Stpm Percubaan 2008 Sabah Chemistry Paper 2

Section A [40 marks] Answer all questions 1. (a) The following figure shows the principal energy levels of the hydrogen atom.

(i)

The figure shows that as the quantum numbers of the energy level increases, the separate lines of the energy levels becomes closer until they converge, at n = ∞ State the magnitude of the nuclear force towards the electron in the hydrogen atom at this point.

………………………………………………………………………………….... [ 1 mark ] (ii)

Calculate the minimum energy required to ionise a hydrogen atom in its ground state.

[1 mark ] (iii)

Calculate the frequency of the electromagnetic radiation emitted if the electron at n = 2 makes a transition to n = 4.

[ 2 marks ]

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(b) Hydrogen combines with chlorine to form hydrogen chloride. The mass spectrum of hydrogen bromide is shown below. Intensity P2

100

50

P1

P4

P3

30 (i)

0

35

40

45

m/e

Identify the ions that causes the peaks P1 and P3

…………………………………………………………………………………… (ii) Calculate the ratio of the peak heights, P2 and P4. State one significance of this ratio value.

[ 3 marks ] (c) The figure below shows the plot of pV/RT against pressure for hydrogen pV/RT 1.0 (i)

Pressure/atm Give a reason why hydrogen gas deviates from the ideal behaviour at high pressure.

……………………………………………………………………………………. …………………………………………………………………………………….

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(ii) Unlike hydrogen gas, hydrogen chloride gas shows a very marked negative deviation from ideal behaviour at medium pressure. Give two reasons. 1. ……………………………………………………………………………………. ……………………………………………………………………………………. 2. ……………………………………………………………………………………. …………………………………………………………………………………… [ 3 marks ] 2.

Chlorine dioxide gas, ClO2, which is a reddish yellow gas. It is used for the disinfection and the principal agent used in the decontamination of buildings in the United states after the 2001 anthrax attack. (a) Chlorine dioxide gas can be prepared in the laboratory by the following reactions: 2AgClO3 (s) + Cl2 (g)

2AgCl (s) + 2ClO2 (g) + O2 (g); ∆H = zero

[ ∆Hf (AgClO2) = -25kJ mol-1; ∆Hf (AgCl) = -127 kJ mol-1] (i)

Calculate the value of ∆Hf (ClO2)

(ii)

From the value obtained in a (i) above; comment on the relative stability of chlorine dioxide.

…………………………………………………………………………………... …………………………………………………………………………………... [ 3 marks ] (b) Chlorine dioxide is a strong oxidizing agent in acidic solution. This is why it is used as a bleaching agent. ClO3 (aq) + 4H+ (aq) + 5e

Cl- (aq) + 2H2O ; EӨ = + 1.50 V

Predict the reactions if any, of chlorine dioxide in acidic solution with (i) aqueous iron (II)

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(ii)

Aqueous hydrogen sulphide, H2S

[4 marks ] (iii )

(c)

Draw a labeled diagram to show how the redox potential of Ө E (ClO2/ Cl ) might be measured in the laboratory.

[2 marks] Chlorine dioxide undergoes disproportionation in alkaline solution to give a mixture of chlorate (III) and chlorate (V) ions. Write an ionic equation for the reaction. …………………………………………………………………………………... [ 1 mark ]

3.

The first ionization energies of some Group 2 and group 3 elements are given as follows. Group 2 Ionisation energy Group 13 Ionisation energy -1 elements (kJmol ) elements (kJmol-1) Be 900 B 798 Mg 726 Al 575 Ca 592 Ga 575 Sr 546 In 556 Ba 502 Tl 589 (a)

Define the first ionization energy of an element.

………………………………………………………………………………………… ………………………………………………………………………………………… [ 1 mark ]

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(b) The first ionization energy of the Group 2 elements decreases down the group. State the 2 factors that contribute to the decrease in 1st ionization energy from Be to Ba. 1. ……………………………………………………………………………….. 2. ……………………………………………………………………………….. [2 marks] (c)

Explain why the 1st ionization energy of magnesium is higher than that of aluminium. …………………………………………………………………………………... …………………………………………………………………………………... …………………………………………………………………………………...

