KCSE CHEMISTRY QUESTIONS AND ANSWERS PER TOPIC
Sodium hydroxide can be prepared by the following methods; I and II
(a) Name one precaution that needs to be taken in method I.
(b) Give the name of process A. (c) Give one use of sodium hydroxide.
ANSWERS
(a) Small piece of sodium metal (pea size) with a lot of water
Perform the experiment wearing goggles. (b) Electrolysis (c) Manufacture of paper (soften), soaps and detergents Fractional distillation of liquid air Extraction of aluminium metal Manufacture of bleaching agents eg NaOCl paper, textiles, oil refinery Making herbicides on weed killers Textile industry to soften
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When fuels burn in the internal combustion engine at high temperature, one of the products formed is nitrogen (II) oxide.
(a) Write the equation for the formation of nitrogen (II) oxide. (b) Give a reason why nitrogen (II) oxide is not formed at room temperature. (c) Describe how formation of nitrogen (II) oxide in the internal combustion engine leads to gaseous pollution.
ANSWERS
(a)N2(g) + 02(g) 2NO(g)
(b)Nitrogen atoms in the molecule are joined by strong triple covalent bond that requires a lot of energy to break than provided at room temperature (c) Nitrogen (II) oxide reacts with oxygen in air to form nitrogen (IV) oxide that dissolves in water vapour causing acid rain.
(a)Draw the structure of compound N formed in the following reaction.
(b) Give one use of compound N.
During an experiment, chlorine gas was bubbled into a solution of potassium iodide.
(a)State the observations made. (b)Using an ionic equation, explain why the reaction is redox
ANSWERS
(a)Solution turned from colourless to dark brown
Greenish yellow / pale green colour of Cl2 disappears Brown solution / black solid is deposited (b) Cl2 (aq) +2I – (aq) → I2 (aq)+2CI- (aq) Explanation; Iodine oxidation state changes from -1 to 0 hence oxidation while Cl2 0.5 changes from 0 to -1 hence reduction / increase is ON and decrease is ON or movement of electrons Cl2 gains e’s where lose
a)Complete the nuclear equation below:
(c)Give one harmful effect of radioisotopes.
Iron (III) oxide was found to be contaminated with copper (II) sulphate. Describe how a pure sample of iron (III) oxide can be obtained.
ANSWERS
The diagram below represents the set-up that was used to prepare and collect hydrogen chloride gas in the laboratory.
State the purpose of concentrated sulphuric (VI) acid in the wash bottle.
Write an equation for the reaction between dry hydrogen chloride gas and heated iron.
ANSWERS
It is a drying agent.
Fe(s) + 2HCI(g) → FeCI2(s) +H2(g)
Aluminium is both malleable and ductile.
(a)What is meant by? (i) Malleable: (ii)Ductile (b)State One use of aluminium based on: (i)malleability (ii)ductility
ANSWERS
(a) (i) Can be hammered into sheets.
(ii)Can be drawn into wires. (b)(i) Making of sufurias/ motor vehicle parts/ aeroplane parts,window / door flames, cups, plates, packaging materials, pans, making sheets/ roof. (ii)electricity cables/ wires.
When 8.53g of sodium nitrate were heated in an open test-tube, the mass of oxygen gas produced was 0.83 g . Given the equation of the reaction as;
2NaNO 3 (s) -> 2NaNO 2 (s) + O 2 (g) Calculate the percentage of sodium nitrate that was converted to sodium nitrite. (Na = 23.0, N = 14.0, O = 16.0)
Ammonium ion has the following structure
Label on the structure:
(a) covalent bond; (b) coordinate (dative) bond.
Starting with sodium metal, describe how a sample of crystals of sodium hydrogen carbonate may be prepared.
ANSWERS
React sodium with water to get sodium hydroxide. Bubble into this solution excess carbon (IV) oxide to get sodium hydrogen carbonate
The set up below was used to investigate the reaction between dry hydrogen gas and copper
(a) Name substance A.
(b) State the observation made in the combustion tube. (c) Explain the observation stated in (b) above.
ANSWERS
(a) Fused anhydrous calcium chloride
Cao: fused CaCl2 (b) Black CuO changes to brown Cu metal Formation of colourless liquid on the cooler parts of the combustion tube. (c) Copper (II) oxide is reduced by hydrogen to copper metal while hydrogen is oxidized to water /CuO reduced to Cu /H2 Oxidized to H2O
The figure below shows an energy cycle.
(a)Give the name of the enthalpy change ΔH1.
(b)Determine the value of ΔH3.
