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[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.

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

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

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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