Name: ___________________________
962/2
TRIAL STPM 2014/ TERM 2 CHEMISTRY One and half hours (1 ½ hours)
For examiner’s use
Instruction to candidates: DO NOT OPEN THIS QUESTION PAPER UNTIL YOU ARE TOLD TO DO SO.
Section A 1 – 15
There are fifteen questions in Section A. For each question, four choices of answers are given. Choose one correct answer and indicate in on the box provided. Answer all questions.
Section B 16
Answer all questions in Section B. Write your answer in the spaces provided.
17
Answer any two questions in Section C. All working should be shown. For numerical answers, units should be quoted wherever appropriate
Section C
Total
___________________________________________________________________________ This question paper consists of 11 printed pages
1
Section A [ 15 marks ] Answer all questions in this section. 1. The electrode potential, E, of a half-cell is given by
E =Eɵ -
log
where Eɵ is the standard electrode potential and n the number of electrons transferred when the oxidised species changes into the reduced species. Given that E = 0.00 V at equilibrium and Eɵ/V
Half-cell reaction Zn2+(aq) + 2e Ag+(aq) + e
Zn(s) Ag(s)
-0.76 +0.80
What is the equilibrium constant, Kc, for the reaction 2Ag+(aq) + Zn(s) —> Zn2+(aq) +
2Ag(s) at 25 °C ?
A. 1.31 x 10-53 dm3 mol-1 B. 4.41 x 10- 2 dm3 mol-1 C. 2.76 x 1026 dm1 mol-1 D. 7.61 x 1052 dm3 mol-1 2.
Beryllium, the first element of group of group 2, shows unsual characteristics that does not belong to the group, one of these is, A. B. C. D.
3.
Why silicon tetrachloride but not carbon tetrachloride forms white fumes when comes into contact with moist air? A. B. C. D.
4.
beryllium does not react with the steam beryllium oxide is amphoteric beryllium chloride is ionic beryllium sulphate is soluble
CC14 is chemically inactive. The air must be hot. The carbon in CCl4 is not stable. SiCl4 molecules undergo hydrolysis.
Which property is true of the elements in Group 14 of the Periodic Table on descending the group? A The electrical conductivity of the elements decreases. B The electronegativity of the elements increases. C The stability of the +4 oxidation state of the elements increases. D The catenation of the elements decreases.
2
5.
An electrochemical cell is shown below.
The value of the standard reduction potentials, E°, for both half-cells are
X2+(aq) + 2e- = Y3+(aq) + e- =
E°/ V
X(s) Y2+(aq)
+0.34 +0.77
and the Nernst equation is
If a reducing agent Z is added to beaker II, the graph representing the change in the e.m.f. of the above cell is
3
6. The first ionisation energy generally increases across a period in the Periodic Table. Which statement explains why the first ionisation energy of sulphur is lower than that of phosphorus?
A.
The electrons in the p orbitals of sulphur experience greater electrostatic repulsion than phosphorus. The sulphur atom has more electrons than phosphorus atom. The S-S bond is weaker than the P-P bond. The size of S8 molecule is bigger than P4 molecule.
B. C. D.
7. Which statement explains why an aqueous solution of aluminium carbonate is not formed when an aqueous solution of aluminium ions is added to an aqueous solution of carbonate ions? A. B. C. D.
A carbonic acid is formed. The lattice energy of aluminium carbonate is high. The solubility of aluminium carbonate is very low. [A1(H20)6]3+ ions react with carbonate ions in aqueous solution.
8. The first six successive ionisation energies (IE) for elements P, Q, R and S are as follows. IE P
Q
R
S
-1
(kJ mol ) First
1 420 740
1 280 780
Second
2 950 1 500 2 400 1 660
Third
4 600 7 000 3 700 3 000
Forth
7 500 10 300 5 500 4 200
Fifth
9 400 18 200 9 340 16 200
Sixth
53 300
19 820
Which of the following statements is true?
A. B. C. D.
P is a transition metal. Q is an alkali metal. R is a Group 16 element. S is a Group 14 element.
9. The values of the lattice energy and the hydration energy for three sulphate salts P, Q, and R which are elements in Group 2 of the Periodic Table are given below. Which sequence is in the order of decreasing solubility?
A. P, Q, R
B. P, R, Q
C.
Q, R, P
4
D.
R, P, Q
10. The oxidation states of several D-block elements represented by letters which are not the usual symbols for the elements concerned in the Periodic Table are as follows: Element
Oxidation state
X
-
-
3
-
-
-
-
Y
-
2
3
4
-
-
-
Z
-
2
3
4
5
6 7
Which statement is not true? A. X forms colourless hydrated ions. B. Y forms a compound with the molecular formula K2 YO4 C. Z2O is an oxidising agent. D. X, Y and Z form octahedral complexes. 11. The arrangement of electrons in the d-orbitals of four complex ions of iron are as follows.
