科目：化学 Chemistry (9701)
年份：2020 夏季
试卷：Paper 41
总页数：18
章节：19 Lattice energy, 20 Electrochemistry, 21 Further aspects of equilibria, 22 Reaction kinetics, 23 Entropy and Gibbs free energy, 24 Transition elements, 25 Benzene and its compounds, 26 Carboxylic acids and their derivatives, 27 Organic nitrogen compounds, 29 Organic synthesis, 30 Analytical chemistry
Answer all the questions in the spaces provided.
1. (a) An aqueous solution of cobalt(II) contains the $[\text{Co(H}_2\text{O})_6]^{2+}$ complex ion.
(i) Define the term complex ion.
[1]
(ii) Samples of $[\text{Co(H}_2\text {O})_6]^{2+}$ are reacted separately with aqueous sodium hydroxide and with an excess of aqueous ammonia.
Give the following information about these reactions.
- the reaction of $[\text{Co(H}_2{\text{O}})_6]^{2+}$ with aqueous sodium hydroxide
colour and state of the cobalt-containing species ...
ionic equation ...
type of reaction ...
- the reaction of $[\text{CO(H}_2\text{O})_6]$ with an excess of aqueous ammonium
colour and state of the cobalт-containing species ...
ionic equation ...
type of reaction .(c) The $[\text{Co}(\text{NH}_3)_3\text{Cl}l_3]$ complex shows stereoisomerism.
Complete the three-dimensional diagrams to show the two isomers of $[\text{Co}(\text{NH}_{3})_{3}\text{Cl}l_3]$.
Suggest the type of stereoisomerism.
type of stereoisomerism ..... [2]
(d) Compound X, $C_6H_{18}N_4$, is a tetradentate ligand.
(i) Suggest why one molecule of X can form four dative bonds.
[1]
(ii) $C_6H_{18} N_4$ reacts with aqueous cobalt(II) ions, $[\text{Co}(\text{H}_2\text{O})_6]^{2+}$, in a 1:1 ratio to form a new complex ion.
Construct an equation for this reaction.
[1]
[Total: 13]2 (a) (i) Describe and explain the trend in the solubility of the Group 2 hydroxides down the group. [4] Group 2 hydroxides decompose on heating to give the corresponding metal oxide and water vapour. (ii) Suggest which of Mg(OH)$_2$ and Sr(OH)$_2$ will decompose at a lower temperature. Explain your answer. [Total: 6]
3 The overall reaction for photosynthesis is shown.
$$6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$$
Water is oxidised in this process according to the following half-equation.
$$2H_2O \rightarrow O_2 + 4H^+ + 4e^{-}$$
half-equation 1
(a) (i) Use these equations to deduce the half-equation for the reduction of carbon dioxide in this process.
[2]
(ii) Draw a fully labelled diagram of the apparatus that should be used to measure the standard electrode potential, $E^\circ$, of $\text{O}_2(\text{g})$ in half-equation 1 under standard conditions. Include all necessary chemicals.
[4]
(iii) For the cell drawn in (a)(ii), use the Data Booklet to calculate the $E^\circ_{\text{cell}}$ and deduce which electrode is positive.
$$E^\circ_{\text{cell}} = \dots\dots\dots\dots\ V$$
identity of 4 (a) The molecular formulae of three nitrogen-containing compounds are given. S $C_6H_5CONH_2$ T $C_6H_5NH_2$ U $C_6H_5CH_2NH_2$ Describe and explain the relative basicities of S, T and U. most basic > least basic [3] (b) Compound U can be prepared by two different methods as shown. (i) Suggest reagents and conditions for reaction 1 and for reaction 2. reaction 1 ... reaction 2 ... [2] (ii) State the type of reaction in reaction 1 and name the mechanism in reaction 2. type of reaction in reaction 1 ... mechanism of reaction 2 ... [2] [Total: 7]
5 (a) Benzene reacts with bromine in the presence of an aluminium bromide catalyst, $AlBr_3$, to form bromobenzene. This is a substitution reaction. No addition reaction takes place. (i) Explain why no addition reaction takes place. $AlBr_3$ reacts with bromine to generate an electrophile, Br$^+$. (ii) Draw the mechanism of the reaction between benzene and Br$^+$ ions. Include all relevant arrows and charges. [3] (iii) Write an equation to show how the $AlBr_3$ catalyst is reformed. [1] (b) Suggest why bromination of phenol occurs more readily than bromination of benzene. [2]
(c) (i) There are four different carbocations with the same formula, $C_4H_9^+$. One structure is given in the table.
Suggest the structural formulae of the three other carbocations.
| structure 1 | structure 2 | structure 3 | structure 4 |
| :---: | :---: | :---: | :---
$CH_3CH_2CH_2CH_2^+$
(ii) Benzene reacts with each of these carbocations in separate Friedel-Crafts alkylation reactions.
