Reactivity 3: What are the mechanisms of chemical change?IB Chemistry HL: Topic test

20 questions, 54 marks

IB Chemistry HL

Reactivity 3: What are the mechanisms of chemical change? topic test

Total 54 marks

Name

Class

Date

1
A student compares four aqueous solutions at 298 K, each with a total concentration of 0.100 mol dm\u207b\u00b3: hydrochloric acid (a strong acid), ethanoic acid (a weak acid), sodium hydroxide (a strong base) and ammonia (a weak base).
(a)
Which solution has the highest pH?
[1 mark]
A0.100 mol dm⁻³ sodium hydroxide
B0.100 mol dm⁻³ ammonia
C0.100 mol dm⁻³ hydrochloric acid
D0.100 mol dm⁻³ ethanoic acid
(b)
Which statement correctly compares the ethanoic acid and hydrochloric acid solutions, both at 0.100 mol dm⁻³?
[1 mark]
AThey have the same pH because they have the same concentration
BThe hydrochloric acid has a lower pH, because it ionises completely while ethanoic acid only partially ionises
CThe ethanoic acid has a lower pH, because it is a carboxylic acid
DThey have the same [H⁺], but different pOH
(c)
Calculate the pH of the 0.100 mol dm⁻³ hydrochloric acid solution, assuming complete ionisation.
[2 marks]
Total for question 1: 4 marks
2
When chlorine gas is bubbled into cold, dilute sodium hydroxide solution, a disproportionation reaction occurs: Cl\u2082(aq) + 2NaOH(aq) \u2192 NaCl(aq) + NaClO(aq) + H\u2082O(l), used to make household bleach.
(a)
What are the oxidation states of chlorine before and after this reaction?
[1 mark]
A0 in Cl₂; −1 in NaCl and NaClO
B0 in Cl₂; +1 in NaCl and NaClO
C0 in Cl₂; −1 in NaCl (Cl⁻) and +1 in NaClO (ClO⁻)
D−1 in Cl₂; 0 in both products
(b)
What term describes this type of redox reaction, in which the same element is both oxidised and reduced?
[1 mark]
ANeutralisation
BSimple displacement
CCombustion
DDisproportionation
(c)
Write the two half-equations that show chlorine being simultaneously oxidised and reduced in this reaction.
[2 marks]
Total for question 2: 4 marks
3
2-methylpropane (isobutane), (CH\u2083)\u2083CH, is reacted with a small amount of bromine vapour in UV light, producing a mixture of monobromination products. There are nine primary hydrogens and only one tertiary hydrogen in each molecule, yet 2-bromo-2-methylpropane, (CH\u2083)\u2083CBr, is formed as the major product.
(a)
Write equations for the initiation step and the two propagation steps that lead to the formation of 2-bromo-2-methylpropane.
[3 marks]
(b)
Explain, in terms of the mechanism, why the tertiary product is formed as the major product even though a purely statistical mechanism based on the number of hydrogens present would predict mostly primary substitution.
[4 marks]
Total for question 3: 7 marks
4
A voltaic cell is set up with a magnesium electrode in 1.00 mol dm\u207b\u00b3 MgSO\u2084(aq) and a silver electrode in 1.00 mol dm\u207b\u00b3 AgNO\u2083(aq), connected by a wire and a salt bridge containing KNO\u2083(aq). E\u2296(Mg\u00b2\u207a/Mg) = \u22122.37 V; E\u2296(Ag\u207a/Ag) = +0.80 V.
(a)
Describe the operation of this cell: identify the anode and cathode, write the half-equations, calculate E⊖cell, and explain the direction of electron flow in the external circuit and of ion movement in the salt bridge.
[6 marks]
(b)