The arrangement of electrons in the above table shows that A. the complex ion [Fe(H20)6]3+ has the highest degree of paramagnetism among the complex ions B. the H20 ligand is a stronger ligand than the CN- ligand C. the magnitude of paramagnetism is not directly proportional to the number of unpaired electrons D. the splitting of the 3d orbitals in the hexaaqua complex is greater than in the hexacyano complex 12. The reaction between the complex ion and the ligand as follow, [Fe(H20)6]2+ + 6CN- = [Fe(CN)6]4- + 6H20 Complex ion Stability constant
[Fe(CN)0] 4- [Fe(CN)6]31.0 x 1023 1.0 x 1033
Which statement is not true? A. The stability constant for the above reaction is
B. The complex ion [Fe(CN)6]3 is more stable than the complex ion [Fe(CN)6]4-. C. The complex ions of [Fe(CN)6]3- and[Fe(CN)6]4- are octahedral in shape. D. The lower the oxidation state of the metal, the more stable is the complex formed between the metal and its ligands. 5
13. The standard electrode potentials of Xm+(aq)/X(s) and yn+(aq)/y(s) half-cells are +0.80 V and -0.76 V respectively. What can be deduced from the given data? 1. Metal Y can extract metal X from an aqueous solution of its ions. 2. Metal Y is more inclined to form positive ions than metal X. 3. The yn+ ion is a stronger oxidising agent than the Xm+ ion. A. 1
B. 1 and 2
C. 2 and 3
D. 1, 2 and 3
14. The standard electrode potential for some half cells at 298 K are given below. Half cell reaction 2H+(aq) + 2e- H2(g) Cr3+(aq) + eCr2+(aq) Cr3+(aq) + 3eCr(s) 2+ Cr (aq) + 2e Cr(s) + 02(g) + 4H (aq) + 4e- 2H20(g)
F°(V) 0.00 -0.41 -0.74 -0.91 + 1.23
Based on the data above, which of the following statement(s) is/are true? 1. 2. 3. A.
Chromium metal dissolves in dilute acids to form Cr3+. Chromium metal dissolves in dilute acids to form Cr2+. Acidified Cr3+ ion is stable in air. 1
B. 1 and 2
C. 2 and 3
D. 1, 2 and 3
15. The diagram below shows the structure of silicate sheet.
Which of the following statements is/are true? 1. 2. 3. A.
The basic unit is [Si2O5]6-. The structure shows a pyroxene. In this structure, all the tetrahedral share two vertices with the neighbouring unit. 1
B. 1 and 2
C. 2 and 3
D. 1, 2 and 3
Answer for Section A
1.
4.
7.
10.
13.
2.
5.
8.
11.
14.
3.
6.
9.
12.
15.
6
Section B [15 marks] Answer all questions in this section. 16. The carbonates of Group 2 in the Periodic Table decompose on heating to form the corresponding metal oxide and carbon dioxide. A general equation for the reaction is MCO3(s)
MO(s) + CO2(g)
The thermal stability of these carbonates can be compared in the laboratory using the apparatus in the diagram below. The test tube on the left contains a sample of a metal carbonate and the tube on the right contains limewater.
(a) (i) State the measurement that you would make in this experiment. ………………………………………………………………………………… (1 mark) (ii) Suggest three ways to make sure that, when carrying out this experiment, the thermal stabilities of the different carbonates are compared fairly …………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… (3 marks) (b) (i) State the trend in the thermal stability of the metal carbonates as the group is descended. ……………………………………………………………………………………… ( 1 mark) (ii) Explain this trend in stability. ………………………………………………………………………………………….. ………………………………………………………………………………………….. ………………………………………………………………………………………….. ( 3 marks) 7
17.(a)
When solid sodium chloride is heated with concentrated sulphuric acid, a white fume, X, is given off. When solid potassium bromide is heated with concentrated sulplric acid, a white fume, Y, is given off together with a coloured fume, Z. (i)
Explain the origin of gas Z.
...................................................................................................................................................... ....................................................................................................................................................... ..................................................................................................................................................... (2 marks)
(ii)
When solid potassium bromide is heated with concentrated phosphoric acid, gas Z is not produced. Explain why it is so.
.................................................................................................................................................... .................................................................................................................................................... ..................................................................................................................................................... (2 marks) (b)
Aluminium oxide show amphoteric properties that react both with hydrochloric acid and sodium hydroxide. Write balanced equation for the reaction between aluminium oxide and (i)
Hydrochloric acid
..................................................................................................................................................... (ii)
Sodium hydroxide
..................................................................................................................................................... (2 marks) (ii)
Explain why the reaction between aluminium with dilute hydrochloric acid is slow, but becomes rapid after a few minutes.
