8 & 9 Acids and Reactions of acids
8.1 The pH scale runs continuously from 1 (strong acid) through 7 (neutral) to 14 (strong alkali).
- if a substance dissolves in water (pH = 7) then it may change the pH of the water making an acid or an alkali.
8.2 substance dissolving solution pH indicator ions
non-metal oxide acid below 7 red LOTS -H+(aq) in excess
water neutral 7 green FEW- H+ (aq) = OH-(aq)
metal oxide alkali above 7 purple LOTS - OH- (aq) in excess
(remember group I = alkali metals)
neutral solutions may have lots of ions but H+(aq) = OH-(aq)
8.3 Sulphur dioxide and oxides of nitrogen (pollution caused by burning fossil fuels) react with water in the atmosphere to produce acid rain.
Acid rain can damage buildings, stones, metals (speeds up corrosion/rusting) by chemical reactions - and can damage plant and animal life by lowering pH - (make it more acidic).
8.4 Acids are usually called 'ACIDS' sulphuric acid H2SO4; hydrochloric acid HCl ; nitric acid HNO3 vinegar
Alkalis are usually called HYDROXIDES = sodium hydroxide NaOH; potassium hydroxide KOH
calcium hydroxide Ca(OH)2- lime water, soaps, toothpaste, cleaners
8.5 Diluting an acid or alkali with water reduces the acidity or the alkalinity (makes less acid or alkaline) and shifts the pH closer to 7 - decreases concentration of H+(aq) or OH-(aq).
8.6 All acids have excess H+(aq) and when an electric current is passed through an solution the H+(aq) are attracted to the -ve electrode where they gain electrons (reduction) to produce hydrogen gas.
Hydrogen gas burns with a pop (explodes) when a lighted splint is added to it.
8.7 Calculations involving
mass in question = mole
mass of 1 mole
concentration is written as "mol/litre" which is a division
concentration = mole
9.1 When acids react the pH will increase, moving towards pH = 7 which is neutral
When alkalis react the pH will decrease, moving towards pH = 7 which is neutral
ACID + ALKALI is a NEUTRALISATION reaction
ACID + METAL OXlDE is a NEUTRALISATION reaction
ACID + CARBONATE is a NEUTRALISATION reaction
H+(aq) forming water is a NEUTRALISATION reaction
a base neutralises an acid - a base that dissolve is called an alkali
- other examples
Reducing acidity in soil or lakes by adding lime;
treatment of acid indigestion.
9.2 Acids react to form salts. hydrochloric acid (HCl) produces chlorides (Cl-)
sulphuric acid (H2SO4 produces sulphates (SO4)2-
nitric acid (HNO3) produces nitrates (NO3-)
- the positive metal ion comes from the substance which neutralises the acid replacing
the hydrogen ion in the acid
9.3 Acid reacts with
(i) ALKALI (metal hydroxide) -----> SALT + WATER
(ii) METAL OXIDE- ------> SALT + WATER
(iii) CARBONATE -------> SALT + WATER + CARBON DIOXIDE
(iv) METALS (not copper, silver, gold)------->SALT + HYDROGEN
Acid rain attacks buildings and rocks which are carbonate containing stone and structures made of iron.
(i) ACID + ALKALI (metal hydroxide) -----> SALT + WATER
hydrochloric acid + sodium hydroxide ----> sodium chloride + water
HCl + NaOH ----> NaCl + H2O
remove spectator ions
H+ + OH- ----> H2O
(ii) ACID + METAL OXIDE -----> SALT + WATER
nitric acid + lithium oxide ----> lithium nitrate + water
HCl + Li2O ----> LiNO3 + H2O
(iii) ACID + CARBONATE -----> SALT + WATER + CARBON DIOXIDE
sulphuric acid + calcium carbonate ----> calcium sulphate + water + carbon dioxide
H2SO4 + CaCO3 ----> CaSO4 + H2O + CO2
remove spectator ions
An acid reacts with a carbonate by (hydrogen ion plus carbonate ion)
2H+(aq) + CO32-(aq) -----> CO2(g) + H2O(l)
When an acid reacts with a metal 2H+(aq)+ 2e-------> H2(g) (reduction)
and the metal is oxididsed
silver nitrate + sodium chloride -----> silver chloride + sodium nitrate
soluble soluble insoluble soluble
Ag+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq)-----> Ag Cl(s) + Na+(aq) + NO3-(aq)
spectator ions NO3-(aq) and Na+(aq) do not take part in the reaction so are called spectator ions
9.5 It is easier to prepare a salt by neutralising an acid with an insoluble base
- once the acid is neutralised (no reaaction occurring -no gas produced or no more solid reacting because no more H+ left) excess solid can be filtered away. Water then evaporated.
With two solutions reacting it is difficult to find the neutralisation point and therefore easy to add too much.
These notes are for the Scottish National Chemistry course taught in fourth year in most Scottish schools.
The notes available here are concise notes. They are NOT to be considered as material to learn from - they are for revision.
The notes are arranged under the following topics
National 3 Unit 1
National 4 and 5 Unit 1 Rates of Reaction
National 4 and 5 Unit 2 Fuels and Homologous Series
National 4 and 5 Unit 3 Metals