KCSE CHEMISTRY QUESTIONS AND ANSWERS PER TOPIC
Compound L reacts with hydrogen bromide gas to give another compound whose structure is
(a) Give the structural formula and name of compound L
(b) Write an equation for the reaction, which takes place between ethane and excess chlorine gas
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State and explain one disadvantages of using hard water in boilers
Expected Response
On boiling hard water deposits CaCO3 which forma fur/ scales in the boiler. This reduces conductivity
Ethanoic acid and ethanol react as shown in the equation below:
Other than warming, how would the state of equilibrium be established within a short time?
Soap dissolves in water according to the equation below;
NaSt(aq) —> Na+(aq) + St- where St- is the stearate ion. (a) Write the formula of the scum formed when soap is used in hard water. (b) Write the ionic equation for the reaction that occurs when sodium carbonate is used to remove hardness in water.
Two organic compounds P and Q decolourise acidified potassium manganate (VII) solution but only P reacts with sodium metal to give a colourless gas. Which homologous series does compound P belong? Give a reason.
ANSWERS
P is in alkanol R - OH. The alkanol reacts with sodium metal to produce the colourless gas.
Sodium hydroxide can be prepared by the following methods; I and II.
(a) Name the 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 soap.
(a) What is meant by a strong acid?
(b) In an experiment 40cm3 of 0.5M hydrochloric acid was reacted with excess sodium carbonate and the volume of carbon (IV) oxide produced recorded with time. In another experiment, the same volume and concentration of ethanoic acid was also reacted with excess sodium carbonate and the volume of carbon (IV) oxide produced recorded with time. On the grid below, sketch and label the curves if the volumes of carbon (IV) oxide were plotted against time.
Complete the nuclear equation below:
(c) Give one harmful effect of radioisotopes.
(a) What name is given to the process by which alcohol is formed from a carbohydrate?
(b) Explain why the solubility of ethane in water is lower than that of ethanol.
ANSWERS
(a) Fermentation.
(b) Ethane remains in molecular form while ethanol forms hydrogen bonds with water.
The figure below shows how the rate of the following reaction varies with the time.
i) Which of the two curves represent the rate of the reverse reaction? Give a reason
ii) What is the significance of point X and Y on the figure? b) State and explain the effect of an increase in pressure on the rates of the following reactions.
i) On a grid plot a graph of volume of gas produced (vertical axis) against time
ii) From the graph, determine the rate of the reaction at: (I) 15 seconds (II) 120 seconds (III) Give a reason for the difference between the two values.
ANSWERS
(a)Curve 1
The concentration of products are increasing The rate of reaction is increasing. At time x equilibrium has been established, the rate of forward reaction is equal to the rate of reverse reaction and this has a value of Y. (b)(i) Increasing pressure increases rate of reaction. Molecules are brought closer, more collision of gases particles. Increasing pressure has no effect on liquids.
The melting and boiling points of zinc are 419°C and 907°C respectively. One of the ores of zinc blende. To extract zinc, the ore is first roasted in air before feeding it into a furnace.
a. i) Write the formula of the main zinc compound in zinc blende. ii) Explain using an equation why it is necessary to roast the ore in air before introducing it into the furnace b. The diagram below shows a simplified furnace used in the extraction of zinc. Study it and answer the questions that follows:
i) Name two other substance that are also introduced into the furnace together with roasted ore.
ii) The main reducing agent in the furnace is carbon(II) oxide. Write two equations showing how it is formed. iii) In which physical state is zinc at point Y in the furnace? Give a reason iv) Suggest a value for the temperature at point X in the furnace. Give a reason. v) State and explain one environmental effect that may arise from the extraction of zinc from zinc blende vi) Give two industrial uses of zinc.
a. 50cm3 of 1M copper (II)sulphate solution was placed in a 100cm3 plastic beaker. The temperature of the solution was measured. Excess metal A powder was added to the solution, the mixture stirred and the maximum temperature was repeated using powder of metals B and C. The results obtained are given in the table below:
i. Arrange the metal A, B, C and copper in order of reactivity starting with the least reactive. Give reasons for the order.
ii. Other than temperature change, state one other observation that was made when the most reactive metal was added to the copper(II) sulphate solution.
b. The standard enthalpy change of formation of methanol is -239 kJmol-1
i) Write the thermol chemical equation for the standard enthalpy change of formation of methanol.
iii) The calculate enthalpy change in part B(ii) (II) above differ from the standard enthalpy change of formation of methanol. Give a reason.
