Chemistry: Sulfuric Acid and Marking Scheme

Topics: Sulfuric acid, Sodium hydroxide, Hydrochloric acid Pages: 14 (2946 words) Published: July 28, 2013
Set 4 ACID AND BASES PAPER 2 : STRUCTURE

Marking Scheme: Acid – Bases & Salts 2010

No. Answer 1(a) Neutralisation (b) 2KOH + H2SO4  K2SO4 + 2H2O Correct formulae of reactants and products Balanced equation (c) i. H+ , OH- , K+ and SO42ii. K+ and SO42(d) 5 cm3 (e) Mol H2SO4 = 0.005 mol 1 mol H2SO4 --> 2 mol KOH 0.005 mol H2SO4  0.01 mol KOH Molarity of KOH = 0.4 mol dm-3 // MaVa = 1 MbVb 2 Mb = 2 x 1 x 1 000 25 = 0.4 mol dm-3 (f) 10 cm3 HNO3 is monoprotic acid // H2SO4 is diprotic acid

Marks 1 1 1 1 1 1 1 1 1

1 1 11

2(a) i. water ii ethyl benzene , [any organic solvent] (b) (c) i. solution X ii. ionise hydrogen chloride and produce hydrogen ions i. carbon dioxide ii. channel / pass through the gas into lime water lime water turns cloudy Solution X contain freely hydrogen ions and chloride ions Solution Y contain hydrogen chloride molecule

1 1 1 1 1 1 1 1 1 9

(d)

Chemistry Perfect Score Module Form 4 2010 Marking Scheme Set 4

27

Set 4

Marking Scheme: Acid – Bases & Salts 2010

No. 3(a)

Answer (i) sodium hydroxide / potassium hydroxide r ; formula (ii)2NaOH + H2SO4  Na2SO4 + 2H2O / 2KOH + H2SO4  KSO4 + 2H2O - Correct formulae of reactants and products - Balanced chemical equation (iii) neutralisation Y (i) W / X (ii)calcium carbonate is added to solution X/W Gas released turns lime water chalky // Magnesium/ zinc added to solution X/W Gas released gives a pop sound with a lighted splinter W is a strong acid but X is a weak acid W completely dissociates in water produce high concentration of hydrogen ions // X partially ionise in water Produce low concentration of hydrogen ions

Marks 1

1 1 1 1 1 1 1

(b) (c)

(d)

1 1

10 4(a) (b) To prevent sucking back of the solution i. no change in Beaker A . Effervescence occur in Beaker B ii. hydrogen chloride in methyl benzene in Beaker A do not contain hydrogen ion hydrogen chloride in water in Beaker B produces hydrogen ion which react with magnesium to produce hydrogen gas i. hydrogen chloride molecules ii. hydrogen ions and chloride ions i. carbon dioxide gas. Pass the gas evolved into lime water Lime water turns cloudy/chalky/milky ii. in the presence of water , hydrogen chloride dissociates/ ionises produce hydrogen ion which react with sodium carbonate solution to produce carbon dioxide gas. iii. 2H+ + CO32-  CO2 + H2O 1 1 1 1 1 1 1 1 1

(c) (d)

9

Chemistry Perfect Score Module Form 4 2010 Marking Scheme Set 4

28

Set 4

Marking Scheme: Acid – Bases & Salts 2010

No. 5(a) (i) (ii) (b)

Answer Blue to red H+ 1. No 2. No water / H+ ions 1. Ethanoic acid is weak acid, hydrochloric acid is strong acid 2. The concentration of H+ ions in ethanoic acid is lower / The concentration of H+ ions in hydrochloric acid is higher. CaCO3 + 2HCl  CaCl2 + CO2 + H2O 1. [ Material : name of reactive metal / name of carbonate salt] 2. [To test gas produced] 3. [Observation] Example 1 Procedure: 1. Magnesium ribbon is added into the aqueous solution X 2. Lower a lighted wooden splinter Observation: A “pop” sound is heard Example 2 Procedure: 1. Calcium carbonate is added into the aqueous solution X 2. Passing the gas through lime water Observation: Lime water turns milky

Mark s 1 1 1 1 2 1 1 2 1 1 1 2 1 1 1 2 1 1 1 2 1

(c)

(d) (e)

6(a)

(b)

i. Q ii. R iii. P iv. U i. Acid molecules that ionise partially in water Produce low concentration of hydrogen ion ii. malic acid is weak acid but hydrochloric acid is a strong acid malic acid partially ionise in water but hydrochloric ionise completely in water less number of hydrogen ion in malic acid i. hydrogen ii. bring burning splinter near the mouth of the test tube pop sound is produced iii. Zn + 2HCl  ZnCl2 + H2 correct formulae of reactants and products balanced chemical equation

1 1 1 1 1 1 1 1 1 1 1 1 1

(c)

Chemistry Perfect Score Module Form 4 2010 Marking Scheme Set 4

29

Set 4 PAPER 2 : ESSAY

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