Form Four Chemistry NECTA Review Papers: 2025 CSEE Paper with Interactive Answers

Hints for matching the parts

• Anode: the electrode joined to the positive terminal of the cell. Negative ions move to it and lose electrons (oxidation).
• Cathode: the electrode joined to the negative terminal of the cell. Positive ions move to it and gain electrons (reduction).
• Electrolyte: the liquid or solution in the beaker that contains ions and is split up by the current.
• Cell: the battery that supplies the electric current.
• Ammeter: connected in series in the circuit to measure the current.
• Electrons: the charge carriers that move through the wires and electrodes.
• Current: the flow of charge in the circuit, conventionally from the positive terminal of the cell.
• Electrolysis: the overall decomposition of the electrolyte by electric current.

Suggested answer and marking guide

Economic activities, any six

• Agriculture: Water is needed for irrigation and for crop growth, and livestock keeping needs water for drinking and cleaning.
• Fishing: Fish live in rivers, lakes and the sea, so fishing and fish farming depend on water.
• Hydroelectric power: Falling water turns turbines to generate electricity that is sold or used in industries.
• Water transport: Boats, ships and ferries carry people and goods across lakes, rivers and the sea.
• Industry: Factories use water as a raw material, solvent, coolant and for cleaning, for example in brewing and textile making.
• Tourism: Beaches, lakes, rivers and waterfalls attract tourists and support boat trips, hotels and sport fishing.
• Mining: Water is used to wash and separate minerals and to cool equipment.
• Construction: Water is mixed with cement and sand to make concrete and mortar.

Suggested answer and marking guide

(a)(i) Two points

• Like group I: Hydrogen has one electron in its outer shell and can lose it to form the H⁺ ion.
• Like group VII: Hydrogen needs one more electron to fill its shell. It can gain an electron to form the hydride ion, H⁻, with reactive metals such as sodium, and it exists as diatomic molecules, H₂, like F₂ and Cl₂.

(a)(ii) Why group I

Elements are placed in groups by the number of electrons in the outer shell. Hydrogen has one outer electron, like group I elements, and it usually reacts by losing that electron to form H⁺.

(b) Element G

• G has four occupied shells (2:8:8:7), so it is in Period 4
• G has seven outer electrons, so it is in group VII, the halogens
• H has atomic number 17, so its arrangement is 2:8:7: Period 3, group VII

Reactions of sodium with G and H

• Similarity: Both G and H are halogens, so sodium reacts with each to form an ionic sodium halide: 2Na + H₂ (as Cl₂) → 2NaCl, and in the same way 2Na + G₂ → 2NaG.
• Difference: Sodium reacts more vigorously with H, which is higher in the group, than with G. Reactivity of the halogens falls down the group because the outer shell is further from the nucleus and attracts an extra electron less strongly.

Suggested answer and marking guide

(a) Variables

• Independent (manipulated) variable: The temperature of the water, which the technician changes on purpose.
• Dependent (responding) variable: The solubility of sodium chloride, which is the amount that dissolves at each temperature. More sodium chloride is added until no more will dissolve.
• Controlled (constant) variables: The volume of water, the type of solute (sodium chloride) and the stirring, which are kept the same.

(b) Steps of a scientific investigation, any six

• Identify and state the problem
• Make observations and collect background information
• Form a hypothesis
• Plan and carry out an experiment to test the hypothesis
• Collect and record data accurately
• Analyse and interpret the data
• Draw a conclusion
• Report the findings

Suggested answer and marking guide

(a) Advantages, any three

• Clean burning: It burns to give only water, so it does not release carbon dioxide or smoke.
• High energy: It releases a large amount of energy per gram.
• Renewable source: Hydrogen can be made from water.
• Light: It is the lightest gas, so the fuel adds little mass to the vehicle.

Disadvantages, any three

• Flammable and explosive: It forms an explosive mixture with air, so leaks are dangerous.
• Hard to store: It needs high pressure or very low temperature, and the tanks are heavy and costly.
• Expensive to produce: Making hydrogen from water needs a lot of energy.
• Large volume: It has a very low density, so a large tank is needed.
• Lack of facilities: There are few filling stations and few engines designed for it.

(b) Reasons for the preferred materials

• Zinc granules contain small amounts of impurities such as copper, which speed up the reaction. Pure zinc reacts very slowly with dilute acid
• Dilute hydrochloric acid gives a steady, easily controlled reaction and so a smooth flow of hydrogen
• Concentrated hydrochloric acid is volatile, so hydrogen chloride fumes would escape and make the hydrogen impure
• Concentrated acid reacts too vigorously with zinc, which is dangerous

Suggested answer and marking guide

(a) Steps, in order

1. Connect the rubber tubing firmly from the gas tap to the burner and check that there are no leaks
2. Close the air hole at the base of the burner
3. Light a match or splint and hold it near the top of the barrel
4. Turn on the gas tap slowly. The gas lights with a yellow, luminous flame
5. Open the air hole slowly so that air mixes with the gas
6. Adjust the air hole until a blue, non-luminous flame is obtained

(b) Uses and the property concerned, any four

• Heating substances and liquids: The flame is very hot.
• Heating glass apparatus and crucibles: It does not deposit soot, because combustion is complete.
• Flame tests for metal ions: It is almost colourless, so it does not hide the colour given by the metal salt.
• Sterilising wire loops and needles: It is very hot, so it kills germs quickly.
• Cooking and heating in homes and laboratories: It is smokeless, so it keeps surroundings clean.

