KCSE 2023 Chemistry Paper 1 Marking Scheme

1 The atomic numbers of nitrogen and fluorine are 7 and 9 respectively.

(a) Draw electron dot \(\cdot\) and cross \(\times\) diagrams to illustrate bonding in the molecules of:

(i) nitrogen; (1 mark)

(ii) fluorine; (1 mark)

(b) With reference to the diagrams in (a), state why fluorine is more reactive than nitrogen. (1 mark)

Nitrogen has a tripple bond // Fluorine has a single bond

Single bond is weaker than tripple bond hence earsier to break.

2. Classification of types of salts includes normal and acid salts.

(a) Give the name of another type of salt.

Accept: Base (1 mark)

Double salt // Basic salt

(b) (i) Describe how an acid salt can be prepared using 0.1 M sulphuric(VI) acid and 0.1 M sodium hydroxide. (1 mark)

Mix/react equal volumes of 0.1M H₂SO₄ & NaOH

(ii) Write an equation for the reaction in b(i). (1 mark)

H₂SO₄ (aq) + NaOH (aq) → NaHSO₄ (aq) + H₂O (l)

3 Complete Table 1 by writing the formulae of all the products formed when the metal nitrates are heated strongly. (3 marks)

4 Zinc metal is extracted from zinc blende ore through the following processes: concentration, roasting and reduction.

(a) Explain how the ore is concentrated. (2 marks)

The ore is ground/crushed, water and oil added, air blown thro' the mixture. The earthly material is wetted and sinks. The ore floats on the surface.

(b) Write an equation for the reaction that takes place when the ore is roasted. (1 mark)

2 ZnS_(s) + 3 O_2(g) → 2 ZnO_(s) + 2 SO_2(g)

4 Zinc metal is extracted from zinc blende ore through the following processes: concentration, roasting and reduction.

(a) Explain how the ore is concentrated. (2 marks)

The ore is ground/crushed, water and oil added. air blown thro' the mixture. The earthly material is wetted and sinks. The ore floats on the surface.

(b) Write an equation for the reaction that takes place when the ore is roasted. (1 mark)

2 ZnS_(s) + 3 O_2(g) → 2 ZnO_(s) + 2 SO_2(g)

5 Figure 1 shows how the solubility of oxygen varies with changes in temperature.

(a) Explain using kinetic theory why the solubility varies as shown. (2 marks)

With increase in temperature, oxygen molecules gain kinetic energy and escape from the solution, hence solubility decreases. Oxygen molecules are repelled.

(b) With reference to Figure 1, explain the pollution effect that may be caused by a large increase in the temperature of sea water. (1 mark)

Less dissolved oxygen in sea water results to the death of marine life (animals/plants).

6 A flow diagram for production of sulphuric(IV) acid is shown in Figure 2.

(a) State the optimum conditions necessary for carrying out the reaction in unit II. (1 mark)

High temp of between 450-500°C.
Vanadium (V) oxide/Platinum catalyst.
2-3 atm pressure.

(b) Stage II takes place in two steps. Describe the steps. (2 marks)

SO₃ is dissolved in conc H₂SO₄ to make oleum.
Oleum is then diluted with water.

OR

SO_3(g) + H₂SO_4(l) → H₂S₂O_7(l)

H₂S₂O_7(l) + H₂O_(l) → 2H₂SO_4(l)

7 The following equilibrium exists in aqueous ammonia:

NH_3(aq) + H₂O_(l) ⇌ NH_4(aq)⁺ + OH^-_(aq)

(a) With reference to this equilibrium, explain why ammonia is a weak base. (1 mark)

Ammonia dissociates partially.
Reaction is reversible.
Equilibrium position lies to the left hence very few OH^- ions in the solution.

b) When aqueous ammonia is added drop-wise to aqueous copper(II) sulphate, a blue precipitate is formed. The blue precipitate redissolves giving a deep blue solution. Write the formula of the substance responsible for the:

(i) blue precipitate; (1 mark)

Cu(OH)₂

(ii) deep blue colour. (1 mark)

[Cu(NH₃)₄]²⁺ // Cu(NH₃)₄

8.The molecular formula of two straight chain hydrocarbons is C₄H₆.