.................................................................................................................................................... .................................................................................................................................................... (1 marks)
8
Section C Answer any two questions in this section. 18.(a) Magnesium metal is manufactured by electrolyzing molten magnesium chloride. The magnesium chloride can be obtained from sea water. Magnesium hydroxide is precipitated and then converted into magnesium oxide by heating. The magnesium oxide is converted into anhydrous magnesium chloride. The table below contains some enthalpy data. Process Mg(s) Mg(g) Mg(g) Mg+(g) + e + Mg (g) Mg2+ (g) + e O2(g) 2O(g) O(g) + e O- (g) O-(g) + e O2- (g) Mg(s) + ½ O2(g) MgO(s) (i) (ii) (iii) (iv)
(b)
∆Hθ /kJmol-1 +150 +736 +1450 +496 -142 +844 -602
Suggest why the second ionization energy for magnesium is greater than the first. (2 marks) Suggest why the oxygen atom has a negative electron affinity and why the electron affinity of the O- ion has a positive value. (2 marks) Use the above data to calculate the lattice enthalpy of magnesium oxide. (2 marks) Suggest qualitatively how the magnitude of the lattice enthalpy of magnesium chloride compares with that of magnesium oxide. Explain your answer. (4 marks)
The half equations relating the interconversion of the species Cr2+ (aq), Cr3+(aq) and Cr2O72-(aq) are given below. Cr3+(aq) + e Cr2+ (aq) 2Cr2O7 (aq) + 6e + 14H+(aq) (i) (ii)
2Cr3+(aq) + 7H2O(l)
Use your data booklet to find Eθ for each of the above half equations. (1 mark) 3+ 2+ Write the overall equation for the disproportionation of Cr into Cr and Cr2O72-. Use the Eθ values you have obtained in (b) (i) to show whether or not this disproportionation is spontaneous under standard conditions. (4 marks)
19.(a) The elements of Group 14 form oxides with oxidation numbers of +2 and +4. With suitable examples, (i) (ii)
explain how the stability of the +2 and +4 oxidation states change when descending the group. state the effect of oxidation number on the acid-base properties. [10 marks]
(b) Pure silicon is needed in the manufacture of microchips. Pure silicon can be obtained by heating silicon tetrachloride with zinc. (i) Write an equation for the reaction. (ii) A sample of silicon tetrachloride contains 10% of impurity. Calculate the mass of pure silicon that can be obtained when 8.50 g of the sample is heated with zinc. [5 marks]
9
20.(a)
Chromium is a typical transition metal, although its electronic configuration does not fit the general trend found in the first transition series. Some interconversions found in the chemistry of chromium are shown below. Metal + H2SO4 2+ [Cr(H2O)6] [Cr(H2O)6]3+ CrO42Sky blue step 1 violet step 2 yellow OH-(aq)
H2NCH2CH2NH2 Charcoal catalyst step 4
H3O+(aq)
step 3
[Cr(H2NCH2CH2NH2)3]3+ Deep blue
Cr2O72orange
(i)
State two typical properties of the transition metals, other than the formation of coloured ions, which are shown in the diagram above. ( 2 marks)
(ii)
Using Eθ values from your data booklet to suggest a metal that could be used for reducing Cr3+(aq) to Cr2+(aq) in step 1. Justify your answer by calculating Eθ for your cell. (3marks)
(iii) Explain, by using oxidation numbers, whether or not the conversion in step 3 is a redox reaction. (2 marks) (iv)
b)
The organic compound H2NCH2CH2NH2 that is used in step 4 is 1,2-diaminoethane, often called ethylenediamine. It is a bidentate ligand. Explain the meaning of this term and why this ligand can act as a bidentate whereas H2NNH2 cannot. (3 marks)
Four compounds each contain a different octahedral Co(III) complex. All four compounds have the general empirical formula [CoClx(NH3)y]Clz. Two of the complexes have the same formula but have the ligands arranged differently in space and so are isomers of each other. Treatment of an aqueous solution of each compound with AgNO3(aq) will give a white precipitate of AgCl. The table below contains details of the colours of each complex and of the number of moles of AgNO3 which would react with 1.0 mol of each compound. Number of complex Colour of the solid complex Amount of AgNO3 required per mole of the compound/ mol
