Al Chemistry 1995 Paper 1+2

1995 Paper I Section A Question 1 0 0The first four successive ionization energies of an element A are 578, 1817, 2746 and 10813 kJ mol-1 respectively. To which group in the Periodic Table does A belong? 1The relative atomic mass of bromine is 79.90. It has two isotopes with mass numbers 79 and 81. Calculate the relative abundance of each isotope. (3 marks) 1 0What can you deduce from the fact that the spectral lines in the atomic emission spectrum of hydrogen are not equally spaced? 1In the atomic emission spectrum of hydrogen, the convergence limit for the Lyman series occurs at 3.275x1015 Hz. Calculate the ionization energy of hydrogen, in kJmol-1. (Planck constant, h = 6.626x10-34 Js; Avogadro constant, L = 6.023x1023 mol-1) (3 marks) 2Account for the fact that the carbon-oxygen bond lengths in CO, CO2 and CO32- are 0.113, 0.116 and 0.129 nm respectively. (3 marks) 3The iodination of propanone is catalyzed by hydrogen ions. The overall equation is: CH3COCH3(aq) + I2(aq) CH3COCH2I(aq) + HI(aq) Using four mixtures B, C, D and E, the progress of the reaction was followed by colorimetric measurement. The results are tabulated below. Composition by volume of mixture/cm3

Mixture

Initial rate/moldm-3s1

B C D E

Propanone

Water

1.00 M HCl

10.0 10.0 5.0 10.0

60.0 50.0 65.0 65.0

10.0 10.0 10.0 5.0

0.05 M I2 in KI 20.0 30.0 20.0 20.0

4.96x10-6 5.04x10-6 2.45x10-6 2.47x10-6

0Determine the effects the changes in concentration of each of the reactants (iodine and propanone) and the catalyst (hydrochloric acid) on the reaction rate. Write the rate expression for the reaction. 1For mixture B, calculate the rate constant for the reaction at the temperature of the experiment. (Density of CH3COCH3 = 0.789 gcm-3) (4 marks) 4The reaction between ethanoic acid and ethanol can be represented by the following equation: CH3COOH(l) + C2H5OH(l) CH3COOC2H5(l) + H2O(l) 12.01 g of ethanoic acid are treated with 4.61 g of ethanol in the presence of a catalyst. When the

reaction reaches equilibrium at 298 K, 5.04 g of ethanoic acid are found to have reacted. 0Name a suitable catalyst for this reaction in the forward direction. 1Calculate the equilibrium constant, Kc, for the reaction at 298 K. 2What additional mass of ethanol would be required in order to use up a further 0.60 g of ethanoic acid? 3Would the addition of more of the same catalyst affect the value of Kc? Explain. (7 marks) Question 2 0Explain why phosphorus can form PCl3 and PCl5, while nitrogen can form only NCl3. (2 marks) 1 0What is the essential feature of a ‘redox reaction’? 1Is the reaction below a ‘redox reaction’ or an ‘acid-base reaction’? Explain. CaO + SiO2 CaSiO3 (3 marks) 2Manganate(VI) ions readily undergo disproportionation in aqueous solutions, except under very basic conditions. Write a balanced equation for this disproportionation, and give the oxidation state of manganese in the reaction product(s). (3 marks) 3 i) State whether the following hydrides would react with water under room temperature and atmospheric pressure. If there is a reaction, give the balanced equation. CH4 SiH4 H2S HCl ii) Explain the difference in reactivity with water between CH4 and SiH4 iii) Explain the difference in reactivity with water between SiH4 and H2S. (6 marks) 4For each of the following species, draw a three-dimensional structure showing the bond electron pairs and lone electron pairs of the central atom. State the shape of the species in each case. i) ICl4ii) SCl2 (3 marks) 5 0Show the electronic configuration of a ground state Cr2+ ion by filling in the appropriate boxes below.