(a) Give the general formula of the homologous series to which the hydrocarbons belong. (1 mark)

C_nH_2n-2

(b) Draw the structures of the two compounds and give their names. (2 marks)

9 Water hardness may be temporary or permanent.

(a) Write the formula of an ion that causes temporary hardness. (1 mark)

HCO₃^-

(b) Temporary hardness can be removed by boiling. Give one disadvantage of using this method in industries. (1 mark)

Leaves deposits/CaCO₃/MgCO₃/Scales/fur which reduces heat conductivity/efficiency in boiler/consumes a lot of energy/expense/blockage/clogging.

c) State one method that can be used to remove permanent hardness. (1 mark)

- Addition of Na₂CO₃/Washing soda/Siva ash to precipitate MgCO₃/Mg(HCO₃)₂ (Remove Ca, Mg)

- Use of ion-exchange resins/sodium permutit

10 (a) Figure 3 shows an energy level diagram for the formation of hydrogen fluoride. (1 mark)

Calculate the:

(i) enthalpy change for the formation of hydrogen fluoride; (1 mark)

100 - 370 = -270 kJmol^-1

(ii) activation energy of the reaction. (1 mark)

411 - 370 = +41 kJmol^-1

11 At high temperature, hydrogen and oxygen react as shown in the following equation:

O_2(g) + 2H_2(g) → 2H₂O_(g)

In an experiment, a mixture containing 200 cm³ oxygen and 300 cm³ hydrogen was heated to form water.

(a) Determine which gas was in excess and by how much. (1 mark)

Oxygen was in excess by 200 - 150 = 50 cm³

(b) Calculate the volume of water that was formed. (1 mark)

300 cm³

(c) Explain how one could confirm that the product was pure water. (1 mark)

Determine the boiling point (about 100°C) at 1 atm pressure
Freezing/melting point of 0°C
Density of 1 g/cm³

12 Silk and rubber are examples of natural polymers.

(a) Give the name of another natural polymer. (1 mark)

Starch//wool//Cotton//cellulose//proteins//starch//DNA//

(b) State the sources of: (1 mark each)

(i) silk:

Silk worm

(ii) natural rubber.

Rubber tree/Latex from tree

13 (a) Give the formula of the compound referred to as rust. (1 mark)

Fe₂O₃.xH₂O // Fe₂O₃.H₂O // Fe_xO_y.zH₂O (x=1-10)

(b) Four test tubes were setup to investigate rusting of iron as shown in Figure 4. The setup was allowed to stand for one week.

Explain why:

(i) no rusting took place in test tube III. (1 mark)

Absence of oxygen (boiling removes dissolved oxygen) and oil keeps off the oxygen in the air.

(ii) select the test tube in which there was highest amount of rust. Give a reason. (1 mark)

IV

Salt accelerates rusting

14 (a) Avogadro's constant is \(6.02 \times 10^{23} mol^{-1}\). Determine the total number of ions that are present in \(500 cm^3\) of 2.0 M calcium nitrate. (2 marks)

Moles of \(Ca(NO_3)_2\) = \(\frac{500 \times 2}{1000}\) = 1 mol

Moles of all ions (1 \(Ca^{2+}\), 2 \(NO_3^-\)) = 3 ions = 3.05

Total No of ions = 3 x \(6.02 \times 10^{23}\) = \(1.806 \times 10^{24}\)

(\(1 \times 6.023 \times 10^{23}\)) + (\(2 \times 6.023 \times 10^{23}\)) = \(1.806 \times 10^{24}\)

\(1.806 \times 10^{24}\)

(b) The electrical conductivity of 2.0 M calcium nitrate is higher than that of 2.0 M potassium nitrate. Explain. (1 mark)

More/higher No of ions in \(Ca(NO_3)_2\) than \(KNO_3\) // Higher charge on \(Ca^{2+}\) compared to \(K^+\) (Conductivity depends on the No of ions & charge on the ions)

15Carbon(IV) oxide and methane are gases found in the atmosphere.

