Download the nakuru district sec. schools trial examinations - 2015

THE NAKURU DISTRICT SEC. SCHOOLS TRIAL EXAMINATIONS - 2015
Kenya Certificate of Secondary Education (K.C.S.E)
233/1
CHEMISTRY MARKING SCHEME
1. The elements X and Y have electronic configuration 2:8.3 and 2.6 respectively
(a)
To which group and period does Y belong?
 Group VI, period 2
(1 mark)
(b) If the two reacts, what is the formula of the compound formed
 A2B3
2. The empirical formular of a compound is CH2 and it has a molecular mass of 42.
(a) What is the molecular formular of this compound?
 (1.2 + 2)n = 42
n = 42
14
C3H6
n=3
(1 mark)
(1 mark)
(b) To which group of hydrocarbons does the compound formed in (a) above belong?
 alkenes
(c) Draw the structural formula of the third member of this series and give its IUPAC name
(1 mark)
but–1– ene
or
but-2-ene
3. Excess carbon (II) oxide gas was passed over heated sample of an oxide of iron. Study it and answer the
questions below using data provided below.
to fume chamber
dry CO(s)
dish
heat
oxide or iron
Mass of empty dish = 10.98 g
Mass of empty dish + oxide of iron = 13.14g
Mass of empty dish + Residue = 12.66g
Determine the simplest formula of the oxide of iron .(Fe=56, 0=16)
(3 marks)
Mass of Iron 12.66- 10.98=1.68 
element
Fe
O
Mass of oxygen 13.14-10.98 = 2.16g 
(1)
mass gm
1.68
0.48
Mass of oxygen be 2.16 – 1.68 = 0.48g 
RAM
56
16
No. of moles 1.68 0.48 FeO1
56
16
mole ratio 0.03
0.03
0.03 0.03
1
1
Nakuru District Sec. Schools Trial Examination @ 2015 Chemistry Paper 1 Marking scheme
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4. Study the table below and answer the questions that follows.
Substance
A
B
C
Boiling
point
10
2660
1420
Melting point Solubility in
water
-39
Insoluble
1610
Insoluble
801
Soluble
Electrical conductivity
solid
Molten
Poor
Poor
Good
Good
Poor
Good
With reasons, select a substance
(a) With an eletrodent bond
 C 1 has high m.p and B.P and only conducts electricity in molten state. 1
(2 marks)
(b) Likely to be a metal
(2 marks)
 B  1 conducts electricity in both solid and molten state and has high m.p and B.P 1
1
5. The use of materials of lead in roofing and in water pipes is being discouraged. State
(i) One reason why these materials have been used in the past.
(1 mark)
 Lead piping had unique ability to resist pin hole leaks
 lead does not rust/corrode forms a protective layer on its surface
6. State and explain what is observed when chlorine gas is bubbled through a solution of potassium bromide
solution
(2 marks)
 The colourless solution of KBr. turns red due to bromine liquid being displaced
(b) Write an ionic equation for the above reaction
 Cl2(s) + 2Br-(aq)
Br2(aq) + 2Cl-(aq)
(1 mark)
7. The table below gives the rate of decay for a sample of radioactive element K. Study it and answer the
questions that follows:
Mass of K(g)
72
9
Number of days
0
120
(a) Define the term half-life as used in radioactivity
(1 mark)
 Half life is the time taken for a given mass of radioactive isotope to reduce to half its original
mass.
(b) Determine the half-life of element K
t½
t½
t½
 72
36
18
9g 1
3 t ½ = 120 days
(2 marks
t ½ = 120 days
3
= 40 days 1
8. Explain in terms of structure and bonding why silicon (IV) oxide exists as a solid of high melting point
while sulphur (IV) oxide exists as a gas.
(2 marks
 SiO2 has giant atomic structure 1 with strong covalent bonds between hence exists as a solid
while SO2 has simple molecular structure 1 with weak van das Waals force of attraction
between the molecules
Nakuru District Sec. Schools Trial Examination @ 2015 Chemistry Paper 1 Marking scheme
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9. A current of 2 amperes was passed through a solution of copper (II) sulphate for 15 minutes. Calculate
the mass of copper metal deposited at the cathode. (Cu=64.0, IF = 96500 coulombs) (3 marks)
Mass in gm = Q x RAM
96500 x charge
= 64 x 2 x 15 x 50 1
96500
= 115200
96500 x 2
1
= 0.59689 g 1
10. In the Haber’s process, the industrial manufacture of ammonia is given by the following equation:
N2(s) + 3H2(g)
2NH3(g)
∆ H = -97kj/mol
(a) Name one source of hydrogen gas used in this process
(1 mark)
 Cracking of long chained alkanes – natural gas or coal
 natural gas
 electrolysis of brine
(b) Name the catalyst used in the above reaction
 finely divided iron
(1mark)
(c) What is the effect of increasing temperature of the yield of ammonia? Explain
(1 mark)
 The yield decreases. The reaction is exothermic hence backward reaction is favoured.
