Acids, bases and salts

You should Define be acid, able acid to: anhydride, base, alkali, salt, acidic, Relate basic, acidity amphoteric and alkalinity and neutral to the oxides. pH scale. Discuss the strengths of acids and alkalis on the basis of their completeness of ionisation. QUICK POINTS

Organic acids are generally weak (e.g., ethanoic (acetic) acid).

For weak acids, a large number of their molecules remain undissociated in solution, hence ionisation is incomplete.

Non-metals form acidic oxides, while metals form basic oxides. Amphoteric oxides show both basic and acidic properties, while neutral oxides show neither.

Non-metallic oxides dissolve in water to form acids.

Metallic oxides are basic oxides. Those that dissolve in water are called alkalis.

Acids combine with bases to form salt and water only.

Salts can be formed from the reaction of acids with carbonates, hydrogen-carbonates, alkalis and metals.

The pH of a substance indicates how acidic or basic (alkaline) its aqueous solution is. The pH scale goes from 0 – 14, with a pH less than 7 described as acidic, and a pH greater than 7 described as basic or alkaline. Litmus (pH paper) or indicator solutions, such as methyl orange and phenolphthalein, can determine whether substances are alkaline or acidic.

TYPICAL PH SCALE

An acid is a substance which dissolves in water or aqueous solution to produce H+ ions, which exist as hydroxonium ion (H3O+) in solution. An acid is formed when an acid anhydride (acidic oxide) dissolves in water.

The strength of an acid depends on how completely it ionises or breaks up into ions when in solution. Mineral acids, such as hydrochloric, sulphuric and nitric acids, completely dissociate in solution and are called strong acids.

QUICK CHECK

What makes a solution acidic?

Alkalis can also be strong or weak. NaOH is a strong alkali, while aqueous ammonia is weak, as it is only slightly dissociated in aqueous solution.

Dibasic and tribasic acids can form both normal and

acid salts. A dibasic acid dissolves in solution to produce 2 moles of H+ ions. Sulphuric acid (H SO 4) and Carbonic acid (H 2CO3) are examples of dibasic 2 acids. Tribasic acids ionise to produce 3 moles of H+ ions in solution. Phosphoric acid (H PO ) is an example of a tribasic acid. 3 4

Soluble salts can be prepared from the action of an acid on a metal, carbonate and/or base. Insoluble salts are obtained from ionic precipitation reactions between two soluble salt solutions. Anhydrous salts must be prepared by direct combination of the elements involved.

A salt is formed when all or part of the hydrogen in an acid is replaced by a metal ion or the ammonium ion. Salts generally consist of a negative ion from an acid and a positive ion from a base or metal. For example, NaCl; the Na+ is from a metal or base (NaOH) and Cl- is from the acid (HCl). If all the replaceable hydrogen ions are removed, a normal salt is formed, e.g., NaCl. If only part

of acid the salt replaceable is formed, hydrogen e.g., NaHCO3. ions is removed, then an QUICK CHECK

Why would a solution of hydrochloric acid and ethanoic acid, having the same concentration, be of different strengths? FURTHER PRACTICE

A suspension of barium sulphate is normally ingested by patients before taking an X-ray. Explain why it is safe for patients to have barium sulphate in their bodies.

How could indigestion caused by excess acid be cured? Explain how the indigestion remedy works. Francine Taylor-Campbell is an independent contributor.