(a)State one disadvantage of carbon(IV) oxide in the atmosphere. (1 mark)

Causes global warming // Green house effect // Acid rain

(b) Name one source of emissions of methane into the atmosphere. (1 mark)

Production of natural gas/fossil fuel/coal mining/decomposition of manure and biogas (fermentation of animals)

Cracking long chain alkanes

(c) State how emissions of carbon(IV) oxide into the atmosphere can be reduced. (1 mark)

Reduction of use of fossil fuels/burning of wood/charcoal

Tree planting

Mass of D = 5.70 - 4.55

(16) 5.70g of a chloride of element D was found to contain 4.55g of chlorine.

(a) Determine the empirical formula of the chloride (D = 27.0; Cl = 35.5). (2 marks)

No of moles

D: 1.15/27 = 0.0426

Cl: 4.55/35.5 = 0.128

Mole ratio

D: 0.0426/0.0426 = 1

Cl: 0.128/0.0426 = 3

EF = DCl₃

(b) Given that the relative formula mass of the chloride is 267, determine its molecular formula. (1 mark)

(DCl₃)_n = 267

135.5n = 267

n = 2

MF = D₂Cl₆

Rg. (DCl₃)₂

(17)Table 2 shows the boiling points of ethane, butane and ethanol. (H = 1.0; C = 12.0; O = 16.0)

Compound: CH₃CH₃, CH₃CH₂CH₂CH₃, CH₃CH₂OH

Relative molecular mass: 30, 58, 46

Boiling point, °C: -88.6, -0.5, 78.5

Give reasons for the following:

(a) boiling point of butane is higher than that of ethane; (1 ½ marks)

Butane has a higher mass/larger molecules/longer carbon chain, more carbon atoms hence has stronger van der Waals forces compared to ethane.

(b) boiling point of ethanol is higher than that of butane. (1 ½ marks)

Ethanol has hydrogen bonding which is stronger than weak van der Waals forces in butane.

Reactivity of the metals with water

Ca > Mg > Be

(18)Small pieces of beryllium, magnesium and calcium metals were placed in test tubes containing cold distilled water mixed with phenolphthalein indicator as shown in Figure 5.

State and explain the observations that were made in each of the following test tubes:

Test tube I; Do not react

No rxn the soln remains colourless (1 mark)

Reject: No observable change.

Test tube II; (1 mark)

Bubbles/Pink colour forms after some time.
Mg reacts slowly with water to form Mg(OH)₂

Test tube III. (1 mark)

Bubbles/Effervescence/Soln turns pink
Vigorous rxn

(19)A laboratory assistant wanted to investigate the effect of an electric current on substances.

Figure 6 shows arrangement of the apparatus used.

Explain why:

(a) in A, the bulb did not light up and no electrolysis took place;

Hexane is a non-conductor, has no free ions!!

Hexane is a non-electrolyte, it is molecular.

(b) in B, the bulb lit but no electrolysis took place;

Mercury is a metal / good conductor of electricity has delocalized ē hence bulb lights but it cannot be decomposed because it is an element. (Harmful ions)

(c) in C, the bulb lit and electrolysis took place.

Aq. H₂SO₄ is an electrolyte hence contains ions that conducts an electric current / it decomposes / ions are dissociated / discharged

(20)Carbon - 14, \({}^{14}C\) is a radioactive isotope of carbon. It decays to form an isotope of nitrogen (Atomic numbers of: N = 7; C = 6).

(a) Write a nuclear equation for the decay process.

\({}^{14}_6C \longrightarrow {}^{14}_7N + {}^{0}_{-1}e\)

(b) Archeologists calculate the age of organic matter using the proportion of carbon - 14 present and the half-life of carbon-14.

(i)State what is meant by the term half-life of carbon - 14.

Time taken for a radioactive substance to reduce/decay to half its initial/original mass.

(ii) Given that the half-life of carbon-14 is 5570 years, calculate the age of a piece of bone found to contain \(\frac{1}{16}\) as much carbon-14 as living matter.

