CHEMISTRY PAPER I MARKING KEY

SECTION A (70 marks)

Question 1
a. i. solution B is basic while solution A is acidic 2 marks

ii. -Solution A is a strong acid which will dissociate completely in water to

provide the more ions for conductivity.
-On the other hand, solution B is a weak base which will dissociate
partially in water to give fewer ions for conductivity. Solution A would,
therefore, conduct more electric current.

(3 marks, all keys words in bold to be included in the explanation)

b. - N H +¿/
4
NH ¿3

−¿ ¿
- H 2 O/OH (1 mark for any one)
2−¿→ BaC O3( s) ¿
2+¿+C O(aq ) ¿
c. i. B a(aq) 2 marks

ii. Precipitation reaction/ double decomposition reaction

(1 mark for any one)
Question 2
a. A mole is an amount of substance that contains 6.02 3 ×1023particles.
1 mark
b.
Element Number of moles Simplest ratio Simplest whole
number ratio
Fe 70/56 = 1.25 1.25/1.25 = 1 1x2=2
O 30/16 = 1.875 1.875/1.25 = 1.5 1.5 x 2 = 3
The empirical formula is Fe2 O3
Empirical formula mass = (56 x 2) + (16 x 3) = 160
No. of E.F units = molecular mass/E.F mass = 160/160 = 1
Therefore, the molecular formula is Fe2 O3 4 marks
Question 3
a. These include carbon monoxide, hydrogen sulfide, ammonia, chlorine,
carbon disulfide and hydrogen cyanide. (2 marks for any two gases)

b. i. it changes waste materials into new useful materials

ii. Reduces consumption of fresh raw materials
iii. It saves energy
iv. Reduces air and water pollution
v. It provides jobs to the unemployed (2 marks for any two)

c. i. curve (a) 1 mark

ii. The curve has a horizontal line which indicates that during change of state
of matter temperature remains the same. That only happens when the
substance is pure. (Note: during change of state of matter for a pure
substance added heat is absorbed as latent heat) 2 marks
Question 4
a. Rhombic 1 mark

b. -Making gun powder

-making matches
-Vulcanizing rubber
-in photographic development
-making bleaching agents
-making artificial hair dyes
-manufacturing medicine
-manufacturing sulphuric acid
-manufacturing fire works
-manufacturing germicides and fungicides (3 marks for any three)
c. -Sulphuric acid is a dehydrating agent because it has a strong affinity for
water.
-when it comes into contact with water in grass, it reacts exothermically and
forms hydronium ions.
-this reaction effectively removes water from the grass and dries it.
(3 marks, one mark for each point)
Question 5
a. Springs, wells and boreholes (1 mark for any one)

b. Calcium ions and magnesium ions (1 mark for any one)

c. – washing using hard water is accompanied with wastage of soap due to

scum formation.
-calcium and magnesium carbonates exist as ‘fur’ on electrical appliances
making them inefficient as their conductivity is reduced.
-scum leaves dirty marks on clothes after washing. This lowers the quality of
the fibre.
-calcium carbonate and magnesium carbonate formed in hard water blocks
pipes. These deposits reduces heat conductivity.
(4 marks for any two)
Question 6
a. 2-methylpropan-2-ol 1 mark

b. Tertiary. Because the carbon atom bonded to the hydroxyl group is also
bonded to three other carbon atoms. 2
marks

c. Methanol has hydrogen bonding which are absent in ethane. Hydrogen

bonding are very strong and need more heat energy to break than the weak
Van Der Waal’s forces in ethane. This makes methanol to boil at a higher
temperature than ethane 2 marks
Question 7
a. i. Polymerization 1 mark

ii. .
 2 marks
b. –they do not corrode in air or water
-they do not decompose easily
-they are relatively light
-some are soft and malleable while others are very strong
-they are cheap to produce
-some are transparent (2 marks for any two)
Question 8
a. –carbon dioxide
-Sulphur dioxide
-nitrogen dioxide
-nitrogen monoxide
-methane (1 mark for any one)

b. –use of renewable sources of energy such as solar, wind and hydroelectric

power to minimize emission of greenhouse gases
-afforestation-planting trees where none existed before to reduce carbon
dioxide levels in the atmosphere
-fit factories with acid scrubbers to absorb acidic gases such as carbon
dioxide and Sulphur dioxide
-fit cars with catalytic converters to clean up exhaust gases before they are
released into the atmosphere
-Reafforestation - planting trees to replace the cut down trees to moderate
levels of carbon dioxide in the atmosphere
(4 marks for any two points)
Question 9
a. i. isomers b, c and d are conformers 2 marks
ii. Pentane 1 mark
b. -Branching in hydrocarbons lowers their boiling points.
 -This is because branching makes the molecule more compact and thereby
decreasing the surface area.
-The decreased surface area lowers interactive Van Der Waal’s forces.
-This translate to little amount of energy required to break the forces
during transition from liquid to gaseous phase.
4 marks
Question 10
a. –concentration of reactants for aqueous reactants
-pressure for gaseous reactants
-temperature at which the reaction takes place
-surface area of reactants
-light (type and intensity)
-presence of catalysts (2 marks for any two)

