KCSE CHEMISTRY QUESTIONS AND ANSWERS PER TOPIC
Cotton is a natural polymer. State one advantage and one disadvantage of this polymer.
ANSWER
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(a) A student electrolyzed dilute sodium chloride solution using inert carbon electrodes. Name the products at:
i)Anode : ii)Cathode: (b) If the experiment was repeated using concentrated sodium chloride instead of dilute sodium chloride solution, write the half equation at the anode.
Study the flow chart below and answer the question the follow.
a) Name the process in step I.
b) Identify the reagent in step II. c) Give one use of ethane
ANSWERS
(a) Step 1 is neutralisation.
(b) Step II is soda lime/ mixture of NaOH and CaO. (e) Fuel/making ethene/making hydrogen gas.
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) Rate increases.
(b) Temperature increases the kinetic energy of the particles increasing the number of collisions.
When 20cm3 of 1 M sodium hydroxide was mixed with 20cm3 of 1 M hydrochloric acid, the temperature rose by 6.70C Assuming the density of the solution is 1 g/cm3 and the specific heat capacity of the solution is 4.2Jg-1k-1.
(a) Calculate the molar heat of neutralization; (b) When the experiment was repeated with 1 M ethanoic acid, the temperature changes was found to be lower than that with 1 M hydrochloric acid. Explain.
One of the ores of copper has formula, CuFeS2.
(a) Describe how iron in the ore is removed during concentration of copper metal. (b) State two environmental problems associated with extraction of copper metal.
The table below shows the relative molecular masses and boiling points of pentane and ethanoic acid
Explain the large difference in boiling point between ethanoic acid and pentane.
ANSWERS
The flow chart in Figure 1 represents some stages in the extraction of copper metal. Study it and answer the questions that follow.
Identify:
Describe how an increase in concentration increases the rate of a reaction. (2 marks)
Expected Response
As the concentration increases the number of reactivity of particles per unit volume increases thus affecting collisions. This leads to increase in the rate of reaction
a) Study the information in the table below and answer the questions that follow.
(The letters do not represent the actual symbols of the elements).
c) Neutralization is one of the methods of preparing salts. i) What is meant by neutralization? ii) Describe how you would prepare crystals of sodium nitrate starting with 200cm3 of 2M sodium hydroxide. iii) Write an equation for the reaction that takes place when a solid sample of sodium nitrate is heated.
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
In an experiment to study the rate for reaction between duralumin (an alloy of aluminium, magnesium and copper) and hydrochloric acid, 0.5g of the alloy were reacted with excess 4M hydrochloric acid. The data in the table below was recoded.
Use it to answer the questions that follow. a) i) On the grid provided, plot a graph of total volume of gas produced (vertical axis) again time. ii) From the graph, determine the volume of gas produced at the end of 2 ½ minutes. b) Determine the rate of reaction between the 3rd and 4th minute. c) Give a reason why some solid remained at the end of the experiment d) Given that 2.5cm3 of the total volume of the gas was from the reaction between magnesium and aqueous hydrochloric acid, calculate the percentage mass of aluminium present in 0.5g of the alloy. (Al = 27.0 and Molar gas volume = 24,000cm3 at 298k) e) State two properties of duralumin that make it more suitable than aluminium in aeroplane construction. a) In which homologous series do the following compounds belong i) CH3CC ii) CH3CH2COO
b) Raw rubber is heated with sulphur in the manufacture of natural rubber.
i) What is the name given to the process ii) Why is the process necessary?
c) Study the scheme given below and answer the questions that follow
i) Write an equation for the reaction between propane – 1 – 0 / and potassium metal.
ii) Name processes I and II I II iii) Identify the products A and B iv) Name one catalyst used in process II v) Draw the structural formular of the repeating unit in the polymer C. d) State two industrial uses of methane.
a) Study the standard electrode potentials do the half – cells given below and
answer the questions that follow. (The letters do nor represent the actual symbols of the elements.)
i) Identify the strongest oxidizing agent. Give a reason for your answer.
ii) Which two half – cells would produce the highest potential difference when combined? iii) Explain whether the reaction represented below can take place. b) 100cm3 of 2M sulphuric acid was electrolysed using the set – up represented by the diagram below.
i) Write an equation for the reaction that produces gas L.
ii) Describe how gas K can be identified iii) Explain the difference in : I The volume of the gases produced at the electrodes. II Brightness of the bulb if 100cm3 of 2M ethanoic acid was used in place of sulphuric acid.
