KCSE CHEMISTRY QUESTIONS AND ANSWERS PER TOPIC
Study the flow chart in Figure 5 and answer the questions that follow.
(a) Identify substances K and L. K:
(b) Name one reagent that can be used to carry out process J.
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The following procedure was used to investigate the temperature changes that occur when sodium hydroxide solution is added to dilute hydrochloric acid.
(i) Place the acid in a glass beaker and record its temperature. (ii) Add a known volume of sodium hydroxide solution. (iii) Stir the mixture and record the highest temperature reached. (iv) Repeat steps (ii) and (iii) with different volumes of sodium hydroxide solution. (a) State two factors that must be kept constant in this experiment (b) Explain how the use of a polystyrene cup will affect the results.
ANSWERS
The set-up in Figure 4 can be used to prepare nitrogen (II) oxide. Use it to answer the questions that follow.
(a) Name substance A
(b) When the gas jar containing nitrogen (II) oxide is exposed to air, a brown color is observed. Explain. (c) Write an equation for the reaction which occurred in the flask.
The flow chart in Figure 3 shows the process of obtaining a sample of nitrogen gas. Study it and answer the questions that follow.
(a) Identify X
(b) Write an equation for the reaction with heated copper turnings. (c) Name an impurity in the sample of nitrogen gas.
In an experiment, concentrated nitric(V) acid was reacted with iron(II) sulphate. State and explain the observations made.
ANSWERS
The mixture changed from green to yellow / formation of a brown gas;
Iron(II) ions is oxidized by nitric(V) acid to Iron(III) ions / nitric(V) acid is reduced to nitrogen(1I) oxide which is oxidized by oxygen to nitrogen(IV ) oxide.
Starting with copper, describe how a pure sample of copper(II) carbonate can be prepared.
Using the elements chlorine, calcium and phosphorus:
(a) Select elements that will form an oxide whose aqueous solution has a pH less than 7. (b) Write an equation for the reaction between calcium oxide and dilute hydrochloric acid. (c) Give one use of calcium oxide.
Explain the observation made when chlorine gas is passed through a solution of potassium iodide.
Potassium nitrate liberates oxygen gas when heated. Draw a diagram of a set-up that shows heating of potassium nitrate and collection of oxygen gas.
An oxide of element K has the formula K2O5.
(a) Determine the oxidation number of K. (b) To which group of the periodic table does K belong?
20 cm3 of ethanoic acid was diluted to 400 cm3 of solution. Calculate the concentration of the solution in moles per litre. (C = 12.0 ; H = 1.0 ; 0 =16.0) (Density of ethanoic acid = 1.05 g/cm3)
Copper(II) ions react with excess aqueous ammonia to form a complex ion.
(a) (i) Write an equation for the reaction that forms the complex ion. (ii) Name the complex ion. (b) Explain why CH4 is not acidic while HCl is acidic yet both compounds contain hydrogen.
(a) State one characteristic of a reaction where equilibrium has been attained.
A sample of water is suspected to contain sulphate ions. Describe an experiment that can be carried out to determine the presence of sulphate ions.
(a) State Charles' Law.
(b) Explain why the pressure of a fixed mass of a gas increases, when the volume of the gas is reduced at constant temperature.
ANSWERS
(a)The volume of a fixed mass of a gas is directly proportional to the absolute temperature at constant pressure.
(b)As the volume decreases, there is increased bombardment / collisions of the molecules against the walls of the container, hence increased pressure.
The set-up in Figure 2 was used to prepare a sample of ethane gas. Study it and answer the questions that follow.
(a) Name B
(b) Write an equation for the complete combustion of ethane. (c) State one use of ethane.
The empirical formula of lead(II) oxide was determined by passing excess dry hydrogen gas over 6.69g of heated lead(II) oxide.
(a) What was the purpose of using excess dry hydrogen gas? (b) The mass of lead was found to he 6.21g. Determine the empirical formula of the oxide. (Pb = 207.0 0 = 16.0)
The diagram in Figure 1 shows a section of a dry cell. Study it and answer the questions that follow.
(a) Name the part labelled B.
(b) The part labelled A is a paste. Give a reason why it is not used in dry form. (c) What is the purpose of the zinc container?
ANSWERS
(a)Carbon electrode (Anode) /Graphite electrode.
