Edexcel GCSE 9-1 Chemistry: SC8 - Acids and Alkalis

Paper 1 Specification: https://qualifications.pearson.com/content/dam/pdf/GCSE/Science/2016/Specification/GCSE_Chemistry_Spec.pdf

Describe the use of hazard symbols on containers
(a) to indicate the dangers associated with the
contents
(b) to inform people about safe-working precautions with these substances in the laboratory.

• hazard symbols are intended to warn about the dangers associated with the substance

• let people know about the precautions to take when they use the substance

Recall that acids in solution are sources of…

acids in solution are sources of:

• hydrogen ions

• alkalis in solution are sources of hydroxide ions

Recall that a neutral solution has a pH of 7 and that acidic solutions have lower pH values and alkaline solutions higher pH values

acid: pH 1-6

neutral: pH 7

alkali: pH 8-14

Recall the effect of acids and alkalis on indicators, including litmus, methyl orange and phenolphthalein.

• litmus: red in acid sol, blue in alkali sol

• methyl orange: red in acid sol, yellow in alkali sol

• phenolphthalein: colourless in acid sol, pink in alkali sol

Recall that the higher the concentration of hydrogen ions in an acidic solution, the lower the pH; and the higher the concentration of hydroxide ions in an alkaline solution, the higher the pH

• higher the conc of hydrogen ions in an acidic sol, the lower the pH

• higher the conc of hydroxide ions in an alkaline sol, the higher the pH

Recall that as hydrogen ion concentration in a solution increases by a factor of 10, the pH of the solution decreases by 1.

• hydrogen ion concentration in a solution increases by a factor of 10, the pH of the solution decreases by 1

  • e.g. pH of 0 is 10x10x10x10 = 10,000 times more acidic than a pH of 4

Explain the terms dilute and concentrated, with respect to amount of substances in solution.

dilute:

• lower the conc of hydrogen ions in a certain vol, the more dilute a sol is

• more dilute, the less acidic the sol and the higher the pH

concentrated:

• higher the num of hydrogen ions in a certain vol, the higher their conc

• higher the conc, the more acidic the sol and the lower the pH

• higher the conc of hydroxide ions, the more alkaline the sol and the higher the pH

Explain the terms weak and strong acids, with respect to the degree of dissociation into ions

strong acids:

• fully dissociate into ions in solution

• all their molecules release H+(aq) ions

  • e.g. HCl and NaSO₄

weak acids:

• partially disassociates its ions in solution

• only a few molecules release H+ (aq) ions

  • e.g. ethanoic acid (acetic acid- HC₂H₃O₂ )

Write balanced equations, including the use of the state symbols (s), (l), (g) and (aq)

H₂SO₄ (aq) + CuO (s) —> CuSO₄ (aq) + H₂O (l)

Recall that a base is any substance that reacts with an
acid to form a salt and water only

base: any substance + acid —> salt + water

Explain the general reactions of aqueous solutions of acids with metal oxides to produce salts

acid (aq) + base (s)→ salt (aq) + water (l)

Describe a neutralisation reaction as a reaction between an acid and a base.

hydrogen ions in the acid react with oxides/hydroxide from the base to form water + a salt

Recall the formula for neutralisation

H⁺ (aq) + OH^- (aq) ⇌ H₂O (l)

Explain why, if soluble salts are prepared from an acid and an insoluble reactant:
a) excess of the reactant is added
b) the excess reactant is removed
c) the solution remaining is only salt and water

prepare a soluble salt: fixed vol of acid in a beaker and add an excess of the metal oxide. filter excess base, put sol in an evaporating basin.

• an excess metal oxide ensures that all the acid has neutralised

• an excess reactant is then removed to make sure the remaining solution is only salt and water

• acids reacted with (insoluble) bases to form only salt and water

C0.1 Recall the formulae of elements, simple compounds and ions.

Hydrochloric acid; HCL
Nitric acid; HNO3
Sulphuric acid; H2SO4

Sodium hydroxide; NaOH
Calcium hydroxide; Ca(OH)2

C0.2 Write word equations.

sodium hydroxide + sulphuric acid → sodium sulphate + water

C0.3 Write balanced chemical equations, including the use of the state symbols (s), (l), (g) and (aq).

