Acids and their Properties

Acids

• In pure form, acids are composed of covalent molecules, but dissolve in water to form solutions containing ions.
• Solutions are acidic.
• Acids contain hydrogen, which can be replaced by a metal to form a salt.
  • Example: Zinc replaces hydrogen in hydrochloric acid to form zinc chloride and hydrogen gas: Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
• Acids form hydroxonium ions when dissolved in water
  • An acid dissolves in water; it forms hydrogen ions, they bond with water molecules to form a hydroxonium ion.
  • Example: HCl(g) + H2O(l) → H3O+(aq) + Cl-(aq)
  • H3O+(aq) represented as H+(aq) for simplicity.
• Acids act as proton donors.
  • Hydrogen ions are protons,.
  • Acids donate protons to other substances.
  • Example: HCl(g) + H2O(l) → H3O+(aq) + Cl-(aq)

General Properties of Aqueous Acids

• Acidic properties in aqueous solutions are due to H+(aq) ions, only formed with water.
• Acids have a sour taste.
• Acids are corrosive.
• Acids turn litmus from blue to red.
• Acids conduct electricity, acting as electrolytes.

Acids React With

• Bases to form a salt and water in a neutralization reaction.
  • Example: Mg(OH)2(s) + 2HCl(aq) → MgCl2(aq) + 2H2O(l)
  • Ionically: OH-(aq) + H+(aq) → H2O(l)
• Carbonates or hydrogencarbonates to form a salt, water, and carbon dioxide.
  • Example: CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
  • Ionically: CO32-(s) + 2H+(aq) → H2O(l) + CO2(g)
• Reactive metals (except nitric acid) to form a salt and hydrogen.
  • Example: Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
  • Ionically: Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g)
  • Nitric acid releases nitrogen oxides like NO2 instead of hydrogen, since it acts as an oxidizing agent.

Basicity (Proticity) of Acids

• Basicity describes the number of moles of H+ ions produced per mole of acid.
• Monobasic acids produce one mole of H+ ions per mole of acid.
  • Example: HCl(aq) → H+(aq) + Cl-(aq)
  • Monobasic acids form normal salts only.
• Dibasic acids produce two moles of H+ ions per mole of acid.
  • Example: H2SO4(aq) → 2H+(aq) + SO42-(aq)
  • Dibasic acids can form normal and acid salts.
• Tribasic acids produce three moles of H+ ions per mole of acid.
  • Example: H3PO4(aq) → 3H+(aq) + PO43-(aq)
  • Tribasic acids can form normal and acid salts.

Acid Anhydrides

• Acid anhydrides are non-metal oxides that react with water to form an acid.
  • Example: CO2(g) + H2O(l) → H2CO3(aq)

Acids in Daily Life

• Antacids contain sodium hydrogencarbonate and citric/tartaric acid that neutralizes stomach acid.
  • Carbon dioxide produced makes it fizzy
  • remaining sodium hydrogencarbonate neutralises the stomach acid.
• Baking powder contains tartaric acid and sodium hydrogencarbonate that releases carbon dioxide when heated which causes the mix rise
  • 2NaHCO3(s) → Na2CO3(s) + H2O(l) + CO2(g)
• Fire extinguishers contain carbon dioxide under pressure.
  • It is non-flammable and its high density smothers fire

Bases

• Bases are chemically opposite to acids.
• Bases react with acids to form a salt and water.
• Bases are proton (H+ ion) acceptors.
  • They accept hydrogen ions from acids, forming water: OH-(aq) + H+(aq) → H2O(l)
• Common bases include ammonia, metal oxides, and hydroxides,
  • Examples: magnesium oxide (MgO), copper(II) hydroxide (Cu(OH)2)

Alkalis

• Alkalis are bases that dissolve in water and contain OH- ions, resulting in alkaline solutions.
  • Example: NaOH(s) + water → Na+(aq) + OH-(aq)
• Most bases do not dissolve, therefore, they are not alkalis.

General Properties of Aqueous Alkalis

• Alkalis have a bitter taste.
• Alkalis feel soapy to the touch.
• Alkalis are corrosive.
• Alkalis turn litmus from red to blue.
• Alkalis conduct electricity, indicating they are electrolytes.
• Alkalis react with solutions containing metal ions (excluding potassium and sodium) to form insoluble precipitates.
  • Example: CuSO4(aq) + 2NaOH(aq) → Cu(OH)2(s) + Na2SO4(aq)
  • Ionically: Mn+(aq) + nOH-(aq) → M(OH)n(s)
• Alkalis react with zinc and aluminium to produce a salt and hydrogen.
  • Example: Zn(s) + 2NaOH(aq) → Na2ZnO2(aq) + H2(g)
• Alkalis react with ammonium salts producing a salt, water, and ammonia.
  • Example: NaOH(aq) + NH4Cl(aq) → NaCl(aq) + H2O(l) + NH3(g)
  • Ionically: OH-(aq) + NH4+(aq) → H2O(l) + NH3(g)
• Insoluble bases react slightly with acids.
• Most bases react with acids to form a salt and water.

Recognising Acids and Alkalis

• Indicators distinguish acids and alkalis by changing color.
  • Litmus paper turns red in acidic solutions and blue in alkaline solutions.

The Strength of Acids and Alkalis

• Strong acids and alkalis fully ionize in water, resulting in high concentrations of H+ or OH- ions.
  • Strong acids/alkalis are strong electrolytes.
  • Example: H2SO4(aq) → 2H+(aq) + SO42-(aq) and NaOH(aq) → Na+(aq) + OH-(aq)
• Weak acids and alkalis partially ionize in water, resulting in low concentrations of H+ or OH- ions.
  • Weak acids/alkalis are weak electrolytes.
  • Example: CH3COOH(aq) ⇌ CH3COO-(aq) + H+(aq) and NH3(g) + H2O(l) ⇌ NH4+(aq) + OH-(aq)

The pH Scale

• The pH scale measures the strength of an acid or alkali using a universal indicator.

Amphoteric Oxides and Hydroxides

• Amphoteric substances react with both acids and strong alkalis.
• A strong alkali reacts with an amphoteric oxide/hydroxide to form a salt and water.
• An amphoteric oxide/hydroxide reacts with an acid to form a salt and water.
• Aluminium hydroxide reacts with sodium hydroxide: NaOH(aq) + Al(OH)3(s) → NaAlO2(aq) + 2H2O(l)
• Aluminium hydroxide reacts with hydrochloric acid: Al(OH)3(s) + 3HCl(aq) → AlCl3(aq) + 3H2O(1)

Acids are covalent molecules that dissolve in water to form ions, creating acidic solutions. They contain hydrogen, which can be replaced by a metal to form a salt. Acids act as proton donors, forming hydroxonium ions (H3O+) when dissolved in water.