IB Chemistry HL 1.6 Chemical Bonding and Structure

[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 ]

(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 ]

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

Equilibrium sign required for the mark.

Marking guidance:

Ignore state symbols.

(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 ]

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

Marking guidance:

Accept other methods of carrying out the calculation.
Award [4] for correct final answer.

(iv)

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

[ 3 ]

$\mathrm{HIn} \rightleftharpoons \mathrm{H}^{+}+\mathrm{In}^{-}$ ;

Marking guidance:

Do not accept equation without equilibrium arrow.
(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 ]

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

Marking guidance:

Accept $\mathrm{SO}_{4}{ }^{2-}(a q)+4 \mathrm{H}^{+}(a q)+2 e^{-} \rightleftharpoons \mathrm{H}_{2} \mathrm{SO}_{3}(a q)+\mathrm{H}_{2} \mathrm{O}(l)$ .
For final equation:
$\mathrm{ClO}^{-}(\mathrm{aq})+\mathrm{SO}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{SO}_{4}{ }^{2-}(\mathrm{aq})+2 \mathrm{H}^{+}(\mathrm{aq})+\mathrm{Cl}^{-}(\mathrm{aq})$
Accept $\mathrm{ClO}^{-}(a q)+\mathrm{H}_{2} \mathrm{SO}_{3}(a q) \rightleftharpoons \mathrm{SO}_{4}{ }^{2-}(a q)+2 \mathrm{H}^{+}(a q)+\mathrm{Cl}^{-}(a q)$ .
correct reactants and products;
balancing and cancelling $\mathrm{e}^{-}, \mathrm{H}^{+}$ and $\mathrm{H}_{2} \mathrm{O}$ ;

Apply ECF if incorrect half-equations written.
Ignore state symbols and absence of equilibrium arrow for all equations and accept inclusion of $\mathrm{Na}^{+}$ in any equation.
(ii) Award [2] for all correct, [1] for 2 or 3 correct.

(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 ]

(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 ]

potential (of reduction half-reaction) under standard conditions measured relative to standard hydrogen electrode/SHE / OWTTE; Allow "solute concentration of $1 \mathrm{~mol} \mathrm{dm}{ }^{-3}$ " or " $1 \mathrm{bar} / 1 \mathrm{~atm}$ (pressure) for gases" instead of "standard conditions".

(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 / OWTTE;

[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: 9]

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

Figure for Question Reactivity 3. What are the mechanisms of chemical change? question 4 — IB Chemistry HL

(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 ]

Figure for Answer to Question (c) — IB Chemistry HL

curly arrow going from lone pair/negative charge on O in $\mathrm{HO}^{-}$ to C ;
Do not accept curly arrow originating on H in $\mathrm{HO}^{-}$ .
curly arrow showing Br leaving;
Accept curly arrow either going from bond between C and Br to Br in 2-bromobutane or in the transition state.
Accept if arrow goes from C-Br bond to/or beyond Br .
representation of transition state showing negative charge, square brackets and partial bonds;
Do not penalize if HO and Br are not at $180^{\circ}$ to each other.
Do not award M3 if OH----C bond is represented.
formation of organic product $\mathrm{CH}_{3} \mathrm{CHOHCH}_{2} \mathrm{CH}_{3}$ and $\mathrm{KBr} / \mathrm{Br}^{-}$ ;

(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;

Marking guidance:

Do not accept just stronger attraction.
Reference to carbon atom needed for M2.

(e)

The organic product of the reaction in part (d) (i) can be reduced to:

Figure for Question (e) — IB Chemistry HL

State the two reagents required.

[ 1 ]

hydrogen $/ \mathrm{H}_{2}$ and nickel/ Ni ;

Marking guidance:

Accept other suitable metal catalysts such as platinum/Pt, palladium/Pd.

IB Chemistry HL 1.6 Chemical Bonding and Structure Question Bank

Covered syllabus points

Covered syllabus points

Reactivity 3. What are the mechanisms of chemical change? 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)

Reactivity 3. What are the mechanisms of chemical change? question 2

Question (a)

Question (b)

Question (c)

Question (i)

Question (ii)

Reactivity 3. What are the mechanisms of chemical change? question 3

Reactivity 3. What are the mechanisms of chemical change? question 4

Question (a)

Question (b)

Question (c)

Question (d)

Question (e)