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

Practise IB Chemistry HL 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, buffer and titration evidence.
Assign oxidation states, balance redox half-equations, calculate cell potentials and explain electron flow, cells and electrolysis.
Recognize radicals and write homolytic-fission, propagation and termination steps for alkane substitution.
Predict electrophilic addition and substitution products, distinguish S_N1 and S_N2 mechanisms, and explain benzene substitution.

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: 20]

Bleaches in which chlorine is the active ingredient are the most common, although some environmental groups have concerns about their use.

(a)

Outline, with the help of a chemical equation, why this reaction occurs.

[ 2 ]

chlorine more reactive/more powerful oxidizing agent (than bromine);
$\mathrm{Cl}_{2}(aq)+2\mathrm{Br}^{-}(aq)\rightarrow \mathrm{Br}_{2}(aq)+2\mathrm{Cl}^{-}(aq)$ ;

(b)

In aqueous chlorine the equilibrium below produces chloric(I) acid (hypochlorous acid), HOCl , the active bleach.

\mathrm{Cl}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{HOCl}(\mathrm{aq})+\mathrm{H}^{+}(\mathrm{aq})+\mathrm{Cl}^{-}(\mathrm{aq})

[ 9 ]

(i)

Chloric(I) acid is a weak acid, but hydrochloric acid is a strong acid. Outline how this is indicated in the equation above.

[ 1 ]

(ii)

State a balanced equation for the reaction of chloric(I) acid with water.

[ 1 ]

$\quad \mathrm{HOCl}(\mathrm{aq}) \rightleftharpoons \mathrm{H}^{+}(\mathrm{aq})+\mathrm{ClO}^{-}(\mathrm{aq}) / \mathrm{HOCl}(\mathrm{aq})+\mathrm{H}_{2}\mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{H}_{3}\mathrm{O}^{+}(\mathrm{aq})+\mathrm{ClO}^{-}(\mathrm{aq})$ ;

(iii)

Partial neutralization of chloric(I) acid creates a buffer solution. Given that the $\mathrm{p} K_{\mathrm{a}}$ of chloric(I) acid is 7.53 , determine the pH of a solution that has $[\mathrm{HOCl}]=0.100 \mathrm{~mol} \mathrm{dm}^{-3}$ and $\left[\mathrm{ClO}^{-}\right]=0.0500 \mathrm{~mol} \mathrm{dm}{ }^{-3}$ .

[ 4 ]

$K_{\mathrm{a}}=10^{-7.53}=2.95 \times 10^{-8}\,\mathrm{mol}\,\mathrm{dm}^{-3}$ ;
$K_{\mathrm{a}}=\frac{\left[\mathrm{H}^{+}\right]\left[\mathrm{ClO}^{-}\right]}{[\mathrm{HOCl}]}=\frac{\left[\mathrm{H}^{+}\right](0.05)}{(0.1)} \approx \frac{\left[\mathrm{H}^{+}\right]}{2}=2.95 \times 10^{-8}\,\mathrm{mol}\,\mathrm{dm}^{-3}$ ;
$\left[\mathrm{H}^{+}\right]=2 \times 2.95 \times 10^{-8}=5.9 \times 10^{-8}\,\mathrm{mol}\,\mathrm{dm}^{-3}$ ;
$\mathrm{pH}=-\log\left(5.9 \times 10^{-8}\right)=7.23$ ;

(iv)

Describe, using HIn to represent the indicator in its acid form, why an indicator changes colour when excess alkali is added.

[ 3 ]

$\mathrm{HIn} \rightleftharpoons \mathrm{H}^{+}+\mathrm{In}^{-}$ ;
(weak acid in which the) acid/HIn and conjugate base/In- have different colours / OWTTE;
excess alkali shifts the equilibrium to the RHS/towards the conjugate base;

(c)

Aqueous sodium chlorate(I), NaOCl, the most common active ingredient in chlorine based bleaches, oxidizes coloured materials to colourless products while being reduced to the chloride ion. It will also oxidize sulfur dioxide to the sulfate ion.

[ 6 ]

(i)

Deduce a balanced equation for the reaction between the chlorate(I) ion and sulfur dioxide from the appropriate half-equations.

[ 4 ]

$\mathrm{ClO}^{-}(\mathrm{aq})+2 \mathrm{H}^{+}(\mathrm{aq})+2 \mathrm{e}^{-} \rightleftharpoons \mathrm{H}_{2}\mathrm{O}(\mathrm{l})+\mathrm{Cl}^{-}(\mathrm{aq})$ ;
$\mathrm{SO}_{4}^{2-}(\mathrm{aq})+4 \mathrm{H}^{+}(\mathrm{aq})+2 \mathrm{e}^{-} \rightleftharpoons \mathrm{SO}_{2}(\mathrm{aq})+2 \mathrm{H}_{2}\mathrm{O}(\mathrm{l})$ ;
final equation: $\mathrm{ClO}^{-}(\mathrm{aq})+\mathrm{SO}_{2}(\mathrm{aq})+\mathrm{H}_{2}\mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{SO}_{4}^{2-}(\mathrm{aq})+2 \mathrm{H}^{+}(\mathrm{aq})+\mathrm{Cl}^{-}(\mathrm{aq})$ ;

(ii)

State the initial and final oxidation numbers of both chlorine and sulfur in the final equation.