(Products decompose)
11. The equation below represents changes in the physical states of iron
Fe(s)
Fe (l)
∆H = + 15.4 Kj /mol
Fe (l)
Fe (g) ∆ H = +354 Kj/mol
Calculate the amount of heat required to change 11.2g of solid iron to gaseous state (fe-56)
Fe (l)
5.6 g
11.2 g
Fe (g) ∆ H =15.4 +354
=369.4kj1
369.4kj
?
11.2 g x 369.4 kj = 73.88 kj 1
56
(3 marks)
1 mol
369.4kj
𝟏𝟏.𝟐
= 0.2 mol1
𝟓𝟔
1 mol
369.4
0.2 mol
?
12. When excess chlorine gas is bubbled through dilute sodium hydroxide solution; the resulting
solution acts as a bleaching agent.
(a) Write an equation for the reaction between chlorine gas and sodium hydroxide solution
 NaOH(aq) + Cl2(g)
NaCl(aq) + NaOcl(aq) + H2O(l)
(b) Write an equation to show the bleaching and explain how it occurs.
 NaOCl(aq) + dye
NaCl(aq) + (dye + 0)
 NaOcl decomposes to give atomic oxygen [. 𝟎] that bleaches by oxidation
(1 mark)
(1 mark)
13. Hydrogen gas can be used as a fuel. State one advantage and one disadvantage of using hydrogen
as a fuel
Advantage
 Water the product formed is a non-pollutant
(½)
Nakuru District Sec. Schools Trial Examination @ 2015 Chemistry Paper 1 Marking scheme
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Disadvantage
 It is expensive/hydrogen would explode
(½)
14 Red-hot iron reacts with steam to give tri-iron tetroxide and hydrogen gas. The reaction is reversible
3Fe(s) + 4H2O(s)
Fe304(s) + 4H2(g)
(a) Define dynamic equilibrium
(1 mark)
 Although the reaction appears to have stopped by attaining a state of balance, both forward
and backward reactions are still taking place
(b) State giving reasons how an increase in pressure would affect the equilibrium
(1 mark)
 No effect since volume of gaseous reactants equals the volume of gaseous product.
15. State and explain the observations made when excess ammonia gas reacts with chlorine gas
(3 marks)
 White fumes are produced1
 Ammonia react with chlorine producing hydrogen chloride1 gas which reacts with excess
ammonia to give ammonium chloride 1
16. The following are the ionic and atomic radius of some elements. Use the table below to answer the
questions that follows: The elements are in the same period of the periodic table.
Element
W
X
Y
Z
Atomic radius (nm)
0.160
0.231
0.099
0.144
Ionic radius (nm)
0.065
0.133
0.181
0.195
Which element
(a) Is the most reactive non-metal?...Y1
(b) Has the largest atomic number? …Z1
(c) Is the most reactive metal? …X1
17
Using (.) and crosses(x) to represent outermost electrons, draw a dot and cross structure showing
the bonding in hydroxonium ion H3O+ (Atomic numbers H=1, O=8)
(2 marks)
Nakuru District Sec. Schools Trial Examination @ 2015 Chemistry Paper 1 Marking scheme
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18 Starting with sodium metal, describe how a sample of crystals of sodium hydrogen carbonate may be
prepared
(3 marks)
 Heat sodium metal (small pieces) in oxygen to form sodium oxide.  ½
 Dissolve the oxide in water to form sodium hydroxide  ½
 bubble excess CO2 into the solution to form sodium hydrogen carbonate  ½
 warm the solution to concentrate  ½
 Allow solution to cool and form crystals ½
 Filter and dry the crystal between pieces of filter papers.  ½
19. Use the flow chart below to answer the questions that follows:
Solution W
Cr 2O72-/H+(aq)
Step11
SO2(g)
H2S(g)
Step 1
Yellow
specks
Water
Step 111
Solution T
(a) Explain the observation made in step II
 oxidizing agent Cr 2O72-/H+(aq turns orange to green
(1 mark)
(b) Name the process that takes place when flower petals are put into solution T
 bleaching
(c) Reaction in step 1 takes place only in presence of moisture. Give a reason for this
 Moisture assists to form ions for reaction to take place
20
(1 mark)
(1 mark)
Use the diagram below to answer the questions that follows:
A
C
B
Sheet of paper
Charged plates
Identify the radiators
A…Alpha radiations 
B…Beta radiations
C...Gamma radiations/rays
Lead block
Aluminum foil
alimi
(3 marks)
21. (a) State Braham’s law
(1 mark)
 At constant temperature and pressure the rate of diffusion of a gas is inversely proportional
to the square root of its density.