1s

2s

2p

3s

3p

3d

4s

4p

4d

1What is the highest possible oxidation state of chromium? 2Give a chromium compound in which chromium is in its highest oxidation state. (3 marks) Question 3

0Arrange the following carbocations in the order of increasing stability. Explain your arrangement. + CH2+ CH2+

(3 marks) 1Arrange the following carboxylic acids in the order of increasing acidity. Explain your arrangement. ClCH2CO2H ClCH2CH2CO2H FCH2CO2H (3 marks) 2 0Give the structure of the major product formed from the following reaction: CH3 + HBr

1Outline a mechanism for the above reaction. (Movement of electron pairs should be indicated by curly arrows.) (3 marks) 3 0Give the reactants and conditions for the preparation of benzenediazonium chloride in the laboratory. 1Write the structure of the product formed in the diazocoupling of naphthalen-2-ol with benzenediazonium chloride. 2If an aqueous solution of benzenediazonium chloride is heated, a solid, which is soluble in dilute NaOH, can be obtained. Suggest a structure for the solid obtained and account for its solubility in dilute NaOH. (5 marks) 4Use equations to show how you would carry out the following conversions in the laboratory. For each conversion, give the reagent(s), conditions and structure of the intermediate compound(s) formed. 0(CH3)3COH to (CH3)3COCH2CH3 O O 1CH2=CH2

to

COCH2CH2OC

(6 marks) Section B Question 4 0Devise an experiment to distinguish between KBr(s) and a mixture containing approximately 40 % KCl(s) and 60 % KBr(s) by mass. (3 marks) 1 0Describe the experimental procedure by which you would recrystallize a sample of impure benzoic acid. 1How would you test the purity of the product obtained in i)? (4 marks)

2You are provided with a Bunsen burner, a beaker, a thermometer and a set of quickfit apparatus. Draw a labeled diagram to show how selected items from the above are set up for the determination of the boiling point of propanone. (3 marks) 3Name the type of hazard warning label which should be displayed on a bottle containing propanone. (1 mark) 4For each of the volumetric analyses i) to iii), state whether an indicator is required. If an indicator is required, select the appropriate one from those given below: Litmus; methyl orange; phenolphthalein; potassium dichromate(VI) solution; starch solution 0ethanedioic acid titrated with sodium hydroxide 1sulphuric(VI) acid titrated with aqueous ammonia 2iron(II) sulphate(VI) titrated with potassium manganate(VII) (3 marks) 5In the volumetric determination of chloride ions with silver nitrate(V) in neutral solutions, potassium chromate(VI) can be used as an indicator. 0Explain the action of this indicator. 1Why is this titration not carried out in strongly acidic or strongly basic conditions? (3 marks) 6Give the observations when i) ammonia solution and ii) sodium hydroxide solution are added dropwise, until in excess, to aqueous solutions containing Pb2+ and Cu2+ ions respectively. These experiments are carried out at room temperature. i) ammonia solution

ii) sodium hydroxide solution

2+

Pb (aq) Cu2+(aq) (3 marks) Section C Question 5 Write an essay on hydrogen bonding. (20 marks) Question 6 Write an essay on transition metal complexes, covering their stability, stereochemistry and the quantitative investigation of their stoichiometry. (20 marks) Question 7 Write an essay on the synthesis and the uses of poly(alkenes). Your essay should include a mechanism for the formation of poly(ethene) and the reasons why poly(alkenes) are widely used. (20 marks)

1995 Paper II Section A Question 1 0 0Assuming that both hydrogen gas and oxygen gas behave ideally, calculate the work done when 1 mol of water is formed from the combustion of hydrogen at 1 atm and 298 K. 1When 1.00 g of hydrogen is burnt completely in a container of constant volume at 298 K, the energy released is 140.3 kJ. Calculate the molar enthalpy change of formation of water at 298 K. (6 marks) 1You are provided with the following thermochemical data:

Ag(s) + aq Ag+(aq) + e1/2N2(g) + 3/2O2(g) + aq + e1/2Cl2(g) + aq + eCl-(aq) Ag(s) + 1/2Cl2(g) AgCl(s)

∆Ho/kJmol-1 +105.56 -207.36 -167.15 -127.07

NO3-(aq)

Calculate the standard enthalpy change for the reaction AgNO3(aq) + HCl(aq) AgCl(s) + HNO3(aq) (4 marks) 2Two miscible liquids A and B form ideal solutions upon mixing. The boiling point of A is higher than that of B. 0Draw the boiling point-composition graph at 1 atm for mixtures of A and B and label the graph in detail. 1Using your graph, explain how liquid B can be separated from a mixture containing A and B, by fractional distillation. (7 marks) Question 2 0

228 88

Ra decays by the emission of β particles. The half-life for the decay is 6.67 years.

0Write a balanced equation to represent the decay of 1A sample containing 0.50 g of

228 88

228 88

Ra .

