Summary of what I learnt (Acids, Bases and Salts)

Acids

An acid is a substance which produces hydrogen ions as the only positive ions when it is dissolved in water. For example, HCI molecules dissolve in water to give hydrogen ions and chloride ions.

Examples:

Strong acids:

1)

Common/ Mineral Name: Hydrochloric acid

Chemical Name: Hydrogen chloride

Chemical Formula: HCI

2)

Common/ Mineral Name: Nitric acid

Chemical Name: Hydrogen nitrate

Chemical Formula: HNO₃

3)

Common/ Mineral Name: Sulfuric acid

Chemical Name: Hydrogen sulphate

Chemical Formula: H₂SO₄

Weak acids:

1)

Name of acid: Acetic acid (ethanoic acid)

Source: vinegar

2)

Name of acid: Citric acid

Source: lemon

3)

Name of acid: Tartaric acid

Source: grape

Strength of acid

The strength of an acid depends on its degree of dissociation/ ionisation in water to form hydrogen ions.

Strong acids:

A strong acid is one that ionises/ dissociates completely in water to produce hydrogen ions (H⁺) and there are no molecules left. The solution contains a high concentration of ions, which means it is a good electrical conductor.

Pure HCI (gaseous): covalent

HCI (aqueous): H⁺ (aq) + Cl^-(aq)

Pure HNO₃ (Liquid): covalent

HNO₃ (aq): H⁺ (aq) + NO₃^- (aq)

Pure H₂SO₄ (l): covalent

H₂SO₄ (aq): 2H⁺ (aq) + SO₄^2- (aq)

Weak acids:

A weak acid is one that only ionises/ dissociates incompletely/ partially in water to produce few hydrogen ions (H⁺). Most of the acid molecules remain as molecules. The solution contains a low concentration of ions, which means it is a poor electrical conductor.

CH₃COOH (aq) = CH₃COO^- (aq) + H⁺ (aq)

Importance of water for acidity:

Pure acids only exist as molecules as the properties of acids are due to the presence of hydrogen ions. When acids are mixed with water, ionisation of acids occurs, and hydrogen ions are produced. Therefore, acids can only behave as acids when they are dissolved in water.

Properties of acids:

An acid forms a colourless solution when dissolved in water. Solutions of acids have the following properties:

-sour taste

-the colour change of indicators (refer below)

-contain hydrogen ions and conduct electricity

-react with metals, carbonates, and bases

Reactions with metals:

Acid +metal -> salt + hydrogen

Example: 2HCI (aq) + Zn (s) -> ZnCl₂ (aq) + H₂ (g)

Observations:

- Effervescence observed. Colourless, odourless gas evolved which extinguished a lighted wooden splint with a ‘pop’ sound.

- Grey solid reduced in size (when in excess) and dissolves to form a colourless solution.

- Heat liberated

Salts of nitric acid are called nitrates. Salts of sulphuric acid are called sulfates, and those of hydrochloric acid are called chlorides. A salt is the product of the neutralisation of an acid and a base, and is generally made up from a cation (comes from base) and anion (comes from an acid).

Examples:

Sulfuric acid:

Zinc Sulfate: ZnSO₄

Sodium Sulfate: NA₂SO₄

Nitric acid:

Potassium nitrate: KNO₃

Copper (II) nitrate: Cu(NO₃)₂

Hydrochloric acid:

Magnesium chloride: MgCl₂

Sodium Chloride: NaCl

Reactions with carbonates:

Acid +carbonate -> salt + carbon dioxide + water

Example: Na₂CO₃ (s) + H₂SO₄ (aq) -> Na2SO4 (aq) + CO₂ (g) +H₂O (l)

Observations:

- Effervescence observed. Colourless, odourless gas evolved which reacts with calcium hydroxide to form a white precipitate.

- White solid dissolves to form a colourless solution.

Reactions with bases:

Acid +base -> salt + water

Example: MgO (s) + H₂SO₄ (aq) -> MgSO₄ (aq) +H₂O (l)

Observations:

- White solid dissolves to form a colourless solution

Example: 2KOH (aq) + H₂SO₄ (aq) -> K₂SO₄ (aq) + 2H₂O (l)

Observations:

- Heat is evolved, however, there is no visible change.

Uses of acids:

Sulfuric acid:

-in making detergents and fertilisers

-in car batteries as battery acid

Ethanoic acid:

-to preserve food

Colour change of indicators:

Litmus Paper:

-an acid turns blue litmus paper red.

Methyl Orange:

-used to identify strong acids

-colour changes at pH 4

-strong acid turns methyl orange red

-Alkali turn methyl orange yellow

pH:

pH values depend on the concentration of acid/base and degree of dissociation. As such, the use of pH In measuring the strength of an acid is limited since its value changes with concentration. The higher the concentration of the hydrogen ion, the lower is the pH value. For instance, a solution with a pH value of 1 has a higher concentration of hydrogen ions as compared to a solution with a pH value of 2.

Bases and Alkalis:

Bases are the oxides or hydroxides of metal that reacts with an acid to form a salt and water only. An alkali is a metal hydroxide which is soluble in water and produces hydroxide ions (OH^-) in water. Take note that all alkalis are bases but not all bases are alkalis.

Examples:

Insoluble base

Copper (II) oxide, aluminium oxide, zinc oxide, zinc hydroxide

Soluble base/ alkali

Sodium oxide, potassium oxide, sodium hydroxide, potassium hydroxide, calcium hydroxide, ammonium hydroxide

Strength of an alkali:

The strength of an alkali depends on its degree of dissociation/ ionisation in water to form hydroxide ions. The properties of alkalis are due to the hydroxides ions.

Strong alkali:

A strong alkali is one that ionises/ dissociates completely in water to produce hydroxide ions (OH^-). There are no molecules left. The common strong alkalis are sodium hydroxide and potassium hydroxide.

NaOH (aq) -> Na⁺ (aq) + OH^- (aq)

KOH (aq) -> K⁺ (aq) + OH^- (aq)

Weak alkali:

A weak alkali is one that ionises/ dissociates incompletely/ partially in water to produce few hydrogen ions (OH^-). Most of the alkali molecules remain as molecules.

Properties of bases:

1) Alkalis have a soapy feel and a bitter taste

2) Alkalis can change the colour of indicators (e.g. turn red litmus paper blue)

3) Bases can react with acids to form salt and water only – Neutralisation

Acid +base -> salt + water

Example: NaOH (aq) + HCI (aq) -> NaCl (aq) + H₂O (l)

Neutralisation: H⁺ (aq) + OH^- (aq) -> H₂O (l)

Reaction with ammonium salts:

Base +ammonium salt -> salt + ammonia + water

Example: NaOH (aq) + NH₄Cl (s) -> NaCl (aq) + NH₃ (g) + H₂O (l)

Observations:

- A colourless, pungent gas is evolved

- The gas turns damp red litmus paper blue

Uses of Bases and Alkalis:

Magnesium hydroxide:

-in toothpaste to neutralise the acids on our teeth produced by bacteria

Ammonia solution:

-used for cleaning glass windows