An alknal has the following composition by mass: hydrogen 13.5%, oxygen 21.6% and carbon 64.9%
(a)Determine the empirical formula of the alcohol(C=12.0; H=1.0’ 0=16.0). (b)Given that empirical formula and the molecular formula of the alkanol are the same,draw the structure of the alkonol
When burning magnesium ribbon is introduced into a gas jar full of nitrogen, it continues to burn producing a greenish yellow powder.
140cm3 of nitrogen gas diffuses through a membrane in 70 seconds. Flow long will it take 200cm3 of carbon(IV) oxide gas to diffuse through the same membrane under the same conditions of temperature and pressure. (3 marks)
Chemical tests were carried out on separate samples of water drawn from the same source. The observations made were recorded as shown in Table 4.
State the inferences made in reactions:
Starting with copper turnings. describe how a sample of copper(II) sulphate crystals can be prepared in the laboratory. (3 marks)
are isotopes of element X. They occur naturally in the ratio of 9:1 respectively.
Calculate the relative atomic mass of element X. (2 marks)
The diagram in Figure 6 Shows radiations emitted by a radioactive sample.
A student investigated the effect of an electric current by passing it through some substances. The student used inert electrodes, and connected a bulb to the circuit. The table below shows the substances used and their
states.
(a) In which experiment did the bulb not light?
(b) Explain your answer in (a) above.
ANSWERS
(a) 1 and 3
(b)In 1 ions K2CO3 are held rigidly within the crystal cannot move (no mobile ions) In 3 sugar exist as molecule hence no mobile ions.
(b) When magnesium burns in air, it forms a white solid and a grey-green solid.
When a few drops of water are added to the mixture, a gas that turns red litmus paper blue is evolved. Identify the (i) white solid. (ii) gas evolved and state its use. (I) Name of gas (II) Use of the gas. ; (c) Two different samples of water (I and II) were tested with soap solution. Sample II was further subjected to two other processes before adding soap. 20 cm3 of each sample of water was shaken with soap solution in a boiling tube until a permanent lather was obtained. The results are shown in the table below
(i) Identify the water sample that had temporary hardness. Explain your answer.
(ii) Explain why the results for sample II are different after distilling but remain unchanged after filtering. (iii) State two disadvantages of using both water samples for domestic purposes.
(a) Other than concentration, state two factors that determine the rate of a reaction.
b) In an experiment to determine the rate of reaction, excess lambs of calcium carbonate were added to 2 M hydrochloric acid. The mass of calcium carbonate left was recorded after every 30 seconds. The results are shown in the table below
i) Write the equation for the reaction that took place
ii) On the grid provided, plot a graph of mass of calcium carbonate vertical axis Against time (iii) Determine the rate of reaction at the 105th second. (c) Why does the curve level off after some time? (d) On the same grid, sketch a curve for the same reaction using 4 M hydrochloric acid and label the curve R.
The set up below can be used to generate a gas without heating. This occurs when substance M reacts with solid N.
a i) Complete the table below giving the names of substance M and solid N if the gasses generated are chlorine and sulphur (IV) oxide.
(ii) Complete the diagram above to show how a dry sample of sulphur (IV) oxide can be collected
(b) Describe two chemical methods that can be used to test the presence of sulphur (IV) oxide. (c) Other than the manufacture of sulphuric (VI) acid, state two uses of sulphur (IV) oxide.
ANSWERS
(b) Presence of SO2
- Use of acidified potassium dichromate (VI) which turns from orange to green. - Bubble gas through acidified potassium manganate (VII) which decolourises /changes i.e from purple to colourless. - Iron (III) sulphate solution - yellow/brown changes to green - Bromine water colour changes from yellow/brown / orange to colourless (c) - Fumigation - Bleaching agent - Preservative . - Disinfectant - Antioxidant
a) The diagram below represents a dry cell. Use it to answer the quest ions that follows.
i) Which of the letters represent
i) Carbon electrode? ii) The electrolyte? ii) One of the substances used in a dry cell is manganese (IV) oxide. State two roles of manganese (IV) oxide in the dry cells b) Below is simplified electrolytic cell used for purification of copper. Study it and answer the questions that follows.
i) Identify the cathode
ii) Write the equation for the reaction at the anode iii) What name is given to L? iv) A current of 0.6 A was passed Through the electrolyte for 2 hours. Determine the amount of copper deposited (Cu=63.5; 1 Faraday = 96,500 coulombs) v) St ate two uses of copper metal
ANSWERS
(a) (i) I F
II G (ii) - Manganese (IV) oxide oxidises hydrogen to water /depolariser . - It increases the surface area of the electrolyte (b) (i) Cathode J.
(v)Uses of copper metal - soldering bits / wires
- Electrical cables and alloys - coins, ornaments/lightening arrestors/ diodes/ - calorimeters. |
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