Ra is kept in a closed container. Calculate the mass of

228 88

Ra

remaining after 5 years. (The decay of a radioisotope can be described by the equation, Nt = Noe-kt, where No and Nt respectively represent the initial number and number at time t of the radioisotope, and k is the decay constant.) (4 marks) 1In an aqueous solution, the decomposition of hydrogen peroxide in the presence of manganese(IV) oxide can be represented by the following equation: 2H2O2(aq)

MnO2

2H2O(l) + O2(g)

0For a given amount of manganese(IV) oxide, outline how you would use a chemical method to determine the concentration of hydrogen peroxide, at different times, in the reaction mixture. 1How would you use the results obtained in i) to show graphically that the decomposition is first order with respect to H2O2 and to find the rate constant of the decomposition? (6 marks) 2The following reversible reaction occurs in an aqueous solution of ammonium ethanoate: CH3COO-(aq) + NH4+(aq) CH3COOH(aq) + NH3(aq) Reaction 1 0Write an expression for the dissociation constant, Ka, of ammonium ion. 1Calculate the equilibrium constant of reaction 1 at 298 K. 2For a 0.10 M solution of ammonium ethanoate at 298 K, calculate 0the concentration of ammonia, and 1the pH of the solution. (At 298 K, the dissociation constants of ethanoic acid and ammonium ion are 1.76x10-5 moldm-3 and 5.59x10-10 moldm-3 respectively.) (7 marks) Question 3 0 0Derive an expression for the molar mass M of an ideal gas, in terms of its density d, pressure P, and absolute temperature T. 1At 98.6 kPa and 300 K, the density of a sample of dry air is 1.146 gdm-3. Assuming that dry air contains only nitrogen and oxygen and behaves ideally, calculate the composition of the sample. (5 marks) 0 0At 298 K, the solubility product of calcium ethanedioate is 2.3x10-9 mol2dm-6. Predict, with explanation, whether or not calcium ethanedioate will be precipitated when equal volumes of 2.5x10-2 M calcium nitrate(V) and 4.0x10-6 M sodium ethanedioate are mixed together at 298 K. 1Explain why calcium ethanedioate is more soluble in dilute hydrochloric acid than in water. (4 marks) 1The electromotive force of a new zinc-carbon dry cell is 1.5 V. When it is producing an electric current, the following changes occur at the two electrodes: Anode: Zn(s) reacts to give Zn2+(aq). Cathode: MnO2(s) and NH4Cl(aq) react to give Mn2O3(s) and NH3(g). 0Write half equations for the reactions at the anode and at the cathode, and the equation for the overall reaction that occurs in the dry cell. 1Write the cell diagram for the dry cell, using the IUPAC convention. 2 Explain why the electromotive force of the dry cell drops, 0after it has been used for some time; 1after it has been stored for a long time without being used. (8 marks) Section B Question 4 0Explain the following facts:

0The boiling points of the halogens increase as the group is descended. 1The bond dissociation energy of F2 is less than that of Cl2. 2Iodine is more soluble in aqueous potassium iodide solution than in water. (6 marks) 0Give the oxidation states of iodine in the two equations below. Balance the equation in each case. 0S2O32-(aq) + I2(aq) S4O62-(aq) + I-(aq) 1I-(aq) + IO3-(aq) + H+(aq) I2(aq) + H2O(l) (4 marks) 1Consider the data given below for the hydrogen halides and answer the questions that follow. Standard enthalpy change of formation/kJmol-1 -269.4 -92.8 -36.8 +26.1

H-F H-Cl H-Br H-I

Bond dissociation energy/kJmol-1 +562 +430 +367 +298

0Explain briefly the trend in the bond dissociation energy of the hydrogen halides. 1At temperatures above 400 K, hydrogen iodide decomposes to produce violet fumes, but, hydrogen chloride and hydrogen bromide do not decompose. Briefly explain this difference and write a balanced equation for the decomposition of HI. (7 marks) Question 5 0The table below lists some properties of the alkali metals. Element

Atomic radius/nm

Ionic radius/nm

Li Na K Rb Cs

0.123 0.157 0.203 0.216 0.235

0.060 0.095 0.133 0.148 0.169

First ionization energy/kJmol-1 520 495 418 403 374

Standard electrode potential/V -3.04 -2.71 -2.92 -2.93 -2.95

Melting point/ °C 180 98 64 39 29

0Explain why the atomic radii increase from Li to Cs. 1Explain why the atomic radius is larger than the ionic radius in each element and why the ratio, atomic radius: ionic radius is greatest in the case of Li. 2Explain why the ionic radius of K+ is larger than that of Ca2+, although they have the same electronic configuration. 3Explain the trend in the first ionization energy from Li to Cs. 4With reference to the standard electrode potentials and the melting points of the elements, explain why the reactivity of the alkali metals with water increases as the group is descended. (12 marks) 1Explain the following facts: i) The thermal stability of sodium hydroxide is higher than that of magnesium hydroxide. ii) The thermal stability of the carbonates of the alkaline earth metals increases as the group is descended.