b. –chemical reaction takes place due to collisions of reacting particles

-when temperature increases particle gain kinetic energy
-the gained kinetic energy increases particle motion of the reactants
-increased particle motion increases the collision frequency and hence
fruitful (successful) collision enabling the reaction to happen faster
4 marks
Question 11
a. Bonding in graphite uses three of the four valence electrons of carbon
atom. This leaves the fourth electron free to move within its structure.
This delocalized electron is responsible for graphite’s conductivity.
2 marks
b. Water is regarded a polar covalent compound. This is due to the unequal
sharing of the bonding electrons between atoms of hydrogen and oxygen.
This makes the oxygen in water molecule partially negative with
hydrogen atoms becoming partially positive. These partial charges attract
the ions of sodium chloride hence dissolving it. 2 marks

c. i. Activation energy refers to the minimum amount of energy required for a

reaction to take place. 1 mark
ii. Enthalpy change refers to the heat energy exchange with the surrounding
at constant pressure. 1 mark
 d. -Bond breaking requires transfer of heat energy from the surrounding into
the reaction to break the bonds in reacting particles. This justifies bond
breaking as endothermic.
-On the other hand, bond making involves the transfer of heat energy to the
surrounding as a result of new bonds formed in products. 2 marks
SECTION B (30 marks)
Question 12
i. Pipette 25 cm3 of the standard solution (base) and transfer it into a conical
flask.
ii. Add 3 drops of phenolphthalein indicator and take not of the initial colour of
the indicator in the base.
iii. Put the flask on a white tile.
iv. Fill the burette clamped to a stand with the solution whose concentration is
to be found (acid) up to the zero mark.
v. Slowly add the acid to the base by opening the tap of the burette while
continuously shaking the conical flask.
vi. Close the tap of the burette when the end-point is reached. i.e. the colour of
the indicator changed from pink to colourless.
vii. Note and record the volume of the acid used to neutralize the base.
viii. Repeat the experiment 3 times and find the average volume of the acid used
to neutralize the base.
ix. Write the balanced chemical equation of the reaction to get the mole ratio of
the reactants in the reaction.
x. Use the following equation to determine the concentration of the acid:
C1 V 1 C 2 V 2
= ,
n1 n2
Where C1= molarity of base, V1=volume of base, n1 =number of moles of
base from equation, C2 = molarity of acid, V2 =volume of acid and n2
=number of moles of acid from equation
n2 C 1 V 1
C 2=
n1 V 2

(10 marks for all the steps)

Question 13
a. -Put 50 cm3 of distilled water in the beaker.
-Using a thermometer, measure the temperature of the water and record it.
-Add 4 sodium hydroxide pellets into the water and stir.
-Using a thermometer note and record the temperature of the solution.
-Compare the temperature of the water to that of the solution.
-If the final temperature of the solution is higher than the initial temperature
of the water then dissolving of sodium hydroxide in water is an exothermic
reaction that releases heat energy to the surrounding environment.
(5 marks for all the steps)
b. -Pour 50 cm3 of the dilute acid into a beaker.
-Add the pieces of the metal of the salt to be formed and observe what
Happens. (The reaction produces hydrogen gas seen as bubbles going off)
-Keep on adding the metal until some remain unreacted indicating that the
reaction is complete.
-Filter off the excess metal and solid impurities and collect the filtrate in a
conical flask.
-Boil the filtrate to evaporate excess water to obtain the crystals of the
soluble salt.
(5 marks for all the steps)
Question 14

i. Arrange the apparatus as shown above.

ii. With one of the acids to be tested in the beaker, dip the electrodes in the
beaker.
iii. Observe what happens to the bulb.
iv. Note and record the brightness of the bulb as bright or faint.
v. Repeat procedure ii. to iv. for the other acids to be tested.
vi. Compare the bright of the bulb for the different acids that have been tested.
vii. Classify the acids that have produced bright light as strong acids while
those that have produced faint light as weak acids.

(4 marks for the correct diagram with at least three labelled parts and
6 marks for description with all the necessary details)

THE END