KCSE CHEMISTRY QUESTIONS AND ANSWERS PER TOPIC
100 g of a radioactive substance was reduced to 12.5 g in 15.6 years. Calculate the half – life of the substance
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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
The reaction between how concentrated sodium hydroxide and chlorine produces sodium chlorate (V), sodium chloride and water
(a) Write the equation for the reaction (b) Give one use of sodium chlorate (V)
ANSWERS
A certain carbonate, GCO3, reacts with dilute hydrochloric acid according to the equation given below:
GCO3(s) + 2HCL(aq)= GCl2 (aq) + (CO2(g) + H2O(l) If 1 g of the carbonate reacts completely with 20 cm3 of 1 M hydrochloric acid ,calculate the relative atomic mass of G (C = 12.0 = 16.0)
Determine the oxidation state of sulphur in the following compounds:
a) H2S……………………… b) Na2S……………………
Give the name and draw the structural formula of the compound formed when one mole of ethane reacts with one mole of chlorine gas.
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?
Using dots (.) and crosses(x) to represent electrons, show bonding in the compounds formed when the following elements react: (si = 14, Na = 11 and Cl = 17)
a) Sodium and chlorine b) Silicon and chlorine
Calcium oxide can be used to dry ammonia gas.
a) Explain why calcium oxide is not used to dry hydrogen chloride gas b) Name one drying agent for hydrogen chloride gas a) Give the name of each of the processes described below which takes place when salts are exposed to air for sometime. i) Anhydrous copper sulphate becomes wet (1mk) ii) Magnesium chloride forms an aqueous solution (1mk) iii) Fresh crystals of sodium carbonate, Na2CO3. 10H2O (1mk) b) Write the formula of the complex ion formed in each of the reactions described below. (i) Zinc metal dissolves in hot alkaline solution (1mk) (ii) Copper hydroxide dissolves in excess ammonia solution. (1 mk) (c) A hydrated salt has the following composition by mass. Iron 20.2% Oxygen 23.0%, sulphur 11.5%, water 45.3 %. Its relative formula mass is 278. (i) Determine the formula of the hydrated salt.. (3mks) (Fe=56, S=32; O = 16, H =1) (ii) 6.95gm of the hydrates salt were dissolved in distilled water and the total volume made to 250 cm3 of solution. Calculate the concentration of the salt solution in moles per litre.
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)
State one use of sodium hydrogen carbonate.
ANSWERS
(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) Crude oil is a source of many compounds that contain carbon and hydrogen only.
(i) Name the processes used to separate the components of crude oil (ii) On what two physical properties of the above components does the separation depend? b) Under certain conditions, hexane can be converted to two products. The formula of one of the products is C3H3 (i) Write the formula of the other product (ii) Describe a simple chemical reaction to show the difference between the two products formed in (b) above. c) Ethane, C2H2 is another compound found in crude oil. One mole of ethane was reacted with one mole of hydrogen chloride gas and a product p1 and was formed. P1 was then reacted with excess hydrogen gas to form p2. Draw the structures p1 and p2. d) The set-up below was used to prepare and collect ethane gas. Study it and answer the questions that follow.
(i) Name the substance T
(ii) Give the property of ethane that allows it to be collected as shown in the set up. e) One of the reactions undergone by ethane is addition polymerization. Give the name of the polymer and one disadvantage of the polymer it forms. (2 marks) Name the polymer. Disadvantage of the polymer
The flow chart below shows some reactions starting with lead (II) nitrate. Study it and answer the questions that follow.
(i) State the condition necessary in step 1.
(ii) Identify: I Reagent K II Gas Q III Acidic products S and R (iii) Write: I The formula of the complex ion formed instep 3. II The equation of the reaction in step 4 b) The use of materials made of lead in roofing and in water pipes is being discouraged State: (i) Two reasons why these materials have been used in the past. (ii) One reason why their use is being discouraged c) (i) The reaction between lead (II) nitrate and concentrated sulphuric acid starts but stops immediately. Explain
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)
a) The diagram below illustrates how Sulphur/extracted by the Frisch process.
Label the pipe through which superheated water is pumped in
b. The equation below shows the oxidation of sulphur dioxide to sulphur trioxide in the contact process.
(i) Name one catalyst for this reaction
(ii) State and explain the effect on the yield of sulphur trioxide when: I The temperature increased II The amount of oxygen is increased (iii) Describe how sulphur trioxide is converted to sulphuric acid in the contact process. (c) State two disadvantages of having sulphur dioxide in the environment (d) Ammonia sulphate is a fertilizer produced by passing ammonia gas into concentrated sulphuric acid. (i) Write the equation for the reaction (ii) Calculate the mass in Kg of sulphuric acid required to produce 25kg of the fertilizer (S = 32.0; O= 16.0; N = 1.0)
a) The set-up below was used to collect gas F, produced by the reaction between water and calcium metal.
(i) Name gas F
(ii) At the end of the experiment, the solution in the beaker was found to be a weak base. Explain why the solution is a weak base. (iii) Give one laboratory use of the solution formed in a beaker. (b) The scheme below shows some reactions starting with calcium oxide. Study it and answer the questions that follow.
(i) Name the reagents used in steps 2 and 4
Step 2 ………. Step 4 ………. (ii) write an equation for the reaction in step 3. (iii) Describe how a solid sample of anhydrous calcium sulphate is obtained in Step 5 |
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