KCSE CHEMISTRY QUESTIONS AND ANSWERS PER TOPIC
(a) State one characteristic of a reaction where equilibrium has been attained.
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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
When a student was stung by a nettle plant, a teacher applied an aqueous solution of ammonia to the affected area of the skin and the
student was relieved of pain. Explain.
ANSWERS
A mixture contains ammonium chloride, copper (II) oxide and sodium chloride. Describe how each of the substances can be obtained form the mixture.
ANSWERS
Describe an experimental procedure that can be used to extract oil from nut seeds.
ANSWERS
Crush grind using a pestle and mortar, add suitable solvent of propanone ethanol alcohol and stir to dissolve oil. Filter the mixture to obtain a solution of the oil. Leave the solution in the sun for propanone to evaporate leaving the oil.
Hydrogen sulphide is a highly toxic and flammable gas. It is normally prepared in a fume chamber.
(a) Name two reagents that can be used to prepare hydrogen sulphide in the laboratory. (b) One of the uses of hydrogen sulphide is to produce sulphur as shown in the following equation: 2H2S(g) + S02 (g) -> 3S(s) + 2H20(1) Identify the reducing agent in this reaction and give a reason for your answer. (c) Other than production of sulphuric (VI) acid, state one commercial use of sulphur.
ANSWERS
(a)Iron (II) sulphide or conc sulphide / copper sulphide (Accp formula: Fes/ HCl)
Hydrochloric acid or lead (II) sulphide/ HNO3 (b)Hydrogen sulphide The sulphur changes from -2 to zero/ (it reduces SO2 to S) i.e. +4 to 0 / sulphur lost e’s in the H2S to form sulphur (c)Vulcanization of rubber Manufacture of sulphur drugs Manufacture of gun powder/ match sticks / explosives/ fungicides
Charcoal is a fuel that is commonly used for cooking. When it burns it forms two oxides.
(a) Name the two oxides. (b) State one use of the two oxides.
ANSWERS
(a)Carbon (IV) oxide /CO2/ carbon dioxide
Carbon (II) oxide/ CO/ carbon monoxide (b)Fire extinguisher/ photosynthesis Refrigeration Solvay process Fizzy drinks Food preservation Extraction of metals Manufacture of methanol Manufacture of fuel (water, gas)
Describe how a solid sample of potassium sulphate can be prepared starting with 200cm3of 2M potassium hydroxide.
Given the following substances: wood ash, lemon juice and sodium chloride.
(a) Name one commercial indicator that can be used to show whether wood ash, lemon juice and sodium chloride are acidic, basic or neutral. (b) Classify the substances in 15(a) above as acids, bases or neutral.
ANSWERS
(a) Litmus
Phenolphthalein indicator (b) Wood ash- Basic Lemon Juice - acidic Sodium Chloride- neutral
The set-up below was used to collect a dry sample of a gas
Give two reasons why the set-up cannot be used to collect carbon (IV) oxide gas.
ANSWERS
Draw and name the isomers of pentane.
60 cm3 of oxygen gas diffused through a porous partition in 50 seconds. How long would it take 60cm3 of sulphur (IV) oxide gas to diffuse through the same partition under the same conditions? (S = 32.0, 0 = 16.0)
In an experiment on rates of reaction, potassium carbonate was reacted with dilute sulphuric (VI) acid.
(a) What would be the effect of an increase in the concentration of the acid on the rate of the reaction? (b) Explain why the rate of reaction is found to increase with temperature.
ANSWERS
(a)The rate of reaction increases. This is because when the concentration is high: the number of collisions between particles is also high hence reacts faster,
(b)Increase in temperature results in increase in the kinetic energy of the particles. This makes particles move faster and collide frequently leading to faster rate of reaction.
A water trough, aqueous sodium hydroxide, burning candle, watch class and a graduated gas jar were used in an experimental set up to determine the percentage of active part of air. Draw a labeled diagram of the set up at the end of the experiment.
A compound whose general formula is M(OH)3 reacts as shown by the equation below.
(a) What name is given to compounds which behave like M(OH) 3 in the two reactions.
(b) Name two elements whose hydroxides behave like that of M.
ANSWERS
(a) Amphoteric
(b)Lead, Zinc and Aluminium
The atomic number of sulphur is 16. Write the electron arrangement of sulphur in the following:
1.
(a) Alkanes are said to be saturated hydrocarbons.
(b) When the alkane, hexane, is heated to high temperature, one of the products is ethene.
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