The lead-acid battery is a secondary cell. On discharge: Pb(s) + SO₄²⁻(aq) → PbSO₄(s) + 2e⁻ (negative electrode) and PbO₂(s) + 4H⁺(aq) + SO₄²⁻(aq) + 2e⁻ → PbSO₄(s) + 2H₂O(l) (positive electrode). Deduce the electrode reactions during recharging, and explain why this cell is described as 'secondary' rather than 'primary'.
[6 marks]
Total for question 4: 12 marks
5
1-bromopropane, CH\u2083CH\u2082CH\u2082Br, is heated under reflux with aqueous sodium hydroxide, forming propan-1-ol: CH\u2083CH\u2082CH\u2082Br + OH\u207b \u2192 CH\u2083CH\u2082CH\u2082OH + Br\u207b.
(a)
What type of species is the hydroxide ion in this reaction, and what mechanism operates for a primary halogenoalkane such as 1-bromopropane?
[1 mark]
AA nucleophile; SN2 (bimolecular) mechanism
BAn electrophile; SN1 mechanism
CA nucleophile; SN1 mechanism
DAn electrophile; SN2 mechanism
(b)
Which bond breaks, and by what process, in this substitution?
[1 mark]
AThe C–Br bond, by homolytic fission
BThe C–Br bond, by heterolytic fission, with both electrons going to bromine to form Br⁻
CThe C–H bond, by heterolytic fission
DThe O–H bond of the nucleophile, by homolytic fission
(c)
Describe, in words, the movement of electron pairs as the hydroxide ion attacks 1-bromopropane in this SN2 mechanism.
[2 marks]
Total for question 5: 4 marks
6
Ammonium chloride, NH\u2084Cl, is a salt formed from a weak base (NH\u2083, Kb = 1.8 \u00d7 10\u207b\u2075) and a strong acid (HCl). A student dissolves NH\u2084Cl in water to make a 0.100 mol dm\u207b\u00b3 solution at 298 K (Kw = 1.00 \u00d7 10\u207b\u00b9\u2074).
(a)
What is the approximate pH of this ammonium chloride solution?
[1 mark]
ApH = 7 (neutral)
BpH > 7 (basic)
CpH < 7 (acidic)
DThe pH cannot be predicted from this information
(b)
Which equation correctly represents the hydrolysis reaction responsible for this pH?
[1 mark]
ACl⁻(aq) + H₂O(l) ⇌ HCl(aq) + OH⁻(aq)
BCl⁻(aq) + NH₄⁺(aq) ⇌ NH₃(aq) + HCl(aq)
CNH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)
DNH₄⁺(aq) + H₂O(l) ⇌ NH₃(aq) + H₃O⁺(aq)
(c)
Given Ka(NH₄⁺) = Kw/Kb, calculate Ka for the ammonium ion, and state whether NH₄⁺ is a stronger or weaker acid than propanoic acid (Ka = 1.3 × 10⁻⁵).
[2 marks]
Total for question 6: 4 marks
7
Molten lead(II) bromide, PbBr\u2082(l), is electrolysed using inert graphite electrodes with a direct current of 2.00 A for 1930 seconds (1 F = 96500 C mol\u207b\u00b9).
(a)
Deduce the half-equations at each electrode and identify the products formed.
[3 marks]
(b)
Calculate the mass of lead deposited at the cathode.
[4 marks]
Total for question 7: 7 marks
8
Two ways of introducing chlorine into carbon compounds are compared. In method A, ethane reacts with chlorine gas in UV light: C\u2082H\u2086 + Cl\u2082 \u2192 C\u2082H\u2085Cl + HCl. In method B, ethene reacts with chlorine gas in the dark at room temperature: C\u2082H\u2084 + Cl\u2082 \u2192 C\u2082H\u2084Cl\u2082 (1,2-dichloroethane).
(a)
Using equations, describe the initiation, propagation and termination steps of the free-radical mechanism for method A, and explain why this reaction requires UV light while method B does not.
[6 marks]
(b)
Using curly arrows described in words, explain the mechanism of the reaction in method B, including how the non-polar Cl₂ molecule becomes able to act as an electrophile.
[6 marks]
Total for question 8: 12 marks

End of questions