In each reaction an organic compound with formula $C_{10}H_{14}$ is formed. The number of peaks observed in the carbon-13 NMR spectrum of each compound is given.
Suggest the structures for the three other compounds.
number of peaks in carbon-13 NMR = 8
number of peaks in carbon-13NMR = 6
number of peaks in carbon-15 NMR = 7
number of peaks in carbon-17 NMR = 8
[Total: 14]6 (a) Compare and explain the relative acidities of 2-chloropropanoic acid, 3-chloropropanoic acid, and propanoic acid. Explain your answer.
| most acidic | least acidic |
| :---: | :---: |
explanation ..... [3]
(b) (i) The numerical values of $K_a$ for methanoic acid, HCO$_2$H, and pyruvic acid, CH$_3$COCO$_2$H, are given.
$$\begin{array}{c|c}
\text{acid} & K_a \\
\hline
\text{HCO}_2\text{H} & 1.78 \times 10^{-4} \\
\text{CH}_3\text{COCO}_2\text{H} & 4.07 \times 10^{-3}
\end{array}$$
An equilibrium mixture containing the two acid-base pairs is formed.
$$\text{HCO}_2^- + \text{CH}_3\text{COCO}_�^- \rightleftharpoons \text{HCO}_2\text{H} + \text{CH}_3\text{ COCO}_�^-$$
Use the $K_a$ values to calculate the equilibrium constant, $K_{eq}$, for this equilibrium.
$$K_{eq} = ...$$
(ii) (iii) Ethanedioic acid, HO$_2$CCO$_2$H, has two dissociation constants, $K_{a1}$ and $K_{a2}$, whose p$K_a$ values are 1.23 and 4.19.
Suggest equations to show the two dissociations that give rise to these p$K_a$ values.
$$pK_{a1} = 1.23 \quad pK_{a2} = 4.19$$
(iv) State the mathematical relationship between p$K_a$ and the acid dissociation constant $K_a$.
(c) Three tests were carried out on separate samples of the organic acids shown in the table. The following results were obtained.
- $\checkmark$ = observed change
- $x$ = no observed reaction
| test | reagent(s) and conditions | HCO$_2$H | CH$_3$COCO$_2$H | HO$_2$CCO$_2$h | observed change |
| :--- | :---: | :---: | :---: | :-: | :-: |
| 1 | ... | ✓ | ✗ | ✗ | |
| 2 | ... | ✗ | ✓ | ✗ | |
| 3 | ... | ✓ | ✗ |✓ | |
Complete the table w(d) A sample of pyruvic acid, CH$_3$COCO$_2$H, is analysed by carbon-13 NMR spectroscopy. Three peaks are observed. Complete the table by: • circling the carbon atom responsible for the chemical shift • stating the hybridisation of the circled carbon atom. | chemical shift ($\delta$) | carbon atom responsible for chemical shift | hybridisation of the circled carbon atom | | :--- | :---: | :---: | | 27 |  | | | 163 | | [2]
(e) An ester of pyruvic acid, F, is dissolved in CDCl$_3$ and analysed by proton NMR spectroscopy.
The proton NMR spectrum of F is shown.
Use the proton NMR spectrum of F to complete the table.
| chemical shift ($\delta$) | group responsible for the peak | splitting pattern | number of $^1\text{H}$ atoms responsible for the peak |
| :--- | :---: | :---: | :---: |
| 1.3 | | | |
| 2.2 | | | |
| 4.0 | | | |(f) Deuterium oxide, D$_2$O, where D is $^2\text{H}$, can be used as a solvent in proton NMR spectroscopy. The proton NMR spectrum of alanine in CDCl$_3$ has 4 peaks. The proton NMR spectrum of alanINE in D$_2$O has 2 peaks.
alanine
$$\text{NH}_2 \\
\text{H}_3\text{C} \longrightarrow \text{C} \longrightarrow \text{\text{COOH}} \\
\text{H}$$
On the diagram of alanine, circle the protons that show peaks in both NMR spectra. Explain your answer.
[2]
(g) The ionic product, $K_w$, for D$_2$O has a value of $1.35 \times 10^{-15}\text{mol}^2\text{dm}^{-6}$ at 298 K.
(i) Write the expression for the $K_w$ of D$_2$O.
$$K_w =$$
[1]
(ii) Calculate the pH of pure, neutral D$_2$O at 298 K. Assume [D$^+$] is equivalent to [H$^+$] for pH calculations.
pH = ..... [2]
[Total: 23]7 (a) Silver carbonate, Ag$_2$CO$_3$, is sparingly soluble in water. The numerical value of the solubility product, $K_{\text{sp}}$, for silver carbonate is $6.3 \times 10^{-12}$ at $25^{\circ}\text{C}$.
