IB Chemistry SL Reactivity 3 What Are the Mechanisms of Chemical Change Questions

Practise IB Chemistry SL 1.6 with evidence-led questions on bonding, molecular structure and material properties.

Exam points

Identify Brønsted–Lowry acids and bases, calculate pH and interpret strong/weak acid-base and titration evidence.
Assign oxidation states, balance redox half-equations and explain electron flow, cells and electrolysis.
Recognize radicals and write homolytic-fission, propagation and termination steps for alkane substitution.
Identify nucleophiles and electrophiles, predict substitution and addition products, and classify Lewis acids and bases.

Covered syllabus points

3.1 Proton transfer reactions 3.2 Electron transfer reactions 3.3 Electron sharing reactions (Radicals)3.4 Electron-pair sharing reactions

[Maximum number: 3]

Two hydrides of nitrogen are ammonia and hydrazine, $\mathrm{N}_{2} \mathrm{H}_{4}$ . One derivative of ammonia is methanamine whose molecular structure is shown below.

(a)

Hydrazine reacts with water in a similar way to ammonia. Deduce an equation for the reaction of hydrazine with water.

[ 1 ]

$\mathrm{N}_{2} \mathrm{H}_{4}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{N}_{2} \mathrm{H}_{5}{ }^{+}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq})$ $\text { Accept } \mathrm{N}_{2} \mathrm{H}_{4}(\mathrm{aq})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{I}) \rightleftharpoons$ $\mathrm{N}_{2} \mathrm{H}_{6}{ }^{2+}(\mathrm{aq})+2 \mathrm{OH}^{-}(\mathrm{aq})$ .

Equilibrium sign must be present.

(b)

Outline, using an ionic equation, what is observed when magnesium powder is added to a solution of ammonium chloride.

[ 2 ]

bubbles

OR

gas

OR

magnesium disappears
$2 \mathrm{NH}_{4}^{+}(\mathrm{aq})+\mathrm{Mg}(\mathrm{~s}) \rightarrow \mathrm{Mg}^{2+}(\mathrm{aq})+2 \mathrm{NH}_{3}(\mathrm{aq})+\mathrm{H}_{2}(\mathrm{~g})$

Marking guidance:

Do not accept "hydrogen" without reference to observed changes. Accept "smell of ammonia".

Accept $2 \mathrm{H}^{+}(\mathrm{aq})+\mathrm{Mg}(\mathrm{s}) \rightarrow \mathrm{Mg}^{2+}(\mathrm{aq}) +\mathrm{H}_{2}(\mathrm{~g})$

Equation must be ionic.

[Maximum number: 8]

Chlorine undergoes many reactions.

(a)

2.67 g of manganese(IV) oxide was added to $200.0 \mathrm{~cm}^{3}$ of $2.00 \mathrm{moldm} \mathrm{m}^{-3} \mathrm{HCl}$ .

\mathrm{MnO}_{2}(\mathrm{~s})+4 \mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{Cl}_{2}(\mathrm{~g})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})+\mathrm{MnCl}_{2}(\mathrm{aq})

[ 1 ]

(i)

Deduce, referring to oxidation states, whether $\mathrm{MnO}_{2}$ is an oxidizing or reducing agent.

[ 1 ]

oxidizing agent AND oxidation state of Mn changes from +4 to +2 / decreases $\boldsymbol{}$

(b)

Chlorine gas reacts with water to produce hypochlorous acid and hydrochloric acid.

\mathrm{Cl}_{2}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{HClO}(\mathrm{aq})+\mathrm{HCl}(\mathrm{aq})

[ 3 ]

(i)

Hypochlorous acid is considered a weak acid. Outline what is meant by the term weak acid.

[ 1 ]

(ii)

State the formula of the conjugate base of hypochlorous acid.

[ 1 ]

$\mathrm{ClO}^{-}$

(iii)

Calculate the concentration of $\mathrm{H}^{+}(\mathrm{aq})$ in a HClO(aq) solution with a pH=3.61.

[ 1 ]

« $\left[\mathrm{H}^{+}\right]=10^{-3.61}=$ » $2.5 \times 10^{-4}$ «mol dm ${ }^{-3}$ »

(c)

State the type of reaction occurring when ethane reacts with chlorine to produce chloroethane.

[ 1 ]

«free radical» substitution/ $\mathrm{S}_{\mathrm{R}}$

Marking guidance:

Do not accept electrophilic or nucleophilic substitution.

(d)

Predict, giving a reason, whether ethane or chloroethane is more reactive.

[ 1 ]

chloroethane AND C-Cl bond is weaker/ $324 \mathrm{~kJ} \mathrm{~mol}^{-1}$ than C-H bond $/ 414 \mathrm{~kJ} \mathrm{~mol}^{-1}$

OR

chloroethane AND contains a polar bond

Marking guidance:

Accept "chloroethane AND polar".

(e)

Write the equation for the reaction of chloroethane with a dilute aqueous solution of sodium hydroxide.

[ 1 ]

$\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}(\mathrm{l})+\mathrm{OH}^{-}(\mathrm{aq}) \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}(\mathrm{aq})+\mathrm{Cl}^{-}(\mathrm{aq})$

OR
$\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}(\mathrm{l})+\mathrm{NaOH}(\mathrm{aq}) \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}(\mathrm{aq})+\mathrm{NaCl}(\mathrm{aq})$

Marking guidance:

Accept use of $\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}$ and $\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH} / \mathrm{C}_{2} \mathrm{H}_{6} \mathrm{O}$ in the equation.

(f)

Deduce the nucleophile for the reaction in d(iii).

[ 1 ]

hydroxide «ion»/ $\mathrm{OH}^{-}$

Marking guidance:

Do not accept NaOH.

[Maximum number: 1]

Antioxidants are a type of food additive.

Antioxidants such as BHT and 3-BHA prevent the unwanted reactions of free radicals. State what a free radical is.

[Maximum number: 4]

(a)

Ethanol is a primary alcohol that can be oxidized by acidified potassium dichromate(VI). Distinguish between the reaction conditions needed to produce ethanal and ethanoic acid.

Ethanal:
Ethanoic acid:

[ 2 ]

(b)

Deduce the half-equation for the oxidation of ethanol to ethanal.

[ 1 ]

$\mathrm{CH_3CH_2OH \rightarrow CH_3CHO + 2H^+ + 2e^-}$

Half-equation required. Do not accept:
$\mathrm{C_2H_5OH+2[O]\rightarrow CH_3CHO+H_2O}$

Accept e for $e^-$ .

(c)

Deduce the overall redox equation for the reaction of ethanol to ethanal with acidified potassium dichromate(VI) by combining your answer to part (c) (iii) with the following half-equation:

\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}(\mathrm{aq})+14 \mathrm{H}^{+}(\mathrm{aq})+6 \mathrm{e}^{-} \rightarrow 2 \mathrm{Cr}^{3+}(\mathrm{aq})+7 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})

[ 1 ]

$\mathrm{3CH_3CH_2OH(aq)+Cr_2O_7^{2-}(aq)+8H^+(aq) \rightarrow 2Cr^{3+}(aq)+3CH_3CHO(l)+7H_2O(l)}$

Award [1] for correct reactants and products.
Award [1] for correct balancing;
M2 can only be scored if M1 is correct.
Ignore state symbols.

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Covered syllabus points

Question 1

Question (a)

Question (b)

Question 2

Question (a)

Question (i)

Question (b)

Question (i)

Question (ii)

Question (iii)

Question (c)

Question (d)

Question (e)

Question (f)

Question 3

Question 4

Question (a)

Question (b)

Question (c)