[ 2 marks ] (d) The 1st ionization energy of boron and barium do not show a marked difference, however, barium compounds are ionic whereas boron compounds are predominately covalent. Give an explanation for this observation. …………………………………………………………………………………... ………………………………………………………………………………....... …………………………………………………………………………………... [ 2 marks ] (e)

Burning of magnesium in air produces magnesium oxide and a little magnesium nitride (i) Draw a Lewis structure for magnesium nitride.

(ii)

When magnesium nitride is warmed with water, a gas that turns moist red litmus paper blue is liberated. Write a balanced equation for the reaction between magnesium nitride and water.

………………………………………………………………………………….. [ 3 marks]

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4.

An organic compound X of molar mass of 136.9g mol-1 has the following composition by mass: C, 35.2%; H, 6.5 %; Br, 58.3% (a)

Determine the molecular formula of X.

[ 2 marks ] (b) When X is boiled under reflux with aqueous sodium hydroxide, 2-methyl-1propanol is formed. (i)

Draw the structural formula of X.

(ii)

State the mechanism of the reaction.

…………………………………………………………………………………... [ 2 marks ] (c)

When X is treated with an alcoholic solution of sodium hydroxide, an alkene Y is formed. Y reacts with hydrogen bromide to form Z which is an isomer of X (i)

Y has four isomers. Give the structural formula for the isomer that exhibits geometrical isomerism.

(ii)

Draw the structural formula for Z

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(iii) State one isomer of alkene Y that produces carbon dioxide when heated with concentrated acidified KMnO4? Give an equation for the reaction. Isomer: Equation:

[ 4 marks] (d) Z reacts with aqueous sodium hydroxide to form an alcohol P. Suggest a simple chemical test to differentiate between alcohol P and 2-methyl-1-propanol ………………………………………………………………………………….... …………………………………………………………………………………… …………………………………………………………………………………… [ 2 marks]

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SECTION B (60 marks) 6. 5.

Answer any four questions. (a) The following are all equilibrium processes. In each case, write a chemical (a) What do you understand by the terms equation to show the equilibrium, and use Le Chatelier’s principle to explain the (i) relative atomic mass observations described. (ii) isotope [ 3 marks] (b) The mass spectrum of ethanol contains peaks at m/e values of 15, 28, 31, 45 (i) When potassium nitrate dissolves in water, the temperature drops. If a and 46. Account for the appearance of these peaks as fully as you can. saturated solution of potassium nitrate at 100 ° C is cooled to room [ 5 marks] temperature, crystals are formed. (c) Explain clearly and concisely the meaning of each underline terms and indicate the significance of each of the numerical values. (ii) When a small volume of a concentrated solution of sodium sulphate is added (i) The ionic product of water is equal to 1.0 x 10-14 mol2dm-6 at 250C to a saturated solution of calcium sulphate, a white precipitate is formed. and increases as the temperature increases [ 4 marks] (iii When brown NO2 gas is cooled, colourless N2O4 gas is formed. A gas ) syringe is filled with NO2 gas and the end sealed. 2+ When the gas is (ii) The standard electrode potential of calcium (Ca / Ca) is -2.87V compressed, the brown colour fades. Warming the syringe restores the [ 3 marks] colour. (b)

[ 8 marks ] The following reaction is used industrially to produce a combustible gas from coal. H2O(g) + C (s)

H2(g) + CO (g)

A mixture of powdered coal and steam at a pressure of 1 atm and a temperature of 1300 ° C was allowed to reach equilibrium. It was found that the total pressure had increased to 1.9 atm but the remaining steam had a partial pressure of only 0.1 atm. Use the data given to calculate the partial pressures of hydrogen and carbon monoxide, and hence calculate a value for Kp [ 3 marks ]

(c)

Hydrangeas are flowering plants which commonly have pink flowers. They produce blue flowers in soils which contain high concentrations of Mg 2+ (aq). The pH of well limed soil is 9.0, whereas the pH of peat-based soil is 6.5. The numerical value of the solubility product, Ksp of magnesium hydroxide is 1.8 x 1012

Deduce and explain which of these types of soil favours good cultivation of blue hydrangeas. [ 4 marks] 7.

