2.1.4a-c - Acids, bases, alkalis and neutralisation

2.1.4a-c - Acids, bases, alkalis and neutralisation

Acid-base chemistry starts with a simple particle idea: acids produce hydrogen ions in water, and alkalis produce hydroxide ions in water. In this lesson you will learn the common formulae, how to compare strong and weak acids qualitatively, and how to write balanced neutralisation equations for salts.

Common acids, bases and alkalis

An acid is a substance that releases H+ ions when it is in aqueous solution. Aqueous solution means the substance is dissolved in water, shown by the state symbol (aq).

Acid

For this lesson, an acid is a substance that releases H+ ions in aqueous solution.

The common acid formulae you must recognise are:

acidformulaion left after H+ is released
hydrochloric acidHClCl-
sulfuric acidH2SO4SO4^2-
nitric acidHNO3NO3-
ethanoic acidCH3COOHCH3COO-

The ion left after the acid releases H+ helps you name the salt. Hydrochloric acid forms chlorides, sulfuric acid forms sulfates, nitric acid forms nitrates, and ethanoic acid forms ethanoates.

An alkali is a water-soluble base that releases OH- ions in aqueous solution. Sodium hydroxide and potassium hydroxide contain OH- ions in their formulae, so their aqueous dissociation is direct:

NaOH(aq)→Na+(aq)+OH−(aq)\mathrm{NaOH(aq) \rightarrow Na^+(aq) + OH^-(aq)} KOH(aq)→K+(aq)+OH−(aq)\mathrm{KOH(aq) \rightarrow K^+(aq) + OH^-(aq)}

Alkali

An alkali is a water-soluble base that releases OH- ions in aqueous solution.

The common alkalis are:

alkaliformulauseful ion or product in acid reactions
sodium hydroxideNaOHNa+ and OH-
potassium hydroxideKOHK+ and OH-
ammoniaNH3forms NH4+ in ammonium salts

Ammonia is the unusual one in this list because its formula does not contain OH. In water, some ammonia molecules react with water to produce ammonium ions and hydroxide ions:

NH3(aq)+H2O(l)⇌NH4+(aq)+OH−(aq)\mathrm{NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)}

So the formula to remember is NH3, not NH4OH.

Worked example: connecting formulae to ions

Hydrochloric acid has formula HCl. In aqueous solution, it releases H+ and leaves Cl-. Sodium hydroxide has formula NaOH. In aqueous solution, it gives Na+ and OH-. If the acid and alkali react, H+ and OH- form water, while Na+ and Cl- form the salt sodium chloride.

Strong and weak acids

The strength of an acid is about how far its molecules or formula units dissociate in water. It is not about how concentrated the solution is.

Strong acid

A strong acid dissociates completely, or almost completely, in aqueous solution so that nearly all acid particles release H+ ions.

Hydrochloric acid is treated as a strong acid:

HCl(aq)→H+(aq)+Cl−(aq)\mathrm{HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)}

The single arrow shows that, for this qualitative model, the dissociation goes essentially to completion.

Weak acid

A weak acid only partially dissociates in aqueous solution, so the solution contains mostly undissociated acid particles and a smaller amount of H+ and negative ions.

Ethanoic acid is a weak acid:

CH3COOH(aq)⇌H+(aq)+CH3COO−(aq)\mathrm{CH_3COOH(aq) \rightleftharpoons H^+(aq) + CH_3COO^-(aq)}

The reversible arrow shows that dissociation is incomplete. At any moment, there is a mixture of CH3COOH molecules, H+ ions and CH3COO- ions.

Worked example: choosing the correct language

A student says, "Ethanoic acid is weak because there is not much acid in the bottle." That is not the right reason. A weak acid could be concentrated or dilute. Ethanoic acid is weak because only a small proportion of CH3COOH particles dissociate into H+ and CH3COO- in water.

Another student says, "Hydrochloric acid is strong because it dissociates completely in water." That is the correct idea for this boundary.

Strong versus weak describes relative dissociation. Concentrated versus dilute describes how much solute is dissolved per unit volume.

Neutralisation by H+ and OH-

Neutralisation is the reaction in which hydrogen ions from an acid react with hydroxide ions from an alkali to form water.

Net ionic equation for neutralisation

H+(aq)+OH−(aq)→H2O(l)\mathrm{H^+(aq) + OH^-(aq) \rightarrow H_2O(l)}

This ionic equation is the core of acid-alkali neutralisation. The other ions are still important because they form the salt, but they do not change during the H+ and OH- step.

Worked example: hydrochloric acid and sodium hydroxide

Hydrochloric acid provides H+ and Cl-. Sodium hydroxide provides Na+ and OH-.

