KCSE CHEMISTRY QUESTIONS AND ANSWERS PER TOPIC
Pentane and ethanol are miscible. Describe how water could be used to separate a mixture of pentane and ethanol
Expected Response
20. Add water to the mixture in a separating funnel. Ethanol dissolves while pentane does not. Allow the mixture to separate in two layers. Open the tap to drain the lower aqueous layer. Distil the water ethanol mixture to get ethanol.
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The curves below were obtained when two equal volumes of hydrogen peroxide of the same concentration were allowed to decompose separately. In one case, manganese (IV) oxide was added to the hydrogen peroxide
Which curve represents the decomposition of hydrogen peroxide with manganese (IV) oxide? Explain
Expected Response
19. I – Manganese (iv) Oxide is a catalyst and increases the rate of decomposition of the hydrogen peroxide.
The table below shows the tests carried out on separate samples of water drawn from a well and the results obtained
(a) Identify the cation and the anion present in the water
Cation Anion (b) Write an ionic equation for the reaction which takes place in test (iii) A radioactive isotope X2 decays by emitting two alpha (a) particles and one beta (β) to from 214 Bi 83 (a) What is the atomic number of X2? (b) After 112 days, 1/16 of the mass of X2 remained. Determine the half life of X2
Expected Response
Study the solubility curves below and answer the questions that follows
What happens when a solution containing 40gm of potassium chlorate and 40gm of potassium nitrate in 100gm of water at 900C is cooled to 400C? Explain
When a current of 1.5 amperes was passed though a cell containing m3+ ions of a metal M for 15 minutes, the mass of the cathode increased by 0.26g.
(1 Faraday = 96500 coulombs) (a) Calculate the quantity of electricity used (b) Determine the relative atomic mass of metal M
(a) Name two cations that are present in hard water.
(b) Explain how the ion exchange resin softens hard water.
120g of iodine - 131 has a half life of 8 days and decays for 32 days. On the grid provided, plot a graph of the mass of iodine - 131 against time.
a) The flow chart below shows a sequence of reactions starting with.
Study and answer the questions that follow.
Cu = 63.5, Fe = 56.0
a) When an organic compound Y is reacted with aqueous sodium carbonate, it produces carbon dioxide reacts with propanol to form a sweet smelling compound Z whose formula is.
c) The scheme below was used to prepare a cleaning agent. Study and answer the questions that follow.
i) What name is given to the type of cleaning agent prepared by the method shown in the scheme?
ii) Name one chemical substance added in step II iii) What is the purpose of adding the elements substance names in C(ii) above. iv) Explain how an aqueous solution of the cleansing removes oil from utensils during washing.
Study the information in the table below and answer the questions that follow: A mixture containing 35g of CuS04 and 78g of Pb(N03)2 in l00g of water at 60°C was cooled to 40°C.
(a) Which salt crystallised out? Give a reason
(b) Calculate the mass of the salt that crystallised out.
The set – up below was used during the electrolysis of aqueous magnesium sulphate using inert electrodes
i) Name a suitable pair of electrodes for this experiment ii) Identify the ions and cations in the solution iii) On the diagram label the cathode iv) Write ionic equations for the reactions that took place at the anode. v) Explain the change that occurred to the concentration of magnesium sulphate solution during the experience. vi) During the electolysis a current of 2 amperes was passed through the solution for 4 hours. Calculate the volume of the gas produced at the anode.(1 faraday 96500 coulombs and volume of a gas at room temperature is 24000cm3) vii) One of the uses of electrolysis is electroplating What is meant by electroplating? Give tow reasons why electroplating is necessary.
The table below gives the volume of the gas provided when different volumes of 2M hydrochloric were reacted with 0.6g of magnesium powder at room temperature
(a) Write an equation fro the reaction between magnesium and hydrochloric acid
(b) On the grid provided plot a graph of the volume of gas produced (vertical axis), against the volume of acid added (Note the reaction comes to completion, the volume of the gas produced directly proportional to completion, the acid added). From the graph determine c)
e) Given that one mole of the gas occupied 24000cm3 at room temperature. The table below shows the PH values of solutions I, II, III and IV.
(a) Which solution is likely to be that of calcium hydroxide?
(b) Select the solution in which a sample of aluminium oxide is likely to dissolve. Give reason for your answer
Bromine reacts with ethane as shown below
(a) What condition is necessary for this reaction to occur?
(b) Identify the bonds which are broken and those that are formed
The graph below shows the mass of a radioactive isotope plotted against time
(a) Using the graph, determine the half – life of the isotope
(b) Calculate the mass of the isotope present after 32 days
(a) What is meant by molar heat of combustion?
(b) State the Hess's Law. c) Use the following standard enthalpies of combustion of graphite, hydrogen and enthalpy of formation of propane.
