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C5: Chemical Changes (Strong and Weak Acids (If you dilute an acid enough…
C5: Chemical Changes
Strong and Weak Acids
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Strong acids - Hydrochloric acid, Nitric acid, sulfuric acid
Weak acids - Ethanoic acid (in vinegar), citric acid, carbonic acid (in rain and fizzy drinks)
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Acids must dissolve in water before they show acidic properties. Acids ionise (split up) to form H+ ions and negative ions.
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Strong acids - Ionise completely in solution. Irreversible reaction water
Weak acids - Ionise partially in solution. Reversible reaction in water so the molecules split up (to form H+ ions) then recombine to form the original molecules until equlibriam is reached.
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In a strong acid the concentration of the acid and the H+ ions are the same because strong acid ionise completely
When the concentration of H+ ions is reduced by a factor of 10 the pH value goes up one unit e.g. 0.10 mol/dm3 = 1.0 pH, 0.010 = 2.0 pH
Displacement Reactions
A more reactive metal will displace a lees reactive metal from an aqueous solution of one of it salts
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Salts From Metals
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When a metal and acid react, they form a salt
Salt is the general name for a compound formed when the hydrogen is partially are wholly replaced by metal (or ammonium) ions
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Pure, dry crystals of salt can be obtained by:
1) Heating the solution until the point of crystallisation is reached.
2) Crystals should appear at the edge of the solution
3) Leave the crystals at room temperature for the rest of the water to evaporate slowly
4) Dab the crystals dry
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Making More Salts
Two other ways to make a salt:
1) reacting an acid and an alkali together
2) reacting an acid with a carbonate
When an acid reacts with an alkali, a neutralisation reaction takes place. Water and salt are the products
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There is no gas given off in neutralisation reaction so the only way to tell when the reactants have completely reacted is by using an acid/base indicator
Calcium carbonate reacts with an acid to make carbon-dioxide, salt and water
Extracting Metals
Metals are found in the Earth's crust. Most metals are chemically combined with other elements, often oxygen or sulfur.
Some metal ores have to be concentrated before they are extracted. For example copper ores are ground up into a powder first
Factors influencing whether it is worth extracting a metal:
How easy it is to extract.
How much metal the ore contains.
The demand the metal
These factors can change over time. For example, finding a new way to extract metals can turn a 'low grade' ore to an economic source of metal
The Reactivity Series
Most metals in ores have been oxidised, so to extract the metals from their oxides, they must be reduced.
The most reactive metals in the series are potassium, sodium, and lithium
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