The neutralisation step is:

H+(aq)+OH−(aq)→H2O(l)\mathrm{H^+(aq) + OH^-(aq) \rightarrow H_2O(l)}

The remaining ions, Na+ and Cl-, form sodium chloride:

HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)\mathrm{HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)}

Worked example: sulfuric acid and potassium hydroxide

Sulfuric acid has formula H2SO4, so one formula unit has two acidic H atoms. Potassium hydroxide has one OH in each formula unit. You need two KOH for one H2SO4:

H2SO4(aq)+2KOH(aq)→K2SO4(aq)+2H2O(l)\mathrm{H_2SO_4(aq) + 2KOH(aq) \rightarrow K_2SO_4(aq) + 2H_2O(l)}

The salt is potassium sulfate because K+ combines with SO4^2-.

Aqueous ammonia also forms salts with acids. For example:

NH3(aq)+HCl(aq)→NH4Cl(aq)\mathrm{NH_3(aq) + HCl(aq) \rightarrow NH_4Cl(aq)}

This is still an acid-base reaction, but the useful full equation shows ammonium chloride formation rather than pretending ammonia has the formula NH4OH.

Acids with bases, carbonates and metal oxides

A base neutralises an acid. In this lesson, the important bases are alkalis, metal oxides and carbonates.

For an acid reacting with an alkali:

acid+alkali→salt+water\mathrm{acid + alkali \rightarrow salt + water}

For an acid reacting with a metal oxide:

acid+metal oxide→salt+water\mathrm{acid + metal\ oxide \rightarrow salt + water}

The oxide ion, O^2-, combines with H+ ions to form water. For example, copper(II) oxide reacts with sulfuric acid to make copper(II) sulfate and water:

CuO(s)+H2SO4(aq)→CuSO4(aq)+H2O(l)\mathrm{CuO(s) + H_2SO_4(aq) \rightarrow CuSO_4(aq) + H_2O(l)}

For an acid reacting with a carbonate:

acid+carbonate→salt+water+carbon dioxide\mathrm{acid + carbonate \rightarrow salt + water + carbon\ dioxide}

The carbonate ion needs two H+ ions overall to form water and carbon dioxide. For example:

CaCO3(s)+2HCl(aq)→CaCl2(aq)+H2O(l)+CO2(g)\mathrm{CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)}

Worked example: predicting products and balancing

Write the equation for magnesium oxide reacting with nitric acid.

  1. Identify the base: magnesium oxide, MgO.
  2. Identify the acid: nitric acid, HNO3.
  3. Name the salt from the metal ion and acid anion: magnesium nitrate, Mg(NO3)2.
  4. Add water because metal oxide + acid gives salt + water.
  5. Balance the equation:
MgO(s)+2HNO3(aq)→Mg(NO3)2(aq)+H2O(l)\mathrm{MgO(s) + 2HNO_3(aq) \rightarrow Mg(NO_3)_2(aq) + H_2O(l)}

The coefficient 2 is needed because magnesium nitrate contains two nitrate ions.

Writing salts and avoiding traps

When writing a full neutralisation equation, use a calm sequence.

  1. Write the formula of the acid and the base.
  2. Decide the salt name from the positive ion in the base and the negative ion from the acid.
  3. Write the salt formula using ion charges.
  4. Add water for alkalis and metal oxides; add water and carbon dioxide for carbonates.
  5. Balance atoms and charges by changing coefficients, not formulae.

Worked example: barium hydroxide and hydrochloric acid

Barium hydroxide is Ba(OH)2 and hydrochloric acid is HCl. The salt uses Ba^2+ and Cl-, so the salt formula is BaCl2. Because this is acid + alkali, the other product is water.

Ba(OH)2(aq)+2HCl(aq)→BaCl2(aq)+2H2O(l)\mathrm{Ba(OH)_2(aq) + 2HCl(aq) \rightarrow BaCl_2(aq) + 2H_2O(l)}

The 2 in front of HCl is needed because BaCl2 contains two chloride ions. The 2 in front of water is needed because Ba(OH)2 contains two OH groups.

Common traps

Do not use "strong" when you mean "concentrated". A dilute strong acid is still strong if it fully dissociates. A concentrated weak acid is still weak if it only partially dissociates.

Do not change formulae to balance equations. For example, potassium sulfate is K2SO4, not KSO4, because two K+ ions are needed for one SO4^2- ion.

Do not forget carbon dioxide in carbonate reactions. Carbonates with acids form salt, water and CO2.

Do not write ammonia as NH4OH. The formula required here is NH3, and acids convert it into ammonium salts such as NH4Cl.

In equation questions, a good final check is to read the products aloud: "salt plus water" for acid with alkali or metal oxide, and "salt plus water plus carbon dioxide" for acid with carbonate. Then check the salt formula and the balancing numbers.