KCSE CHEMISTRY QUESTIONS AND ANSWERS PER TOPIC
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(a) Below is a simplified diagram of the Downs Cell used for the manufacture of sodium.
Study it and answer the questions that follow
(i) What material is the anode made of? Give a reason
(ii) What precaution is taken to prevent chlorine and sodium from re- combination? (iii) Write an ionic equation for the reaction in which chlorine gas is formed (b) In the Downs process, (used for manufacture of sodium), a certain salt is added to lower the melting point of sodium chloride from about 800°C to about 600°C. (i) Name the salt that is added (ii) State why it is necessary to lower the temperature (c) Explain why aqueous sodium chloride is not suitable as an electrolyte for the manufacture of sodium in the Downs process (d) Sodium metal reacts with air to form two oxide. Give the formulae of two oxides
(a) The list below shows the formulae of some organic compounds. Use it to answer the questions that follow.
(i) Select two compounds whichI are not hydrocarbons II Belong to the same homologous series
(ii) Identify the compound that is likely to undergo polymerization. Give a reason for your answer. a. The structures below represents two cleansing agents: R – COO- Na+ R – OSO3- Na+ In the table below, give one advantage and one disadvantage of using each one of them
b. Under certain, ethanoic acid ( C2H4O2) and ethanol (C2H5OH) react to form a sweet smelling compound.
(i) What is the general name of compound to which the sweet smelling compound belong? (ii) Write the formula of the sweet smelling compound (iii) Give one use of ethanoic acid other the formation of the sweet smelling compounds (iv) Write the equation for the reaction dilute ethanoic acid and solid potassium carbonate c. Fibres are either synthetic or natural. Give one: (i) Example of a natural fibre (ii) Advantage of synthetic fibres have over natural fibres
(a) The table below shows the standard reduction potentials for four half- cells.
Study it and answer the questions that follow. (Letters are not the actual symbols of the elements)
(i) Identify the strongest reducing agent
(ii) Write the equation for the reaction which takes place when solid G is added to a solution containing H2+ ions (iii) Calculate the Eθ value for the reaction in (ii) above (b) The diagram below shows the apparatus that can be used to electrolyze acidified water to obtain hydrogen and oxygen gases. Study it and answer the questions that follow
(i) Identify the electrode at which oxidation takes place
(ii) Give a reason why it is necessary to acidify the water (iii) Explain why hydrochloric acid is not used to acidify the water (c) During electrolysis of aqueous copper (II) sulphate, 144750 coulombs of electricity were used. Calculate the mass of copper metal that was obtained Cu = 64 ; 1 Faraday = 96500 coulombs)
When a current of 0.82A was passed for 5 hours through an aqueous solution of metal Z, 2.65 g of the metal were deposited. Determine the charge on the ions of metal Z. ( 1 Faraday = 96500 Coulombs:
Relative atomic mass of Z = 52
ANSWERS
ANSWERS
(a) what condition is necessary for an equilibrium to be established?
(b) When calcium carbonate is heated, the equilibrium shown below is established CaCO3 (s) → CaO(s) + CO2(g) How would the position of equilibrium be affected if a small amount of dilute potassium hydroxide is added to the equilibrium mixture? Explain
ANSWERS
100 g of a radioactive substance was reduced to 12.5 g in 15.6 years. Calculate the half – life of the substance
Methane reacts with oxygen according to the equation given below.
CH4 (g) + 2O2 (g) →CO2 (g) + 2H2O (l), ΔH = 890 KJ MOL-1 Calculate the volume of methane which would produce 111.25 kj when completely burnt.(Molar volume of a gas = 24 litres.)
In the industrial extraction of lead, the ore is first roasted in a furnace. The solid mixture obtained is then fed into another furnace together with coke, limestone and scarp iron. State the function of each of the following in this process:
(a) Coke (b) Limestone (c) Scrap iron
ANSWERS
Zinc oxide reacts with acids and alkalis
a) Write the equation for the reaction between zinc oxide and: i) Dilute sulphuric acid ii) Sodium hydroxide solution b) What property of zinc oxide is shown by the reaction in (a) above?