(5 marks) Question 6 0Write the electronic configuration of a titanium atom in its ground state. (1 mark) 1Give one industrial use of titanium and one industrial use of a compound of titanium. (2 marks) 2In aqueous solutions, TiO2+ is colourless. It can be reduced to give a violet solution containing [Ti(H2O)6]3+. i) Draw the three-dimensional structure of [Ti(H2O)6]3+. ii) Explain the colour change from TiO2+ to [Ti(H2O)6]3+. iii) The violet solution formed should be kept in a sealed vessel or handled in an inert atmosphere. Using the following data, predict, giving a balanced equation, what will happen to the violet solution if it is not kept in a sealed vessel or not handled in an inert atmosphere.

3+

-

2+

[Ti(H2O)6] (aq) + e [Ti(H2O)6] (aq) 2+ + TiO (aq) + 5H2O(l) + 2H (aq) + e[Ti(H2O)6]3+(aq) O2(g) + 2H2O(l) + 4e 4OH (aq) O2(g) + 4H+(aq) + 4e2H2O(l)

Eo/V -0.37 +0.11 +0.40 +1.23 (8 marks)

3You are provided with some tin granules, a source of chlorine gas, and common apparatus and chemicals available in a school laboratory. Briefly describe how you would prepare a sample of anhydrous tin(IV) chloride. Give a balanced equation for the reaction and draw a labeled diagram of the laboratory set-up. (6 marks) Section C Question 7 0Suggest a chemical test to distinguish one compound from the other in each of the following pairs. Your answer should include the reagents used, the observation expected and the chemical equation(s) for each test. Cl

0

Cl

and

O

O 1 CH CH CCH CH 3 2 2 3

and CH CH CH CCH 3 3 2 2

CH3 2

OH O

CH2OH

and O

3 CH CH CNCH CH and CH CH CCH NHCH 3 2 3 2 2 3 2 3

(12 marks) 1An acidic compound D, C4H4O4, on heating to 140 °C dehydrates to give compound E, C4H2O3. D gives compound F upon hydrogenation. 0Give the structures of D, E and F. 1Give the systematic names for D and E. (5 marks) Question 8 0In an experiment, 46.3 g of 1-chlorobutane reacts with 30.0 g of sodium cyanide to give 35.2 g of pentanenitrile. 0Calculate the percentage yield of pentanenitrile. 1Outline the mechanism of the reaction. (Movement of electron pairs should be indicated by curly arrows.) 2Name the type of the reaction and draw a labeled diagram to show the energy profile of the reaction. (8 marks) 1Each of the following conversions can be completed in not more than three steps. Use equations to show how you would carry out each of these conversions in the laboratory. For each conversion, give the reagent(s), conditions, and structure of the intermediate compound(s). NO2

OH

CH3

CH3

i)

ii) CH3CH2CH2OH

CH3 CHCH3 OH

OH CHO

H C CO2H

iii) NO2

NO2 (9 marks)

Question 9 0An acyclic hydrocarbon G, with relative molecular mass of between 60 and 80, contains 85.6 % carbon and 14.4 % hydrogen by mass. 0Determine the molecular formula of G. 1Give all possible structures for the molecular formula determined in i). 2G, on ozonolysis, yields H and J which both produce positive iodoform reactions. Give the structures of G, H and J. 3Give the systematic name of G. (10 marks) 0Identify K, L, M, N, P, R and S in the following reactions:

O2N O2N

0K

NO2

NHNH2

O 1 CH CH CH COH 3 2 2

O L M

2CH2=CHCO2CH3

3HOCH2CH2OH

4

CH3CH2CH2CCl

CH3CH2CO2CH3

N

O

O

C

C OCH2CH2O

n

P

N H

NO2

NHN

excess CH3I

O 5

C

R

N(CH3)2

LiAlH4

CO2CH3 6

S O

NaBH4

(7 marks)