Suggested answer and marking guide

(a) Chemicals that make water hard, any four

• Calcium chloride, CaCl₂
• Magnesium chloride, MgCl₂
• Magnesium sulphate, MgSO₄
• Calcium hydrogencarbonate, Ca(HCO₃)₂
• Calcium sulphate, CaSO₄

(b)(i) Temporary hardness

It is caused by dissolved calcium or magnesium hydrogencarbonates. Boiling decomposes the hydrogencarbonate ions, and the insoluble carbonate that forms is deposited as scale, so the Ca²⁺ ions leave the water.

Ca²⁺(aq) + 2HCO₃⁻(aq) → CaCO₃(s) + H₂O(l) + CO₂(g)

(b)(ii) Permanent hardness

It is caused by dissolved calcium or magnesium sulphates or chlorides, which boiling does not remove. Washing soda (sodium carbonate) supplies carbonate ions, which precipitate the Ca²⁺ and Mg²⁺ ions as insoluble carbonates.

Ca²⁺(aq) + CO₃²⁻(aq) → CaCO₃(s)
Mg²⁺(aq) + CO₃²⁻(aq) → MgCO₃(s)

(c) Advantages of hardness, any three

• Strong bones and teeth: Calcium in hard water helps build and maintain bones and teeth.
• Better taste: Many people find hard water more pleasant to drink.
• Protection of pipes: A thin layer of insoluble carbonate on the inside of lead pipes stops lead dissolving into the water.
• Brewing: Hard water is suitable for brewing some kinds of beer.
• Shells of aquatic animals: Calcium compounds in water help animals such as snails and shellfish to build their shells.

Suggested answer and marking guide

(a) Le Chatelier's principle

When a change in concentration, pressure or temperature is made to a system in dynamic equilibrium, the position of equilibrium shifts in the direction that opposes the change.

(b) Ways to increase the yield of Z

• Increase the temperature: The forward reaction is endothermic (ΔH is positive), so heating shifts the equilibrium to the right, producing more Z.
• Increase the pressure: Two moles of gas (X and Y) form one mole of gas (Z). Higher pressure favours the side with fewer gas moles, so the equilibrium shifts to the right.
• Increase the concentration of X or Y: Adding more reactant makes the equilibrium shift to the right to use up the extra, forming more Z.
• Remove Z as it forms: Taking Z away lowers its concentration, so the equilibrium shifts to the right to replace it.

A catalyst would make the reaction reach equilibrium faster but does not change the yield.

Suggested answer and marking guide

• (a) Combustion in air: Ethanol burns with a pale blue, almost non-luminous flame, giving out heat and forming carbon dioxide and water.
  C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
• (b) Reaction with sodium: Sodium reacts steadily, giving off bubbles of hydrogen gas and forming sodium ethoxide. The reaction is less vigorous than with water.
  2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂
• (c) Concentrated sulphuric acid at 170 °C: The acid removes water from ethanol (dehydration), forming ethene gas.
  C₂H₅OH → C₂H₄ + H₂O
• (d) Acidified potassium dichromate(VI): Ethanol is oxidised, first to ethanal and then to ethanoic acid. The orange dichromate ions change to green chromium(III) ions.
  CH₃CH₂OH + [O] → CH₃CHO + H₂O
  CH₃CHO + [O] → CH₃COOH
• (e) Reaction with ethanoic acid: On warming with a few drops of concentrated sulphuric acid as catalyst, ethanol undergoes esterification, forming ethyl ethanoate (a sweet-smelling ester) and water.
  CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

Suggested answer and marking guide

(a) Equipment: the Frasch pump

• Three concentric pipes sunk into the sulphur deposit, with a perforated lower end
• A boiler to heat water under pressure to about 170 °C
• An air compressor
• Storage tanks or vats to collect and cool the molten sulphur

In the exam, draw in pencil, give the diagram a title and label with straight lines.

(b) The process (Frasch process)

• Superheated water at about 170 °C, under pressure, is forced down the outer pipe. Sulphur melts at about 115 °C, so it melts around the pipe
• Compressed hot air is forced down the inner pipe
• The air mixes with the molten sulphur and forces it up the middle pipe as a froth
• The molten sulphur is pumped into tanks, where it cools and solidifies. It is about 99.5% pure

(c) Rhombic to monoclinic sulphur

The two forms are stable at different temperatures. Rhombic sulphur is stable below the transition temperature of 96 °C and monoclinic sulphur is stable between 96 °C and the melting point (about 119 °C).

• Melt powdered rhombic sulphur gently in an evaporating dish, without letting it boil
• Pour it into a folded filter paper held in a funnel and let a crust form on top
• Open the paper when the crust has formed and pour off the remaining liquid
• Needle-shaped crystals of monoclinic sulphur are seen. Below 96 °C they slowly change back to rhombic sulphur

(d) Commercial uses, any two

• Manufacture of sulphuric acid
• Vulcanisation of rubber
• Making matches, gunpowder and fireworks
• Making fungicides and insecticides
• Making medicines and skin ointments