H}^+\text{(aq)} + \text{Cl}^-\text{(aq)}$

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The notation "+ aq" indicates that excess water is present, showing the acid is forming an aqueous solution. This is a common way to represent the dissolving process in water.

Strong acids and complete dissociation

A strong acid releases all of its hydrogen atoms into solution as H⁺ ions when dissolved in water. This process is called complete dissociation. Common strong acids include hydrochloric acid, sulfuric acid, and nitric acid.

For example, hydrochloric acid completely dissociates in water:

$\text{HCl(aq)} \rightarrow \text{H}^+\text{(aq)} + \text{Cl}^-\text{(aq)}$

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The single arrow (→) indicates the reaction goes to completion, with all HCl molecules breaking apart into ions. This is different from the equilibrium arrow (⇌) used for reversible reactions.

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Recognizing Strong Acids

You need to memorize the three common strong acids:

Hydrochloric acid (HCl)
Sulfuric acid (H₂SO₄)
Nitric acid (HNO₃)

If an acid isn't one of these three, it's likely a weak acid!

Weak acids and partial dissociation

A weak acid only releases a small proportion of its available hydrogen atoms into solution as H⁺ ions. This is called partial dissociation. Ethanoic acid (found in vinegar) is a typical weak acid.

$\text{CH}_3\text{COOH(aq)} \rightleftharpoons \text{H}^+\text{(aq)} + \text{CH}_3\text{COO}^-\text{(aq)}$

The equilibrium sign (⇌) shows that the forward reaction is incomplete - only about one molecule in every hundred dissociates. Most organic acids, like ethanoic acid, are weak acids.

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Common Misconception About Acidic Hydrogens

Not all hydrogen atoms in a molecule necessarily act as acidic hydrogens. In ethanoic acid (CH₃COOH), there are four hydrogen atoms, but only the hydrogen attached to the COOH group is released as H⁺. The three hydrogens in the CH₃ group remain bonded and are not acidic.

Common acids you need to know

Acid	Formula
Hydrochloric acid	HCl
Sulfuric acid	H₂SO₄
Nitric acid	HNO₃
Ethanoic acid (vinegar)	CH₃COOH

Understanding bases and alkalis

What are bases?

Substances classified as bases include metal oxides, metal hydroxides, metal carbonates, and ammonia (NH₃). The key property of a base is that it neutralises an acid to form a salt.

Metal oxides	Metal carbonates	Alkalis
MgO	Na₂CO₃	NaOH
CaO	CaCO₃	KOH
CuO	CuCO₃	NH₃

What makes an alkali special?

An alkali is a specific type of base that dissolves in water, releasing hydroxide ions (OH⁻) into the solution. Not all bases are alkalis - for example, copper oxide is a base but not an alkali because it doesn't dissolve in water.

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Key Distinction: Base vs Alkali

All alkalis are bases, but not all bases are alkalis. Think of alkalis as a special subcategory of bases - they're the bases that dissolve in water to produce OH⁻ ions. Metal oxides like CuO are bases but not alkalis because they don't dissolve in water.

Sodium hydroxide dissolving in water demonstrates alkali behavior:

$\text{NaOH(s)} + \text{aq} \rightarrow \text{Na}^+\text{(aq)} + \text{OH}^-\text{(aq)}$

Common alkalis you should know include sodium hydroxide (NaOH), potassium hydroxide (KOH), and ammonia (NH₃).

Neutralisation reactions

The basic concept of neutralisation

Neutralisation occurs when H⁺ ions from an acid react with a base. The products formed are a salt and neutral water. During this process, the H⁺ ions from the acid are replaced by metal ions or ammonium ions from the base to form the salt.

Neutralising acids with metal oxides and hydroxides

When an acid reacts with a metal oxide or metal hydroxide, the products are a salt and water only. The general pattern is:

$\text{acid} + \text{metal oxide/hydroxide} \rightarrow \text{salt} + \text{water}$

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Worked Example: Copper(II) Oxide with Acids

When copper(II) oxide reacts with sulfuric acid:

$\text{CuO(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{CuSO}_4\text{(aq)} + \text{H}_2\text{O(l)}$

When copper(II) oxide reacts with hydrochloric acid:

$\text{CuO(s)} + 2\text{HCl(aq)} \rightarrow \text{CuCl}_2\text{(aq)} + \text{H}_2\text{O(l)}$

Notice both reactions follow the same pattern: salt + water are the only products.

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Exam Tip

The reactions for neutralising an acid with a metal oxide or hydroxide are essentially the same pattern - they always produce salt plus water. Remember this simple rule and you can write the equation for any such reaction!