(a)

A solid halide reacts with concentrated sulphuric acid to form HX gas which is 8

then oxidized to X2 gas. The solid plays an important role in black-white photography. Identify the solid halide. Explain your answer. [5 marks] (b )

A coordination compound triamminetrichlorocobalt(III) exists as two isomers. (i)

What is the chemical formula of triamminetrichlorocobalt(III)?

(ii)

Draw the structures of the two isomers and name them. [5 marks]

(c) Explain the observations when an aqueous solution of silver nitrate is added to (i)

triamminetrichlorocobalt(III)

(ii)

triamminedichlorocobalt(III) chloride [5 marks]

8.

(a)

(i)

Suggest reasons why magnesium gives the nitride, Mg3N2, in addition to its oxide when burned in air.

(ii)

A 1.00g sample of the powder obtained from burning magnesium in air was boiled with water. The ammonia that was evolved neutralizes 12.00 cm3 of 0.5 mol dm-3 hydrochloric acid. Construct balanced equations for the reaction of magnesium nitride with water. Calculate the percentage of magnesium nitride in the 1.00g sample. [ 6 marks]

(b) NaCl AlCl3 CCl4 SiCl4 Describe the reaction, if any, of each of the four chlorides with water, stating the appropriate pH of any solution formed, and writing balanced equation for any reaction that takes place. Offer an explanation for any differences that occur in their reactivity. [6 marks ] (c) Articles made from copper and its alloys can be cleaned of their oxides tarnish by the use of metal polishes containing aqueous ammonia and an abrasive. Assuming the tarnish copper contains copper(II) ions, suggest with an equation, how 9

the aqueous ammonia might react with the tarnish, and suggest the colour of the resultant solution. [ 3 marks ] 9.

The amino acids tyrosine, lysine and glycine are constituents of many proteins.

(a)

State the reagents and conditions you could use to break proteins down into amino acids. [ 2 marks ]

(b) (c)

(d)

Draw a ring around each chiral centre in the above molecules. [ 2 marks] In aqueous solution amino acids exist as zwitterions. Draw the zwitterionic structure of glycine. [ 1 mark ] For each of the following reactions, draw the structure of the organic compound formed. (i) glycine + excess NaOH(aq). (ii) tyrosine + excess NaOH(aq) (iii) lysine + excess HCl (aq) (iv) tyrosine + excess Br (aq) 2

[ 5 marks ]

(e)

Draw the structural formula of a tripeptide formed from all three of these amino acids, showing clearly the peptide bonds. [ 2 marks ]

(f)

The formula of part of the chain of a synthetic polyamide is shown below.

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(i) Identify the repeat unit of the polymer by drawing square brackets around it on the above formula. (ii) Draw the structures of the two monomers from which the polymer could be made. (iii) Name the type of polymerisation of the polymer shown above. [ 3 marks ]

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This question is concerned with organochlorine compounds. (a) State the conditions needed to produce the two compounds A and B.

[ 2 marks ] (b )

State the reagent needed to carry out the following reaction. X

[ 1mark ]

(c) The three chloro-compounds A, B and C vary in their ease of hydrolysis. (i) Arrange the reactivity of the above compounds in decreasing order (ii) Suggest an explanation for these differences in reactivity

(d )

[ 3 marks ] Y Draw the structural formulae of the organic products X, Y and Z of the following reactions of compound C.

OH in NaOH (aq)

11 Z

COCl CH3NH2

C

H2O

[ 3 marks ]

(e)

Compounds D and E are both ketones. CH3CH2COCH2CH3 D

CH3CH2CH2COCH3

E

(i) State which one of these compound reacts with alkaline aqueous iodine, and draw the structural formulae of the products formed during this reactions. [ 3 marks ]

(ii) The reduction of D with NaBH4 produces just one alcohol, but a similar reduction of E produces two isomers in equal amounts. Explain these observations, drawing structures where appropriate. [ 3 marks ]

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END OF QUESTION PAPER

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