a) Study the table below and answer the questions that follow
i) Which of the compounds is a solid at 10.00C? Explain (1mk) ii) Choose two compounds which are members of the same homologous series and explain the difference in their melting points. (3mks) iii) The compound C3H8O is an alcohol. How does its solubility in water differ from the solubility of C5H12 in water? Explain. (2mks) b) Complete combustion of one mole of a hydrocarbon produced four moles of carbon dioxide and four moles of water only. i) Write formula of the hydrocarbon (1mk) ii) Write the equation for the combustion reaction: (1mk) c) In a reaction, an alcohol J was converted to a hex- 1 – ene. i) Give the structural formula of the alcohol J ii) Name the reagent and conditions necessary for the reaction in c (i) above d) Compound K reacts with sodium hydroxide as shown below? (1mk) i) What type of reaction is represented by the equation above? (1mk) ii) To what class of organic compounds does K belong? (1mk)
The extraction of aluminium from it s ore takes place in two stages, purification stage and electrolysis stage. The diagram below shows the set – up for the electrolysis stage
a)
i) Name the ore from which aluminium extracted. (1mk) ii) Name one impurity, which is removed at the purification stage.(1mk) b) i) Label on the diagram each of the following i) Anode ii) Cathode iii) Region containing the electrolyte. ii) The melting point aluminium oxide is 20540C, but electrolysis is carried out between 800 – 9000C. i) Why is the electrolysis not carried out at 20540C (1mk) ii) What is done lower the temperatures? (1mk) iii) The aluminium which is produced is tapped off as aliquid. What does this suggest about it smelting point? (1mk) c) A typical electrolysis cell uses current of 40,000 amperes.Calulate the mass (in kilograms) of aluminium produced in one hour) (3mks)
a) The diagram below shows incomplete set – up of the laboratory and preparation collection of chlorine gas. Study it and answer the questions that follow.
i) Complete the set – up to show how dry chloride gas may be collected.
ii) The equation for the redox reaction that takes place is
Explain, using oxidation numbers, which species is reduced (2mks)
iii) What is the purpose of water in flask L? (1mk) b) Study the diagram below and answer the questions that follow.
When some hydrogen chlorides gas is allowed into water and the mixture stirred, the bulb lights and gases X and Y are formed.
i) Name: Gas X Gas Y ii) Explain why the bulb does not light before the hydrogen chloride gas is let into water. (2mks) iii) Explain using equations why the volume of gas X is less than that of gas (2mks)
Sodium thiosulphate solution reacts with dilute hydrochloric acid according to the following equation.
In an experiment to study how the rate of reaction varies withconcentration, 10cm3 of 0.4M sodium thiosulphate was mixed with 10cm3. Of 2M hydrochloric acid in a flask. The flask was placed in a white paper marked with across X.The time taken for the cross X become invisible when viewed from above was noted and recorded in the table below. The experiment was repeated three times as the temperature using the volumes in the table and the results recorded as shown in the table below. a) i) On the grid below, plot a graph of the volume of thiosulphate (Vertical axis) against time taken for the cross (X) to become invisible) ii) From the graph determine how long it would take for the cross to become invisible if the experiment was done. (3mks) i) Using 6cm3 of the 0.4M thiosulphate (1mk) ii) Using 6cm3 of 0.2M thiosulphate solution (1mk) b) i) Using values for experiment I.Calculate i) Moles of thiosulphate used (1mk) ii) Moles of hydrochloric acid used (1mk) ii) Explain which of the two reactants in experiment I controlled the rate of the reaction? Explain (1mk) c) Give two precautions which should be taken in experiment I controlled the rate of the reaction? Explain (2mk)
(a) Brine usually contains soluble calcium and magnesium salts. Explain how sodium carbonate is used to purify brine.
b) The diagram below represents a diaphragm cell used to electrolysed pure brine
i) Write the equations for the reactions that take place at
I Cathode II Anode ii) Name: I Product at U II Another material that can be used instead of titanium III The impurity present in the product at U iii) State two functions of the diagram c) Give one industrial use of the product at U.
a) at 250°C , 50g of potassium were added to 100gm of water to make a saturated solution. What is meant by a saturated solution?
b) The table below gives the solubilities of potassium nitrate at different temperatures.
(i) Plot graph of the solubility of potassium nitrate (vertical axis) against temperature
(ii) Using the graph: I Determine the solubility of potassium nitrate at 15°C II Determine the mass of potassium nitrate that remained undissolved given that 80g of potassium nitrate were added to 100cm3 of water and warmed to 40°C. c) Determine the molar concentration of potassium nitrate at 15°C (Assume there is no change in density of water at this temperature) (K = 39. 0; N= 14.0; O = 16.0)
ANSWERS
The react between a piece of magnesium ribbon with excess 2m hydrochloric acid was investigated at 25°C by measuring the volume of hydrogen gas produced as the reaction progressed. The sketch below represents the graph that was obtained.
a) Name one piece of apparatus that may be used to measure the volume of hydrogen gas produced.
b) On the same diagram, sketch the curve that would be obtained if the experiment when excess chlorine gas was bubbled into hot concentrated sodium hydroxide, the following reaction occurred.
In which product did chorine undergo oxidation? Explain
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