Neutralising acids with alkalis

When working with alkalis, the reactants are already in solution. As with metal oxides, the overall reaction produces a salt and water only:

$\text{acid} + \text{alkali} \rightarrow \text{salt} + \text{water}$

The full equation for hydrochloric acid with sodium hydroxide is:

$\text{HCl(aq)} + \text{NaOH(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)}$

The ionic equation is much simpler and shows the essence of neutralisation - H⁺ ions combining with OH⁻ ions to form neutral water:

$\text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{H}_2\text{O(l)}$

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Universal Ionic Equation

This ionic equation applies to all acid-alkali neutralisation reactions, regardless of which specific acid or alkali you're using. It captures the fundamental process: hydrogen ions and hydroxide ions combine to form water.

Neutralising acids with carbonates

Metal carbonates also neutralise acids to form a salt and water, but there is an additional product - carbon dioxide gas. The general pattern is:

$\text{acid} + \text{metal carbonate} \rightarrow \text{salt} + \text{water} + \text{carbon dioxide}$

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Worked Example: Carbonate Reactions

Zinc carbonate with sulfuric acid:

$\text{ZnCO}_3\text{(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{ZnSO}_4\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$

Magnesium carbonate with hydrochloric acid:

$\text{MgCO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$

The key difference from oxide/hydroxide reactions: carbon dioxide gas is also produced.

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Exam Tip

Remember the equation pattern for carbonate neutralisation - it will help you write the equation for any neutralisation involving a metal carbonate. Just add CO₂(g) to the products compared to the oxide/hydroxide pattern!

Understanding salt formation and naming

The salt formed in neutralisation takes its name from two sources:

The metal (or ammonium) comes from the base
The rest of the salt name comes from the acid

Acid	Salt type	Example salt
Hydrochloric acid (HCl)	Chloride	Sodium chloride (NaCl)
Sulfuric acid (H₂SO₄)	Sulfate	Sodium sulfate (Na₂SO₄)
Nitric acid (HNO₃)	Nitrate	Calcium nitrate (Ca(NO₃)₂)
Ethanoic acid (CH₃COOH)	Ethanoate	Ammonium ethanoate (CH₃COONH₄)

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Pattern for Salt Formation

Notice the pattern: to form the salt, the hydrogen shown in the acid formula is replaced by a metal or ammonium ion.

For example:

H in HCl is replaced by Na to give NaCl
H₂ in H₂SO₄ is replaced by Na₂ to give Na₂SO₄

This helps you predict the formula of any salt formed in a neutralisation reaction!

Special case: sulfuric acid dissociation

Sulfuric acid (H₂SO₄) behaves differently from other acids because it contains two hydrogen atoms. It dissociates in two stages:

First dissociation (complete - strong acid behavior):

$\text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{H}^+\text{(aq)} + \text{HSO}_4^-\text{(aq)}$

This stage releases just one of the two hydrogen atoms as H⁺. This dissociation is complete, so sulfuric acid behaves as a strong acid at this stage.

Second dissociation (partial - weak acid behavior):

$\text{HSO}_4^-\text{(aq)} \rightleftharpoons \text{H}^+\text{(aq)} + \text{SO}_4^{2-}\text{(aq)}$

The hydrogen sulfate ion (HSO₄⁻) formed in the first step only partially dissociates, behaving as a weak acid.

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Two-Stage Behavior

This two-stage behavior is important to understand:

Sulfuric acid first acts as a strong acid (complete dissociation)
The HSO₄⁻ ions then act as a weak acid (partial dissociation)

Other strong acids containing more than one hydrogen atom show similar behavior.

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Key Points to Remember:

Acids release H⁺ ions when dissolved in water. Strong acids completely dissociate; weak acids only partially dissociate.
Common strong acids to memorize: HCl (hydrochloric), H₂SO₄ (sulfuric), and HNO₃ (nitric). Ethanoic acid (CH₃COOH) is a weak acid.
Bases include metal oxides, metal hydroxides, metal carbonates, and ammonia. Alkalis are bases that dissolve in water to release OH⁻ ions.
Neutralisation patterns to learn:
- Acid + metal oxide/hydroxide → salt + water
- Acid + carbonate → salt + water + carbon dioxide
- The ionic equation for acid-alkali reactions: H⁺(aq) + OH⁻(aq) → H₂O(l)
Salt naming: The salt name comes from the metal (or ammonium) in the base and the acid type (chloride from HCl, sulfate from H₂SO₄, nitrate from HNO₃, ethanoate from CH₃COOH).

Acids, Bases and Neutralisation (OCR A-Level Chemistry A): Revision Notes