Sodium hydroxide + sulphuric acid → sodium sulphate + water
NaOH(aq) + HCL (aq) → NaCl (aq) + H2O (l)

C3.6 Investigate the change in pH on adding powdered calcium hydroxide or calcium oxide to a fixed volume of dilute hydrochloric acid.

Titration is used - Soluble base (Ca(OH)2) is needed. Titration must be used if soluble salts are prepared from an acid and a soluble base. Both soluble reactant and acid are mixed in the correct proportion to form only salt and water. pH becomes Neutral

3.10 Recall that alkalis are soluble bases.

Alkalis are soluble bases.

3.11c Explain the general reactions of aqueous solutions of acids with metal hydroxides to produce salts.

acid + metal hydroxide → salt + water. Hydrogen ions from acid react with hydroxide ions from metal hydroxide to produce water. The cation from the metal reacts with the anion from the acid to form a salt

3.14 Explain an acid-alkali neutralisation as a reaction in which hydrogen ions (H+) from the acid react with hydroxide ions (OH-) from the alkali to form water.

In a neutralisation reaction, hydrogen ions from the acid react with hydroxide ions from the alkali. Water, a simple molecular substance containing covalent bonds is formed in the reaction:
H+(aq) + OH-(aq) → H2O(l)

3.16 Explain why, if soluble salts are prepared from an acid and a soluble reactant:
a) titration must be used
b) the acid and the soluble reactant are then mixed in the
correct proportions
c) the solution remaining, after reaction, is only salt and water.

A titration uses a pipette to measure out a fixed volume of acid to put in a conical flask. An alkali is added from the burette.
Titration must be used because the correct proportions of acid and alkali are needed to make sure that the reaction will end up with only salt and water.

3.18 Describe how to carry out an acid-alkali titration, using burette, pipette and a suitable indicator, to prepare a pure, dry salt.

Put alkali into burette. Use a pipette to put a known volume of acid into a conical flask. Put a few drops of a suitable indicator such as phenolphthalein or methyl orange into the acid. Measure the burette start reading. Add alkali to acid. Add alkali to acid until the colour changes - end-point. Record the burette reading. Warm the salt solution to evaporate the water - this will leave crystals of salts behind.

C0.4 H Write balanced ionic equations.

word; Sodium + Hydrochloric acid → Sodium chloride + hydrogen.
symbol; 2Na (s)+ 2HCl (aq) → 2NaCl (aq) + H2 (g)
ionic; 2Na + 2H+ → H2 + 2Na+

3.11 Explain the general reactions of aqueous solutions of acids with (a) metals and (d) metal carbonates to produce salts.

a) Acid + Metals → Salt + Hydrogen
d) Adic + Metal carbonates → Salt + water + carbon dioxide

3.12 Describe the chemical test for (a) hydrogen and (b) carbon dioxide (using limewater).

a) A lighted splint ignites hydrogen with a 'squeaky pop'.
b) Carbon dioxide turns limewater milky or cloudy white

C3.19 Recall the general rules which describe the solubility of common types of substances in water:
a

all common sodium, potassium and ammonium salts are soluble

C3.19 Recall the general rules which describe the solubility of common types of substances in water:
b

all nitrates are soluble

C3.19 Recall the general rules which describe the solubility of common types of substances in water:
c

common chlorides are soluble except those of silver and lead

C3.19 Recall the general rules which describe the solubility of common types of substances in water:
d

common sulfates are soluble except those of lead, barium and calcium

C3.19 Recall the general rules which describe the solubility of common types of substances in water:
e

common carbonates and hydroxides are insoluble except those of sodium, potassium and ammonium.

C3.21 Describe the method used to prepare a pure, dry sample of an insoluble salt.

Mix - mix two solutions that will form an insoluble salt
Filter - filer the mixture to remove the insoluble salt
Wash - wash the salt with distilled water
Dry - leave salt to dry on the filter paper/dried in an oven
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