(i) Write the equation for the formation of propane.
(ii) Draw an energy cycle diagram that links the heat of formation of propane with its heat of combustion and the heats of combustion of graphite and hydrogen. (iii) Calculate the standard heat of combustion of propane. Other than the enthalpy of combustion, state one factor which should be considered when choosing a fuel. (e) The molar enthalpies of neutralization for dilute hydrochloric acid and dilute nitric (V) acid are -57.2kJ/mol while that of ethanoic acid is -55.2kJ/mol. Explain this. observation.
ANSWERS
(a) The amount of heat liberated when one mole of a substance is burnt in excess Oxygen.
(b) The heat evolved are absorbed in a chemical change is the same whether the change occurs in one step or through many steps.
(d) cost
effect on environment availability storage (e) Ethanoic acid is a weak acid therefore heat is used to ionize it before neutralization occurs . It value is therefore lower than that of hydrochloric acid which is fully ionized
(a) Study the flowchart below and answer the questions that follows;
(i) I What observation will be made in Step I
II Describe a chemical test that can be carried out to show the identity of compound C (ii) Give the names of the following I E II substance D (iii)Give the formula of substance B . (iv) Name the type of reaction that occurs in: I Step (II) II Step (IV) (v) Give the reagent and conditions necessary for Step (VI). Reagent: Conditions (b) (i) Name the following structure.
(ii) Draw the structure of an isomer of pentene.
ANSWERS
(a) (i) I The potassium permanganate is decolorized or changes from purple to colourless.
II C is a ethanoic acid (carboxylic acid) Add sodium carbonate, you will see effervescence, test gas evolved with lime water, it will form a white precipitate. (ii) I Polyethene II Substance D - sodium ethoxide
(a) Ethanol can be manufactured from ethene and steam as shown in the equation below:
Temperature and pressure will affect the position of equilibrium of the above reaction. Name the other factor that will affect the position of equilibrium of the above reaction.
(b) The data in the table below was recorded when one mole of ethene was reacted with excess steam. The amount of ethanol in the equilibrium mixture was recorded under different conditions of temperature and pressure. Use the data to answer the questions that follow.
(i)State whether the reaction between ethene and steam is exothermic or endothermic.
Explain your answer. (ii) State and explain one advantage and one disadvantage of using extremely high pressure in this reaction. I Advantage II disadvantage (c) In an experiment to determine the rate of reaction between calcium carbonate and dilute hydrochloric acid, 2g of calcium carbonate were reacted with excess 2 M hydrochloric acid, The volume of carbon (IV) oxide evolved was recorded at regular intervals of one minute for six minutes. The results are shown in the table below.
(i) plot a graph of time in minutes on the horizontal axis against volume of carbon (IV) oxide on the vertical axis.
(ii) determine the rate of reaction at 4 minutes
ANSWERS
(a) Increase or change in amount of reagent either reactants or products.
(Concentration). (b) (i) Exothermic increase in temperature from 250 - 350 at constant pressure the amount of ethanol formed at equilibrium decreases. (ii) I Advantage - it would increase the yield of ethanol ; since increase in pressure will favour side with less moles i.e. the products. II Disadvantage - it would mean investment in equipment to withstand the high pressure and would be expensive.
The set-up below was used by a student to investigate the products formed when aqueous copper (II) chloride was electrolyzed using carbon electrodes.
(a) (i) Write the equation for the reaction that takes place at the cathode.
(ii) Name and describe a chemical test for the product initially formed at the anode when a highly concentrated solution of copper (II) chloride is electrolysed. (iii) How would the mass of the anode change if the carbon anode was replaced With copper metal? Explain. (b) 0.6 g of metal B were deposited when a current of 0.45A was passed through an electrolyte for 72 minutes. Determine the charge on the ion of metal B. (Relative atomic mass of B = 59, 1 Faraday = 96 500 coulombs) (c) The electrode potentials for cadmium and zinc are given below:
Explain why it is not advisable to store a solution of cadmium nitrate in a container made of zinc
A sample of river water is suspected to contain zinc and sulphate ions.
Describe how the presence of zinc ions and sulphate ions can be established.
ANSWERS
The flow chart below shows some processes involved in the extraction of zinc metal:
a) Name oneore from which zinc is extracted.
b) Write the equation of the reaction taking place in unit II c) Name two uses of zinc metal.
The thermal chemical reaction between carbon and sulphur is as shown by the equation below:
When solid magnesium carbonate was added to a solution of hydrogen chloride in methylbenzene, there was no apparent reaction. On addition of water to the resulting mixture, there was vigorous effervescence. Explain these observations
One of the fuels associated with crude oil is natural gas. Name the main constituent of natural gas and write an equation for its complete combustion
Name: Equation: |
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