Acids and Alkalis
Four important acids are: Hydrochloric acid (HCl), Sulphuric acid (H2SO4), Nitric acid(HNO3) and Citric acid.
Acids have a pH below 7 and turn pH paper orange (weak acid) or red (strong acid). Weak acids taste sour and some are corrosive. Acids are neutralised by alkalis to make salt and water.
Acids + alkalis → salt + water
Acids + metals → salt + hydrogen
Acids + metal carbonates → salt + water + carbon dioxide
Four important alkalis are: Sodium hydroxide (NaHO), Calcium carbonate (CaCO3), Calcium hydroxide (Ca(HO)2) (lime), Ammonia (HN3) solution.
Alkalis have a pH over 7and turn pH paper blue/purple (weak/strong alkalis). Alkalis often feel soapy and can be corrosive (worse than an acid). Alkalis neutralise acids to make salts.
The pH Scale
Universal indicator shows the pH with different colours. Water is neutral with a pH value of 7.
Rain water is slightly acidic because of the carbon dioxide (pH 6) and acid rain has a pH value of 5. Citric acid (lemon) and ethanoic acid (vinegar) are pH 3. Hydrochloric acid and Sulphuric acid are pH 1.
Washing up liquid has a pH value of 9, soap pH10/11, ammonia pH 12 and sodium hydroxide pH 14.
Universal indicator solution is a mixture of dyes which changes colour depending on how acid or alkali something is. Litmus paper only indicates whether a liquid is an acid or an alkali - it does not say how strong it is.
Neutralisation
All acids contain hydrogen - this can be replaced by a metal or an ammonium ion. The substance formed is called a salt.
Neutralisation always produces a salt and water: Acid + Alkali --> Salt + Water.
Indigestion is caused by too much hydrochloric acid in the stomach (too much rich food). The excess acid can be neutralised with a medium strength alkali like magnesium oxide:
Hudrochloric acid (2HCl) + Magnesium oxide (MgO) → Magnesium Chloride (MgCl) + Water (H2O)
Soil can be acidic either because of acid rain or because of naturally acidic minerals in the rocks. Plants are sensitive to the level of acidic, some prefer acid soils while others prefer netural or alkaline soils. The wrong pH level will affect plant growth and 'liming' the soil neutralises the acid:
Sulphuric acid (H2SO4) + Calcium hydroxide (Ca(HO)2) → Calcium sulphate (CaSO4) + Water (2H2O)
Calcium hydroxide is commonly known as lime.
Making Salts
Salts are produced when hydrogen atoms in an acid are replaced by a metal. The salt produced depends on the acid used:
chloride salt is produced by hydrochloric acid
sulphate salt is produced by sulphuric acid
nitrate salt is produced by nitric acid
There are four reactions which can be used to prepare salts:
metal + acid --> salt + hydrogen
metal oxide (base) + acid --> salt + water
metal hydroxide + acid --> salt + water
metal carbonate + acid --> salt + water + carbon dioxide
Acids in the Atmosphere
Sulphuric acid and nitric acid can exist in the atmosphere which speed up the corrosion of metals and acid rain causes weathering of rocks.
Rain is naturally a bit acidic due to the CO2 in the air: CO2 + water → Carbonic acid.
However, acid rain is caused by sulphur dioxide and oxides of nitrogen mix with water in the atmosphere. When metals above hydrogen in the reactivity series are exposed to acids, they corrode by chemical reaction which weakens the metal:
Sulphuric acid (H2SO4) + Iron (Fe) → Iron sulphate (FeSO4) + Hydrogen (H2)
Sulphur dioxide comes from the combustion of fuels which contain high sulphur content (e.g. coal). Oxides of nitrogen comes from the action of lightning on nitrogen and oxygen and from reactions between nitrogen and oxygen in car engines.
Acid rain attacks exposed metals and acidic food attacks steel cans. Steel cans are coated with tin (a less reactive metal) to prevent a chemical reaction.
When acid rain falls on rocks containing calcium carbonate (CaCO3) (limestone, chalk and marble), the chemical reaction turns them into a calicum salt, water and carbon dioxide. This weathers the rock.
Nitric acid (2HNO3) + Calcium carbonate (CaCO3) → Calcium nitrate (Ca(NO3)2) + Water (H2O) + Carbon dioxide (2CO2)