(i) Write an expression for the solubility product, $K_{\mathrm{sp}}$, of Ag$_2$CO$_3$, and state its units.
$$K_{\mathrm{sp}} =$$
units = ..... [2]
(ii) Calculate the equilibrium concentration of Ag$^+$ in a saturated solution of Ag$_2$CO$_3$ at $25^{\circ}\text{\text{C}}$.
$$[\text{Ag}^+] = \dots\dots\dots\dots\ldots\ldots\ldots\ldotting\ldots\ldots\ldots\text{mol dm}^{-3}$$ [1]
(iii) Solid Ag$_2$CO$_3$ is stirred at $25^{\circ}\text{"C}$ with $0.050\text{ mol dm}^{-3}\text{AgNO}_3$ until no more Ag$_2$CO$_3$ dissolves.
Calculate the concentration of carbonate ions, $$[\text{CO}_3(b) Silver chloride, AgCl is sparingly soluble in water. The equation for the enthalpy change of solution is shown.
$$\text{AgCl(s)} \rightarrow \text{Ag}^+(\text{aq}) + \text{Cl}^-(\text{aq}) \quad \Delta H_{\text{sol}}^e = +65.5\text{ kJ mol}^{-1}$$
Standard entropies are shown in the table.
| species | AgCl(s) | Ag⁺(aq) | Cl⁻(aq) |
| :--- | :---: | :---: | :---: |
| $S^\circ$/JK$^{-1}$mol$^{-1} | +96.2 | +72.7 | +56.5 |
(i) Calculate the standard entropy change of solution, $\Delta S^\circ$.
$$\Delta S^\circ = ... \text{ J K}^{-1}\text{ mol}^{-1}$$ [1]
(ii) Explain, with the aid of a calculation, why AgCl is insoluble in water at 25°C.
You should use data from this question and your answer to (b)(i).
[Total: 10]8 (a) Explain what is meant by the term buffer solution.
[2]
(b) (i) Write an expression for the acid dissociation constant, $K_a$, for ammonium ions, $\text{NH}_4^+(\text{aq})$.
$$K_a =$$
[1]
(ii) Write two equations to describe how a solution containing ammonium ions, $\text{ NH}_4^+(\text{aq})$, and ammonia, $\text{NH}_3(\text{aq})$, can act as a buffer.
[2]
(iii) The numerical value of $K_a$ for $\text{NH}_4^+(\mathrm{aq})$ is $5.6 \times 10^{-10}$ at 298 K.
A buffer solution was prepared by adding 0.80 dm³ of 0.25 mol dm⁻³ ammonia, an excess, to 0.20 dm³ of 0.120 mol dm⁻³ hydrochloric acid.
Calculate the pH of the buffer solution formed at 298 K.
pH = ... [3]
[Total: 8]9 (a) Manganese(IV) oxide, MnO$_2$, catalyses the decomposition of hydrogen peroxide, H$_2$O$_2$, as shown.
$$2\text{H}_2\text{O}_2(\text{aq}) \xrightarrow{\text{MnO}_2} 2\text{H}_2\text{O}(\text{l}) + \text{O}_2(\text{g})$$
The mechanism involves the formation of the intermediate species, Mn$^{2+}$, in the first step which is subsequently used up in the second step.
State and use relevant electrode potentials, $E^\circ$, to construct two equations to show how MnO$_2$ can catalyse this reaction.
equation 1 ... [3]
(b) The equation for the decomposition of hydrogen peroxide without a catalyst is shown.
$$2\text{H}_2\text{O}_2(\text{aq}) → 2\text{H}_2\text{ O}(\text{l}) + \text{ O}_2(\text{g})$$
Under certain conditions this reaction is found to be first order with respect to hydrogen(c) A four-step mechanism is suggested for the reaction between hydrogen peroxide and iodide ions in an acidic solution. step 1 $H_2O_2 + I^- \rightarrow IO^- + H_2O$ step 2 $H^+ + IO^- \rightarrow HIO$ step 3 $HIO + I^- \rightarrow I_2 + OH^-$ step 4 $OH^- + H^+ \rightarrow H_2O$ Step 1 is the rate-determining step. (i) State what is meant by the term rate-determining step. [1] (ii) Use this mechanism to construct a balanced equation for this reaction. [1] (iii) Deduce the order of reaction with respect to each of the following. $$H_2O_2 = ... \quad I^- = ... \quad H^+ = ...$$ [Total: 7]
10 (a) The electronic configuration of transition element $Q$ is [Ar] 3d² 4s². Predict the likely oxidation states of element $Q$ in compounds. [1] (b) Suggest why transition elements often show variable oxidation states in their compounds, but typical s-block elements such as calcium do not. [1] (c) Many enzymes contain transition element complexes. Describe, with the aid of a suitably labelled diagram, how an enzyme catalyses the breakdown of a substrate molecule. [Total: 5]