\(\frac{1}{2} \longrightarrow \frac{1}{4} \longrightarrow \frac{1}{8} \longrightarrow \frac{1}{16}\)

4 Half lives.

Age = 4 X 5570

= 22,280 years

\((\frac{1}{16}) = (\frac{1}{2})^{5570} \\ \Rightarrow (\frac{1}{2})^4 = (\frac{1}{2})^t \\ \Rightarrow t = 4 \\ \Rightarrow t = \frac{T}{5570} \\ \Rightarrow T = 22,280\)

21 A student used the following procedure when standardising sodium hydroxide. The student placed 25.00 cm³ of the hydroxide in a conical flask and titrated it with 0.1 M hydrochloric acid using phenolphthalein indicator. The average titre was 21.50 cm³ of the acid. State and explain how each of the following steps in the procedure may have affected the titre value:

(a) the student rinsed the conical flask with the sodium hydroxide before using it; (1 mark)

Rinsing leaves some extra NaOH in the conical flask this will increase the titre value.

(b) the student did not rinse the freshly cleaned burette with the hydrochloric acid before filling it; (1 mark)

Some water was is left in the burette after cleaning this dilutes the HCl increasing the titre value.

(c) the student used about 2.0 cm³ of the phenolphthalein indicator. (1 mark)

Phenolphthalein is a weak acid, this reacts with some NaOH and this lowered the titre value.

22 The setup shown in Figure 7 (a) was used to determine the approximate percentages of the major components of air.

(a) Complete Figure 7 (b) to show the setup after it was left standing until no further changes were observed. (1 mark)

Measure the height of gas jar above water level in 7(a), h_i and when there is no further change, in 7(b), h_f.

(b) Explain how the percentages of the major components of air are calculated. (2 marks)

% of Active component = (h_i - h_f) / h_i * 100% ✓ [2]

% of Inactive component = h_f / h_i * 100% ✓ [2]

23 The heat energy value of a fuel is defined as the amount of energy produced by burning 1.0 gram of the fuel. (C=12.0; H=1.0; O=16.0)

(a) Calculate the heat energy values of the fuels, hydrogen and ethanol and complete Table 3.

(b) Other than its heat energy value, state one advantage of using hydrogen and ethanol as fuels:

(i) hydrogen; (1 mark)

Producer water which does not pollute environment ✓ [1]

(ii) ethanol. (1 mark)

It is renewable source of energy // cheap // Easy to store // Easy to transport. Any 1 ✓ [1]

24 Study the structures of substances L and M shown in Figure 8 and complete Table 4 by giving the names that describe the type of structure and bonding in each substance.

Figure 8

Must used the term (Giant)

(2 marks)

25 State one physical property that would suggest the presence of each of the following gases from a leaking gas cylinder:

(a) H₂S (1 mark)

Smell of rotten eggs ✓ [1]

(b) N₂O (1 mark)

Sweet odour/Pleasant smell ✓ [1]

(c) Cl₂ (1 mark)

Green-yellow / Yellow-green gas ✓ [1]

26 (a) Give the names of the type of compounds whose reaction is described as:

(i) esterification; (1 mark)

Alkanoic/carboxylic acid and Alkanol/Alcohol ✓ [1]

(ii) saponification. (1 mark)

Fat/Oil/Ester/Lipids and KOH/NaOH ✓ [1]

Suitable alkali/hydroxide

(b) State the importance of vulcanisation. (1 mark)

Hardening of rubber for use in tyre industry / Making rubber stronger ✓ [1]

Acc. Manufacture Rubber

27 Figure 9 shows a setup used to separate a mixture of two liquids, Q (boiling point 117°C) and R (boiling point 103°C).

Identify three mistakes in this setup. Give a reason in each case.

Mistake 1 (1 mark)

Thermometer is too far inside the column ✓ [1/2]

It should be at the end of the adopter to measure the temp of the vapour. ✓ [1/2]

Mistake 2 (1 mark)

Use of water bath. ✓ [1/2]

The liquids have higher b.p. than water so they will not boil at 100°C ✓ [1/2]

Mistake 3 (1 mark)

Water in and water out in condenser is reversed. ✓ [1/2]

Absence of glass beads to make the fractionating column long enough.

The liquid will not cool sufficiently (No effective condensation) ✓ [1/2]

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