KCSE CHEMISTRY QUESTIONS AND ANSWERS PER TOPIC
The Ph of a sample of soil was found to be 5.0.An agricultural office recommended the addition of calcium oxide in the soil. State two functions of the calcium oxide in the soil.
Expected Response
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Hydrogen peroxide decomposes according to the equation below:
8.5 gm of hydrogen peroxide contained in 100cm3 of solution with water were completely decomposed. Determine the rise in temperature due to the reaction. Specific density of water = 1g/cm3 O =16, H = 1,).
Expected Response
The apparatus shown below shown below was used to investigate the effect of carbon monoxide on copper (II)oxide
a) State the observation that was made in the combustion tube at the end of the experiment.
b) Write an equation for the reaction that took place in the combustion tube c) Why is it necessary to burn the gas coming out of tube K?
Explain why hydrogen forms compounds in which its oxidation state is either + 1 or -1 (Atomic number of hydrogen is 1)
The table below shows the properties of substances K,L,M and N
Select the substances which are likely to be:
a) Copper metal b) Magnesium chloride
Expected Response
(a) Copper metal M
(b) Magnesium chloride K
An element P has a relative atomic mass of 88. When a current of 0.5 amperes was passed through the fused chloride of P for 32 minutes and 10 seconds, 0.44g of P were deposited at the cathode. Determine the charge on an ion of P. (1 faraday = 96500 Coulombs).
Expected Response
The melting point of phosphorous dichloride is – 910C. While that of magnesium chloride is 7150C.In terms of structure and bonding, explain the difference in their melting points.
Expected Response
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 experiments did the bulb not light?
b) Explain your answer in (a) above.
A student investigated a property of acids M and N by reacting equal volumes of acid M and N of the same concentration with equal volumes of 2M potassium hydroxide.
The results were recorded in the table below.
a) Which of the acids is likely to be a weak acid? Explain.
b) Write the equation for the reaction between ethanoic acid and potassium hydroxide.
By using aqueous sodium chloride, describe how a student can distinguish calcium ions from lead ions.
State and explain what would happen if a dry red litmus paper was dropped in a gas jar of dry chlorine.
ANSWERS
A solution was made by dissolving 8.2g of calcium nitrate to give 2 litres of solution. (Ca= 40.0; N=14.0; O= 16.0)
Determine the concentration of nitrate ions in moles per litre.
Explain why burning magnesium continues to burn a gas was bubbled
The burning magnesium produces more heat energy that the burning splint. The heat energy from magnesium is enough to break the sulphur oxygen bond setting free oxygen. Magnesium uses freed oxygen to continue burning.
Explain why a burning magnesium ribbon continues to burn when placed in a gas jar containing carbon (iv) oxide gas but a burning splint is extinguished.
(b) Write an equation for the reaction that takes place in (a) above.
a)what observation would be made if hydrogen sulphide gas was bubbled through a solution of zinc nitrate?
b) write an equation for the reaction that takes place in (a) above
In the presence of U.V light, ethane gas undergoes substitution reaction with chlorine.
(a) What is meant by the term? Substitution reaction: (b) Give the structural formula and the name of the organic product formed when equal volumes of ethane and chlorine react together.
a) State one cause of temporary hardness in water.
b) How does distillation remove hardness from water?
Expected Response
(a) Presence of Ca (HCO3) or mg (HCO3)2 (b) Water vaporizes and distils off leaving behind ions that cause hardness
The diagram below shows an iron bar, which supports a bridge. The Iron bar is connected to a piece of magnesium metal
Explain why it is necessary to connect the piece of magnesium metal to the iron bar.
Expected Response
Magnesium is above iron in the activity series. It supplies electrons to the iron bar Hence prevent it from rusting
When 15 cm3 of a gaseous hydrocarbon, P, was burnt in 100cm3 of oxygen, the resulting gaseous mixture occupied 70cm3 at room temperature and pressure.When the gaseous mixture was passed through potassium hydroxide solution, its volume decreased to 25cm3
a) What volume of oxygen was used during the reaction? b) Determine the molecular formula of the hydrocarbon
a) Name two ores from which copper is extracted
b) During extraction of copper metal, the ore is subjected to froth flotation. Give a reason why this process is necessary. c) Name one alloy of copper and state its use.
ANSWERS
(a) Copper pyrites chalcocite, malachite
(b) To concentrate the ore (c) Brass Batteries
On heating a pale green solid K, carbon (IV) oxide gas and a black solid M were formed. On reacting K with dilute hydrochloric acid, carbon (IV) oxide gas and
green solution S were formed. When excess aqueous ammonia was added to solution S, a deep blue solution was formed. a) Identify the cation in solid K b) Identify the two onions in solution S.
The plots below were obtained when the atomic radii of some elements in groups I and II were plotted against atomic numbers.
Explain:
a) The trend shown by Li, Na and K. b) Why the atomic radii of elements Be, Mg, and Ca are lower than those of Li, Na and K.
ANSWERS
(a)The atomic radii increase with increase in atomic number. This is due to increase in energy levels.
(b) The group II elements have more protons than group I elements hence this increases the nuclear attraction for the outer electrons.
(a) Diamond and graphite are allotropes of carbon. What is meant by an allotrope?
(b) Explain why graphite can be used as a lubricant while diamond cannot.
ANSWERS
(a) These are different forms carbon in the same physical state.
(b) The hexagonal graphite rings have weak Van der Waals forces between the layers that allow the layers to slide over each other , while in diamond the atoms are held by strong Covalent bonds.
The set-up below was used to investigate the products of burning biogas (methane).
Study it and answer the questions that follow.
a) What product will be formed in test- tube Y?
b) State and explain the observations which would be made in Z?
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. |
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