Acids Bases and Salts for the ESAT

Defining Acids and Salts

An acid is defined as a substance capable of donating $H^+$ ions. When these substances dissolve in water, they produce aqueous hydrogen ions, $H^+(aq)$. For example, the dissociation of hydrochloric acid is represented as:

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

When acids react, they often form salts. A salt is a compound produced when the hydrogen ions in an acid are replaced by metal ions or other cations. The name of the salt is derived from the metal and the parent acid.

Acid Name	Acid Formula	Sodium Salt Name	Sodium Salt Formula
Hydrochloric acid	$HCl$	Sodium chloride	$NaCl$
Sulfuric acid	$H_2SO_4$	Sodium sulfate / hydrogensulfate	$Na_2SO_4$ / $NaHSO_4$
Nitric acid	$HNO_3$	Sodium nitrate	$NaNO_3$
Ethanoic acid	$CH_3COOH$	Sodium ethanoate	$CH_3COONa$

Exercise 79: Magnesium Salts

a) Hydrochloric acid: Magnesium chloride, $MgCl_2$ b) Sulfuric acid: Magnesium sulfate, $MgSO_4$ c) Propanoic acid: Magnesium propanoate, $(CH_3CH_2COO)_2Mg$

Reactions with Metals

Metals react with acids to form a salt and hydrogen gas:

$metal + acid \rightarrow salt + hydrogen$

This reaction results in the effervescence of a colourless gas. These reactions only occur if the metal is more reactive than hydrogen. For instance, iron reacts with dilute acids, but gold does not. Very reactive metals may react too violently for safe laboratory practice.

This is a redox reaction. The metal atoms lose electrons (oxidation), while the hydrogen ions gain electrons (reduction).

Exercise 80: Silver and Sulfuric Acid Silver sulfate cannot be prepared this way because silver is less reactive than hydrogen and cannot displace it from the acid.

Exercise 81: Sodium and Hydrochloric Acid Sodium is extremely reactive. The reaction with dilute acid would be too violent and dangerous to perform in a standard laboratory.

Exercise 82: Formula Equations a) $Ca(s) + H_2SO_4(aq) \rightarrow CaSO_4(aq) + H_2(g)$ b) $Mg(s) + 2CH_3COOH(aq) \rightarrow (CH_3COO)_2Mg(aq) + H_2(g)$

Exercise 83: Magnesium and Hydrochloric Acid a) $Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)$ b) $Mg(s) + 2H^+(aq) \rightarrow Mg^{2+}(aq) + H_2(g)$ c) Half-equations: $Mg \rightarrow Mg^{2+} + 2e^-$ and $2H^+ + 2e^- \rightarrow H_2$. This is redox because $Mg$ is oxidised (loses electrons) and $H^+$ is reduced (gains electrons).

Reactions with Carbonates, Hydroxides, and Oxides

Metal Carbonates react with acids to produce a salt, water, and carbon dioxide:

$metal carbonate + acid \rightarrow salt + water + carbon dioxide$

Example: $CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$. This is a neutralisation reaction, not a redox reaction.

Exercise 84: Potassium Carbonate and Nitric Acid a) $K_2CO_3(s) + 2HNO_3(aq) \rightarrow 2KNO_3(aq) + H_2O(l) + CO_2(g)$ b) $K_2CO_3(s) + 2H^+(aq) \rightarrow 2K^+(aq) + H_2O(l) + CO_2(g)$

Metal Hydroxides react with acids to form a salt and water only. This neutralisation is central to titrations.

$metal hydroxide + acid \rightarrow salt + water$

Exercise 85: Potassium Hydroxide and Sulfuric Acid a) $2KOH(aq) + H_2SO_4(aq) \rightarrow K_2SO_4(aq) + 2H_2O(l)$ b) $OH^-(aq) + H^+(aq) \rightarrow H_2O(l)$

Metal Oxides are bases that react with acids to form a salt and water:

$metal oxide + acid \rightarrow salt + water$

Exercise 86: Zinc Oxide Calculation c) $ZnO(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2O(l)$ d) Moles of $H_2SO_4 = 2.0 \times (20 / 1000) = 0.04$ mol. The ratio is 1:1, so 0.04 mol of $ZnSO_4$ forms. $M_r$ of $ZnSO_4 = 65.4 + 32.1 + (16.0 \times 4) = 161.5$. Mass $= 0.04 \times 161.5 = 6.46$ g.

Strong, Weak, Dilute, and Concentrated

A strong acid (e.g. $HCl$, $H_2SO_4$, $HNO_3$) fully dissociates in water. The reaction goes to completion: $HNO_3(aq) \rightarrow H^+(aq) + NO_3^-(aq)$.

A weak acid (e.g. carboxylic acids) only partially dissociates. An equilibrium exists: $CH_3COOH(aq) \rightleftharpoons CH_3COO^-(aq) + H^+(aq)$.

Concentrated and dilute refer to the number of moles of acid per volume. A $6.0$ mol dm$^{-3}$ solution is concentrated, while $2.0$ mol dm$^{-3}$ is dilute. These are relative terms; one can have a concentrated weak acid or a dilute strong acid.

Exercise 87: Comparing Nitric and Ethanoic Acid When added to sodium carbonate, nitric acid will produce much faster effervescence than ethanoic acid of the same concentration. This is because nitric acid is strong and has a higher concentration of $H^+$ ions, leading to a faster rate of reaction.

Oxides of Non-metals

Some non-metal oxides react with water to form acidic solutions. This is the basis of acid rain.

$SO_2(g) + H_2O(l) \rightarrow H_2SO_3(aq)$ (sulfurous acid) $2NO_2(g) + H_2O(l) \rightarrow HNO_3(aq) + HNO_2(aq)$ $CO_2(g) + H_2O(l) \rightarrow H_2CO_3(aq)$ (carbonic acid)

Exercise 88: Phosphorus(V) Oxide $P_4O_{10} + 6H_2O \rightarrow 4H_3PO_4$

The pH Scale and Proticity

pH measures $H^+$ concentration. A change of 1 on the pH scale represents a tenfold (factor of 10) change in $H^+$ concentration.

$[H^+(aq)]$ (mol dm$^{-3}$)	pH
$1.0 \times 10^{-1}$	1.0
$1.0 \times 10^{-2}$	2.0
$1.0 \times 10^{-3}$	3.0

Exercise 89: pH Calculation $3.65$ g $HCl$ in $10$ dm$^3$: $M_r(HCl) = 36.5$, so moles $= 0.1$. Conc $= 0.1 / 10 = 0.01$ mol dm$^{-3}$. Transferring $1.0$ cm$^3$ to $1.0$ dm$^3$ is a 1000-fold dilution. New conc $= 0.01 / 1000 = 1.0 \times 10^{-5}$ mol dm$^{-3}$. pH $= 5.0$.

Acids are classified by how many $H^+$ ions they can donate per molecule:

1. Monoprotic: Donate one $H^+$ (e.g. $HCl$, $HNO_3$, $CH_3COOH$).
2. Diprotic: Donate two $H^+$ (e.g. $H_2SO_4$).
3. Triprotic: Donate three $H^+$ (e.g. $H_3PO_4$).

Exercise 90: Titration Stoichiometry $H_2SO_4 + 2NaOH \rightarrow Na_2SO_4 + 2H_2O$ Moles $NaOH = 2.0 \times 0.025 = 0.05$ mol. Moles acid needed $= 0.05 / 2 = 0.025$ mol. Conc $= 0.025 / 0.0215 = 1.16$ mol dm$^{-3}$.