Under certain conditions, carbon dioxide reacts with water to form methanol (CH3OH) and oxygen as shown below
What would be the effect on the yield of methanol if the temperature of the reaction mixture is increased? Explain
In an experiment a certain volume of air was passed from syringe to syringe over heated zinc powder as shown in the diagram below
The experiment was repeated using excess magnesium powder. In which of two experiments was the change in volume of the air greatest. (Give reasons)
(a).i) Give the name of the following compounds:
b). Describe a chemical test that can be carried out in order to distinguish between
c). Study the flow chart below and answer the questions that follows
i) Name the compounds:
(I). L (II). N ii) Draw the structural formula of compound M showing two repeat units iii) Give the reagent and the conditions used in step I iv) State the type of reaction that take place in: (I). Step 2 (II). Step 3
d). The molecular formula of compound P is C2H2Cl4. Draw the two structural formulae of compound P
a) Which one of the following compounds; urea, ammonia, sugar and copper (II) chloride will conduct an electric current when dissolved in water? Give reasons.
b) The diagram below shows an electrochemical cell. Study it and answer the questions that follows.
i) Show on the diagram using an arrow, the direction of flow of electrons
ii) Name two subsrances that are used to fill the part labeled L c) In an experiment to electroplate iron with silver, a current of 0.5 amperes was passed through a solution of silver nitrate for an hour i) Give two reasons why it is necessary to electroplate iron with silver ii) Calculate the mass of silver that was deposited on iron (Ag = 108, 1 Faraday = 96,500 coulombs) (a) The table below gives the solubilities of hydrated copper (II) sulphate in mol dm3 at different temperatures (i) On the grid provided, plot a graph of solubility of copper (II) sulphate (vertical axis) against temperature. (ii) From the graph, determine the mass of copper (II) sulphate deposited when solution is cooled from 700C to 400C. (Molar mass of hydrated copper (II) sulphate = 250g) (b) In an experiment to determine the solubility of sodium chloride, 5.0cm3 of a saturate solution of sodium chloride weighing 5.35g were placed volumetric and diluted to a total volume of 250cm3. 25.0cm3 of the dilute solution chloride completely reacted with 24cm3 of 0.1M silver nitrate solution. AgNO3(aq) + NaCI(aq) → AgCl(s) + NaNO3(aq) Calculate: (i) Moles of silver nitrate in 24cm3 of solution (ii) Moles of sodium chloride in 25.0cm3 of sodium (iii) Moles of sodium chloride in 250cm3 of solution (iv) Mass of sodium chloride in 5.0cm3 of saturated sodium chloride Solution (Na = 23.0, CI = 35.5) (v) Mass of water in 5.0cm3 of saturated solution of sodium chloride (vi) The solubility of sodium chloride in g/100 water
A sample of river water was divided into three portions. The table below shows the test carried out on the portions and the observations made.
Complete the table by filling in the inferences.
The curves below shows how the electronic conductivity of hydrochloric and ethanoic acids vary with concentration.
Explain why the electrical conductivity of 0.01M hydrochloric acids is higher than that of 0.01M ethanoic acid.
ANSWERS
Nitrogen and hydrogen react to form ammonia gas as shown in the following equation:
a) The figure below shows how the percentage of ammonia gas in the equilibrium mixture change with temperature.
Explain why the percentage of ammonia gas change as shown in the figure.
b) On the axes below, sketch a graph showing how the percentage of ammonia gas in equilibrium mixture changes with pressure.
The use of CFCs has been linked to depletion of the ozone layer.
a) What does CFC stand for? b) Explain the problem associated with the depletion of the ozone layer c) State another environment problem caused by CFCs
ANSWERS
(a)Chlorofluorocarbon
(b)When ozone is depleted, high energy UV radiations reach the earth, which may cause skin cancer to human beings. (c)Global warmings, /green house effect.
The half equations involved in a cell are:
2H2O(l) + 2e = H2(g) + 2OH-(aq): Eθ = - 0.83V O2(g) + 2H2O(l) + 4e = 4OH-(aq): Eθ = + 0.40V a) Write the overall equation for the electrochemical cell. b) Calculate he e.m.f. generated by a battery consisting of ten cells. c) State one environment advantage of using these cells in spacecrafts.
A sample of fertilizer is suspected to be calcium ammonium nitrate. Describe chemical tests for each of the following ions in the sample:
a) Calcium ions; b) Ammonium ions.
ANSWERS
(a)Add a few drops of NaOH to an aqueous solution of the fertilizer. Forms white precipitate insoluble in excess. .
Add a few drops of aqueous sulphuric (VI) acid to another portion of aqueous solution of fertilizer. Forms a white precipitate insoluble in excess. (b)Heat the sample fertilizer in a test tube, and test gas evolved with damp red litmus paper, turn blue. Or add NaOH to the sample fertilizer and heat the mixture; test gas evolved using damp red litmus paper, turn blue.
Some animal and vegetable oils are used to make margarine and soap. Give the reagents and conditions necessary for converting the oils into:
a) Margarine b) Soap
A beaker contained 75.0cm3 of aqueous copper (II) sulphate at 23.7°C. when scrap iron metal was added to the solution, the temperature rose to 29.3°C.
a) Write an ionic equation for the reaction that took place. b) Given that the mass of copper deposited was 5.83g, calculate the molar enthalpy change in kJmol-1. (specific heat capacity of solution = 4.2Jg-1 K-1, density of solution 1.0gcm-3, Cu = 63.5)
The figure below shows an energy cycle.
a) Give the name of the enthalpy change ΔH1.
b) Determine the value of ΔH3. |
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