Table for Question (ii) — IB Chemistry HL

[ 2 ]

Element	Initial oxidation number	Final oxidation number
Chlorine	+I / +1	-I / -1
Sulfur	+IV / +4	+VI / +6

(d)

The standard electrode potential for the reduction of the chlorate(V) ion to the chloride ion is +1.49 V .

[ 3 ]

(i)

Define the term standard electrode potential.

[ 1 ]

(ii)

Referring to Table 14 of the Data Booklet, deduce, giving a reason, whether the oxidation of the chromium(III) ion to the dichromate(VI) ion by the chlorate(V) ion is energetically feasible.

[ 2 ]

yes / energetically feasible;
would have a positive $E_{\text{cell}}$ / chlorate(V) ion stronger oxidizing agent than dichromate(VI) ion;

[Maximum number: 8]

Phosphine (IUPAC name phosphane) is a hydride of phosphorus, with the formula $\mathrm{PH}_{3}$ .

(a)

Deduce, giving your reason, whether phosphine would act as a Lewis acid, a Lewis base, or neither.

[ 1 ]

(b)

Ammonia acts as a weak Brønsted-Lowry base when dissolved in water.

\mathrm{NH}_{3}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{NH}_{4}^{+}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq})

Outline what is meant by the terms "weak" and "Brønsted-Lowry base".

Weak:

Brønsted-Lowry base:

[ 2 ]

(c)

Phosphine is usually prepared by heating white phosphorus, one of the allotropes of phosphorus, with concentrated aqueous sodium hydroxide. The equation for the reaction is:

\mathrm{P}_{4}(\mathrm{~s})+3 \mathrm{OH}^{-}(\mathrm{aq})+3 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{PH}_{3}(\mathrm{~g})+3 \mathrm{H}_{2} \mathrm{PO}_{2}^{-}(\mathrm{aq})

[ 5 ]

(i)

The ion $\mathrm{H}_{2} \mathrm{PO}_{2}^{-}$ is amphiprotic. Outline what is meant by amphiprotic, giving the formulas of both species it is converted to when it behaves in this manner.

[ 2 ]

can act as both a «Brønsted-Lowry» acid and a «Brønsted-Lowry» base OR can accept and/or donate a hydrogen ion/proton/ $\mathrm{H}^{+}$ $\mathrm{HPO}_{2}{ }^{2-}$ AND $\mathrm{H}_{3} \mathrm{PO}_{2}$

(ii)

Oxidation is now defined in terms of change of oxidation number. Explore how earlier definitions of oxidation and reduction may have led to conflicting answers for the conversion of $\mathrm{P}_{4}$ to $\mathrm{H}_{2} \mathrm{PO}_{2}^{-}$ and the way in which the use of oxidation numbers has resolved this.

[ 3 ]

[Maximum number: 2]

The Sun's energy is produced by the fusion of hydrogen nuclei.

Nuclear energy produces ionizing radiation which leads to the formation of free radicals.

Explain why free radicals are harmful to living cells.

[Maximum number: 8]

Some reactions of but-2-ene are given below.

(a)

Deduce the full structural formula of compound A.

[ 1 ]

(b)

Describe the colour change observed when excess but-2-ene reacts with bromine to form compound A.

[ 1 ]

(c)

Compound $\mathbf{C}, \mathrm{C}_{4} \mathrm{H}_{9} \mathrm{OH}$ , can also be formed by reacting compound B, $\mathrm{CH}_{3} \mathrm{CHBrCH}_{2} \mathrm{CH}_{3}$ , with aqueous potassium hydroxide. This reaction proceeds by both $\mathrm{S}_{\mathrm{N}} 1$ and $\mathrm{S}_{\mathrm{N}} 2$ mechanisms. Explain the $\mathrm{S}_{\mathrm{N}} 2$ mechanism, using curly arrows to represent the movement of electron pairs.

[ 4 ]

(d)

Explain why the hydroxide ion is a better nucleophile than water.

[ 2 ]

$\mathrm{OH}^{-}$ has a negative charge/higher electron density;
stronger attraction to the carbon atom with the partial positive charge / OWTTE;

All question bank results loaded

Covered syllabus points

Question 1

Question (a)

Question (b)

Question (i)

Question (ii)

Question (iii)

Question (iv)

Question (c)

Question (i)

Question (ii)

Question (d)

Question (i)

Question (ii)

Question 2

Question (a)

Question (b)

Question (c)

Question (i)

Question (ii)

Question 3

Question 4

Question (a)

Question (b)

Question (c)

Question (d)