(b)To allow movement of ions / to have it as an electrolyte. When dry, the ions are immobile. (c)It is the cathode / negative electrode.
Calculate the values of X and Y in the following nuclear equation.
ANSWERS
Table 1 shows the atomic numbers and the first ionisation energies of three elements. The letters are not actual symbols of the elements. Use it to answer the questions that follow.
(a) Explain the trend in first ionisation energy from A to C.
(b) Write the electronic configuration for the ion of C.
ANSWERS
(a) Ionisation energy decreases down the group 1 elements.
This is because atomic radii increases from A to C (down the group) /outermost electron is far from nucleus hence requires less energy to be lost during reaction. (b)Electron configuration of ion of C- 2.8.8
(a) Write an equation to show the effect of heat on the nitrate of:
(i) Potassium (ii) Silver (b) The table below gives information about elements Ai, A2, A3, and A4 i) In which period of the periodic table is element A2? Give a reason.
(ii) Explain why the atomic radius of:
I. Ai is greater than that of A2; II. A4 is smaller than its ionic radius. (iii) Select the element which is in the same group as A3. (iv) Using dots (•) and crosses(x) to represent outermost electrons, draw a diagramto show the bonding in the compound formed when Ai reacts with A4.
a)Draw the structural formula for all the isomers of C2H3CL3
b) Describe two chemical tests that can be used to distinguish between ethane and ethane.
c)The following scheme represents various reactions starting with propan-1-ol. Use it to answer the questions that follow.
i) Name one substance that can be used in step I.
ii) Give the general formula of X.
iii)Write the equation for the reaction in step IV.
iv) Calculate the mass of propan-1-ol which when burnt completely in air at room temperature and pressure would produce 18dm3 of gas. (C = 12.0; O = 16.0; H = 1.0; Molar gas volume = 24dm3)
The diagram below represents a set up of an electrolytic cell that can be used in the production of aluminium.
(a) On the diagram, label the anode.
(b)Write the equation for the reaction at the anode. (c) Give a reason why the electrolytic process is not carried out below 950°C. (d)Give a reason why the production of aluminium is not carried out using reduction process (e)Give two reasons why only the aluminium ions are discharged (f)State two properties of duralumin that makes it suitable for use in aircraft industry. (g)Name two environmental effects caused by extraction of aluminium.
The set up below can be used to produce sodium hydroxide by electolysing brine.
(i) Identify gas Y.
(ii)Describe how aqueous sodium hydroxide is formed in setup above. (iii)One of the uses of sodium hydroxide is in manufacture of soaps. State one other use of sodium hydroxide. (b) Study the information given in the table below and answer the questions that follow.
(i) Construct an electrochemical cell that will produce the highest
(ii) Calculate the emf of the cell constructed in (i) above. (iii)Why is it not advisable to store a solution containing E+ ions in the container made of H?
(a) Methanol is manufactured from carbon (IV) oxide and hydrogen gas according to the equation:
The reaction is carried out in the presence of a chromium catalyst at 700K and 30kPa. Under these conditions, equilibrium is reached when 2% of the carbon (IV) oxide is converted to methanol
(i)How does the rate of the forward reaction compare with that of the reverse reaction when 2% of the carbon (IV) oxide is converted to methanol? (ii)Explain how each of the following would affect the yield of methanol: I Reduction in pressure II Using a more efficient catalyst (iii) If the reaction is carried out at 500K and 30kPa, the percentage of carbon (IV) oxide converted to methanol is higher than 2% I what is the sign of ΔH for the reaction? Give a reason II Explain why in practice the reaction is carried out at 700K but NOT at 500K (b)Hydrogen peroxide decomposes according to the following equation:
2H2O2(aq) →2H2O(l) + O2 (g)
In an experiment, the rate of decomposition of hydrogen peroxide was found to be 6.0 x 10-8 mol dm-3 S-1. (i)Calculate the number of moles per dm3 of hydrogen peroxide that had decomposed within the first 2 minutes (ii) In another experiment, the rate of decomposition was found to be 1.8 x 10 - 7 mol dm -3S-1. The difference in the two rates could have been caused by addition of a catalyst. State, giving reasons, one other factor that may have caused the difference in two rates of decomposition |
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