1
2
3
4
Golden brown
purple
green
violet
3.0
2.0
1.0
1.0
Suggest the structures of the four complexes with reasoning. Draw the octahedral structures of the two isomers. ( 5 marks)
END OF QUESTIONS
10
1.D
4.D
7.D
10. B&C
13.B
2.C
5.C
8.D
11.A
14.B
3.D
6.A
9.B
12.D
15.A
Term 2 trial 2014 Answer scheme 16. a (i) Time for the first cloudiness/milky to appear in the lime water. (1 ) Reject volume of CO2 (ii)a. constant Bunsen flame/electrical heater setting b. fixed height of test tube above the flame c. fixed moles/mass of carbonate d. fixed volume of limewater e. Same surface area/particle size f. Standardise cloudiness of limewater using the disappearance of a cross
(any 3)
b (i) Increases/more stable (1) (ii) Cation/positive ion becomes larger/charge density reduced Polarising power reduced/distortion of carbonate electron cloud/anion reduced C-O covalent bond stronger (1+1+1) 17. (a) (i) KBr react with Conc. H2SO4 to form HBr / KBr + H2SO4 KHSO4 + HBr HBr formed is oxidised by conc. H2SO4 to Br2 / 2HBr + H2SO4 Br2 + SO2 + 2H2O (ii) Conc. H3PO4 is not an oxidising agent, it does not oxidise HBr to Br2 / KBr + H3PO4 KH2PO4 + (b) (i) Al2O3 (ii) Al2O3
+ +
6HCl 2NaOH +
2AlCl3
+
3H2O
(1) (1)
(1) (1) HBr
3H2O
(1)
2NaAl(OH)4
(1)
(iii) the reaction is slow at first, Impervious oxide layer on the surface of Al is slow. Once the Al is stripped of its protective film, the reaction speed up. (1) 18. (a) (i)
the ion from which the electron is rem oved has a greater net positive charge The electron removed from Mg+ is at a lower energy level/outer electron nearer to attraction of nucleus (1+1) (ii) there is a net attraction by the oxygen nucleus and the electron received to achieve a more stable octet configuration. O- ion has an overall negative charge which will repel any incoming electron So energy is absorb in order to overcome this repulsive force. (1+1+1) (iii) Born Haber cycle Correct numbers used -3888kJ/mol
(1+1+1) 11
(iv) Smaller than MgO Because Cl- ion has only a single charge Cl- ion is larger than O2- ion Therefore weaker attraction between Cl- and Mg2+ or converse
(1+1+1+1)
19. (a) (i) On descending G 14, The stability of +2 oxidation state increase/ +4 oxidation state decrease (1) Example, Carbon is high up in G 14, - CO2 is stable and does not decompose when heated (1) - the oxidation state of carbon in CO2 is +4 (1) Lead at the bottom in G14, - PbO2 decomposes upon heating and form more stable PbO (1) + (1) - the oxidation state of lead in Pb is +2 (1) 2PbO2 2PbO + O2 (1) - Inert pair effect of Pb (1) - Due to the very much different energy level between 6s and 6p orbital (1) (ii) the oxide of element of G 14 with an oxidation state +4 are more acidic in properties than oxides with an oxidation state +2. (1) Example, CO and SiO are neutral (1) CO2 and SiO2 are acidic or other (1) (b) (i) SiCl4(l) + 2Zn(s)
2ZnCl2 + Si
(1) + (1)
(ii) If 10 % impurity, this mean that 90% is SiCl4, The mass of SiCl4 in the sample is X 8.50 = 7.65 g
(1)
RMM of SiCl4 is 28.1 + 4(35.5) = 170.1 The mass of pure Si obtained =
x
7.65 = 1.26 g
20.(a) (i) Variable/different/several oxidation state/number Complex ion formation (1+1) (ii) Correct metal , ½ equation and E value scores (3) Eg Mg2+ + 2e Mg -2.38V Cr3+ + 3e Cr -0.41V E = Eoxidation, (Mg/Mg2+) + E reduction,,(Cr3+/Cr) = +2.38 + (-0.41) = +1.96V (iii) Not a redox reaction Both contain Cr(VI) 6/6+ (1+1) (iv) forms 2 (dative/covalent) bonds/has 2 lone pairs/donates 2 pairs of electrons Longer chain in enthylenediamine/hydrazine is too short Dative bonds/lone pairs too close/repel in hydrazine (1+1+1) (b) Complex 1 - 3Cl- ions outside ie [Co(NH3)6]3+ Complex 2 - 2 Cl- ions outside ie [Co(NH3)5Cl]2+ Complex 3 and 4 - One Cl- ion outside complex ie [Co(NH3)4Cl2]+ Diagram of complex 3 clearly showing octahedral shape Diagram of complex 4 shown as cis/trans isomer of complex 3 (1+1+1+1+1) 12
(1) + (1)