Nakuru District Sec. Schools Trial Examination @ 2015 Chemistry Paper 1 Marking scheme
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(b) A certain volume of a gas X diffuses through a porous boundary in 20 seconds. How much time
is required for an equal volume of gas Y to diffuse through the same boundary under same
conditions. ) RMM of X = 28, Y=7)
(2 marks)
𝑻𝒊𝒎𝒆 𝒙
√𝑴𝑴𝑿
𝒕𝒊𝒎𝒆 𝒀
√𝑴𝑴𝒀
½
𝟑𝟎
𝑻𝒚
= √𝟒
Ty=
𝟑𝟎
𝑻𝒚
𝟐𝟖
=√ 𝟕 ½
𝟑𝟎
𝟐
Ty = 15 sec. 1
22. (a) Use the information given to draw a labeled diagram of an electrochemical cell that can be
constructed to measure the electromotive force between G and K
(3 marks)
2+
ø
G (aq) + 2e
G(s) E = -0.44v
K2+ (aq) + 2e
K(s) Eø = - 0.76v
½
v
K(s)½
K2+(aq)½
G½
Salt
bridge
½
G2+(aq)  ½
(b) Calculate the Eø value for the cell constructed in (a) above
(1 mark)
Eø = Eø - Eø
cell
R
O
= - 0.44v– (-0.76v)  ½
= - 0.44v + 0.76v
= +0.32v ½
23 The set-up below was used to prepare carbon(II) oxide gas. Study it and answer the questions that
follow.
Nakuru District Sec. Schools Trial Examination @ 2015 Chemistry Paper 1 Marking scheme
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(a) Name the reagents P…Methanoic acid
Q…concentrated sulphuric (VI) acid
(1 mark)
(1mark)
(b) The students carrying out the experiment were unable to collect the gas. Why?
(1 mark)
 the delivery tube collecting the gas was dipped into the solution in the conical flask hence no
gas was collected
(c) State one use of sodium hydrogen carbonate
 raising agent in baking powder/absorb moisture and odours
 effervescent drinks
 tooth paste
24
(1 mark)
The table below shows the first ionization energies of metallic elements Y and Z
Element
Y
Z
Ionization energy Kj/mol
494
418
What does the ionization above suggest about the reactivity of Y compared to that of Z. Explain
 Z  1 is more reactive. It requires less energy to lose the outermost electron to be stable
1
25.
A student set up the experiment below to collect gas K. The glass wool was heated before heating the
zinc powder.
Glass wool
soaked with
water
Excess zinc
powder
Gas K
Test tube
Heat
Heat
(a) Why was it necessary to heat the moist glass wool before heating zinc powder?
 to generate steam which pushes out the air
(1 mark)
(b) What would happen if the zinc powder was heated before heating the glass wool?
 The air would oxidize zinc to zinc oxide hence no gas would be collected.
(1 mark)
(c) What property of gas K makes it possible for it to be collected as shown in the diagram?
 It is less dense than air.
Nakuru District Sec. Schools Trial Examination @ 2015 Chemistry Paper 1 Marking scheme
(1 mark)
Page 7
26. The table below shows the test carried out on a sample of water and the results obtained.
Sample
Test
Observation
A
Addition of sodium hydroxide solution
drop wise until in excess
Addition of excess aqueous ammonia
solution
Addition of dilute nitric(v( acid followed
by barium nitrate solution
White precipitate which dissolves in
excess to form a colourless solution
White precipitate
B
C
White precipitate
(a) Identify the anion present in the water
 SO42-
(1 mark)
(b) Write the ionic equation for the reaction in C
Ba 2+ (aq) + SO42- (aq)
BaS04 (s)

(1 mark)
(c) Write the formula of the complex ion in A
Al(OH)4 or AlO2
(1 mark)
27. A charged glass rod attracts a thin stream of water from a tap.
(a) What does this indicate as concerns the water molecules
 water is a polar molecule
(1mark)
(b) Water is a small molecule with a relative formula mass of 18. This means it would exist in the form
of a gas at r.t.p, but does not. Explain
(2 marks)
 because of the hydrogen bonds 1 which hold the water molecules
H
O
H
1
hydrogen bond
H
H
28. A piece of burning magnesium was inserted in a gas jar of carbon(IV) oxide
(a) State and explain the observations made
(2 marks)
 The reaction is exothermic ½ producing heat which decomposes carbon(IV)oxide to black½
carbon specks and oxygen gas. Oxygen gas supports½ continued burning of magnesium to
produce white½ magnesium oxide.
(b) Write an equation for the reaction in (a) above
 2 Mg(s) + CO2(g)
2MgO(s) + C(s) 1
29. State three observations made when a piece of potassium metal is dropped into cold water giving a
reason for each
(3 marks)
 Potassium will float – less dense than water1
 Potassium will melt into a silvery ball since reaction is exothermic1
 Produce a hissing sound – hydrogen gas produced
 A lilac(purple) flame is produced due to production of potassium vapour
Nakuru District Sec. Schools Trial Examination @ 2015 Chemistry Paper 1 Marking scheme
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