Acids
Organic acids: naturally occurring acids such as citric acids, found in oranges.
Mineral acids: man-made (most of them), such as nitric and sulphuric acid.
What is an acid?
All acids produce hydrogen ions in aqueous solutions. Thus, an acid is a substance which produces hydrogen ions, H+, when dissolved in water.
All acids contain hydrogen but not all compounds that contain hydrogen are acids.
E.g. ammonia and methane contain hydrogen but are not acids because they do not produce hydrogen in the water. One of the properties of acids are that they produce hydrogen ions.
Properties of acids
1) Have a sour taste
2) Dissolve in water to form solutions which conduct electricity.
3) Turns blue litmus paper red.
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4) Reacts with reactive metals to form hydrogen and salt.
Metal+acid --> Salt+hydrogen
E.g. When magnesium ribbon is added to dilute sulphuric acid, bubbles of hydrogen gas is seen.
Magnesium+dilute sulphuric acid --> magnesium sulphate+hydrogen sulphate.
Magnesium sulphate: Salt produced from the reaction. (above)
Sulphates: Salt, but only referred to this name when formed from sulfuric acid.
Nitrates when formed from nitric acid, chlorides when formed from hydrochloric acid.
Experiment: If a lighted splint is placed at the mouth of a test tube, a 'pop' sound can be heard and the lighted splint will extinguish. This shows that hydrogen gas has been produced.
Some acid and metal reaction DO NOT produce hydrogen.
(a) When unreactive metals (like copper and silver) are added to dilute acids.
(b) Concentrated nitric acid reacts with metals (like copper) but instead of hydrogen, a nitrate (salt), water and nitrogen dioxide gases are formed.
(c) Lead does not react with dilute sulfuric acid or dilute hydrochloric acid because a layer of lead chloride and lead sulphate is formed from the initial reaction between lead and dilute acid. The layer is insoluble in water and quickly forms a coating around the metal. This prevents the metal from further attack by the acid.
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5) Acids react with carbonates to form salt, carbon dioxide and water.
Carbonate+acid --> Salt+water+carbon dioxide
Sodium chloride and dilute hydrochloric acid produces sodium chloride, water and carbon dioxide.
Sodium carbonate+dilute hydrochloric acid --> sodium chloride+water+carbon dioxide
Test for carbon dioxide: Bubble carbon dioxide through lime water. Carbon dioxide forms a white precipitate with lime water.
6) Acids react with metal oxides and hydroxides to form salt and water only.
Metal oxide+acid --> salt+water
Metal hydroxide+acid --> salt+water
*All metal oxides and hydroxide react with acids in the same way.
Role of water in acidity
Hydrogen chloride exists as covalent molecules. In the absence of water, e.g. in organic solvents, they do not become acids.
Acids only display their properties when they are dissolved in water (aq), because acids dissociate in water to produce hydrogen ions which allow the acidic properties.
The hydrogen ions produced allow the acids to react with metals like magnesium:
hydrogen ions+magnesium --> hydrogen gas+magnesium ions
Dissociation does not occur when hydrogen chloride is dissolved in an organic solvent. Thus, it does not react with magnesium.
No reaction occurs as well when solid citric acid is mixed with anhydrous sodium carbonate. When a few drops of water is added to the mixture, bubbles of carbon dioxide is formed.
hydrogen ions+carbonate ions --> water+carbon dioxide
Uses of acids
Sulphuric acid: making detergent, making fertilisers, in car batteries
Ethanoic acid: to preserve food (in vinegar), in making adhesives such as glue
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