(a) Give the systematic names for the following compounds
(b) Study the flow chart below and use it to answer the questions that follow
(i) Identify the organic compound K
(ii) Write the formula of M (iii) Give one reagent that can be used in (a) Step I (b) Step II (iv) Write the equation of the reaction in step III (c) The structure below represents a type of a cleaning agent
Describe how the cleansing agent removes grease from a piece of cloth
(a) distinguish between a neutron and proton
(b) What is meant by a radioactive substance? (c) State two dangers associated with radioactive substance in the environment
(i) What is the atomic
a. Mass of Y b. Number of Y (ii) What name is given to the type of reaction undergone by the isotopes of hydrogen? (e)(i) What is meant by half-life of a radioactive substance (ii) 288 g of a radioactive substance decayed to 9 g in 40 days. Determine the half-life of the radioactive substance
(a) the set below can be used to produce sodium hydroxide by electrolyzing brine
(i) Identify gas Y
(ii) Describe how aqueous sodium hydroxide is formed in the above set-up (iii)One of the uses of sodium hydroxide is in the manufacturing of soaps state one other use of sodium hydroxide (b).study the information given in the table below and answer the question that follows
(i) Construct an electrochemical cell that will produce the largest emf
(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 a container made of H?
ANSWERS
(a)(i)Gas Y is chlorine
(ii) sodium and hydrogen ions migrate to the cathode . The hydrogen ions are preferentially discharged, liberating hydrogen gas. chlorine and hydroxide ions migrate to the anode . The chloride ions are preferentially discharged liberating chlorine gas. the sodium ions migrate to the cathode through the membrane . the sodium ions combine with the hydroxide ions to form sodium hydroxide . (iii) Glass making/paper manufacture (1), unclogging of drains, etching NaClo3, Purification of bauxite.
(a) (i) what is meant by the term Enthalpy of formation?
(ii) The enthalpies of combustion of carbon, methane and hydrogen are indicated below:
(i) Draw an energy cycle diagram that links the enthalpy of formation of methane to enthalpies of combustion of carbon, hydrogen and methane
(ii) Determine the enthalpy of formation of methane (b) An experiment was carried out where different volumes of dilute hydrochloric acid and aqueous sodium hydroxide both at 25°C were mixed and stirred with a thermometer. The highest temperature reached by each mixture was recorded in the table below
(i) On the grid provided. plot a graph of highest temperature (vertical axis) against volume of hydrochloric acid ( horizontal axis)
(ii) Using your graph , determine the (a) Highest temperature reached (b) Volume of acid and base reacting when highest temperature is reached
(iii) Calculate the amount of heat liberated during the neutralization process .(specify heat capacity is 4.2 j g -1 k-1 and the density of solution is 1.0 g cm-3)
(c) The molar enthalpy of neutralization between hydrochloric acid and ammonia solution was found to be -52.2 kJ mol-1, while that of hydrochloric acid and sodium hydroxide was – 57.1 Kj mol-1. Explain the difference in these values.
ANSWERS
(a)(i)This is the heat absorbed or evolved when one mole of any substance is formed from its constituent elements in their normal states.
(c)The molar heat of neutralisation between a strong acid and a weak base is low because some of the heat is used to ionise (1) the weak base before neutralization.
For strong acid and strong base they are completely ionised.
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
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.
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
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
The curves below represent the change in mass when equal masses of powered zinc and zinc granules were reacted with excess 2M hydrochloric acid. Study them and answer the question below.
Which curve represents the reaction with zinc granules? Explain your answer.
ANSWERS
X - Zinc granules
The gradient of the graph is less steep because there is less surface area.
a) Write an equation for the reaction
b) Using the bond energies given below, calculate the enthalpy change for the reaction in (a) above |
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