KCSE CHEMISTRY QUESTIONS AND ANSWERS PER TOPIC
ANSWERS
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An alcohol 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)=16.0). b) Draw the structural formulae of the compound,
a) What name is given to a compound that contains carbon and hydrogen only?
b) Hexane is a compound containing carbon and hydrogen. (i) What method is used to obtain hexane from crude oil? (ii) State one use of hexane c) Study the flow chart below and answer the questions that follow.
(i) Identify reagent L.
(ii) Name the catalyst used in step 5. (iii) Draw the structural formula of gas J. (iv) What name is given to the process that takes place in step 5? d) (i) write the equation for the reaction between aqueous sodium hydroxide and aqueous ethanoic acid. (ii) Explain why the reaction between 1g of sodium carbonate and 2M hydrochloric acid is faster than the reaction between 1g of sodium carbonate and 2M ethanoic acid.
Explain why the boiling point of ethanol is higher than that of hexane.
(Relative molecular mass of ethanol is46 while that of hexane is 86)
ANSWERS
(a) The list below shows the formulae of some organic compounds. Use it to answer the questions that follow.
(i) Select two compounds whichI are not hydrocarbons II Belong to the same homologous series
(ii) Identify the compound that is likely to undergo polymerization. Give a reason for your answer. a. The structures below represents two cleansing agents: R – COO- Na+ R – OSO3- Na+ In the table below, give one advantage and one disadvantage of using each one of them
b. Under certain, ethanoic acid ( C2H4O2) and ethanol (C2H5OH) react to form a sweet smelling compound.
(i) What is the general name of compound to which the sweet smelling compound belong? (ii) Write the formula of the sweet smelling compound (iii) Give one use of ethanoic acid other the formation of the sweet smelling compounds (iv) Write the equation for the reaction dilute ethanoic acid and solid potassium carbonate c. Fibres are either synthetic or natural. Give one: (i) Example of a natural fibre (ii) Advantage of synthetic fibres have over natural fibres
a) Study the table below and answer the questions that follow
i) Which of the compounds is a solid at 10.00C? Explain (1mk) ii) Choose two compounds which are members of the same homologous series and explain the difference in their melting points. (3mks) iii) The compound C3H8O is an alcohol. How does its solubility in water differ from the solubility of C5H12 in water? Explain. (2mks) b) Complete combustion of one mole of a hydrocarbon produced four moles of carbon dioxide and four moles of water only. i) Write formula of the hydrocarbon (1mk) ii) Write the equation for the combustion reaction: (1mk) c) In a reaction, an alcohol J was converted to a hex- 1 – ene. i) Give the structural formula of the alcohol J ii) Name the reagent and conditions necessary for the reaction in c (i) above d) Compound K reacts with sodium hydroxide as shown below? (1mk) i) What type of reaction is represented by the equation above? (1mk) ii) To what class of organic compounds does K belong? (1mk)
answers
a) State how burning can be used to distinguish between ethane and ethyne.
Explain your answer. b) Draw the structural formula of the third member of the homologous series of ethyne. c) The flow chart below shows a series of reactions starting with ethanol. Study it and answer the questions that follow.
ii) Write the equation for the combustion of ethanol
iii) Explain why it is necessary to use high pressure to change gas B into the polymer iv) State one use of methane
An organic compound with the formula C4 H10 O reacts with potassium metal to give hydrogen gas and white solid
(a) Write the structural formula of the compound (b) To which homologous series does the compound belong? (c) Write the equation for the reaction between the compound and potassium metal
(a) what is meant by heat of vaporization
(b) The boiling points of ethanol, propanol and butanol are 780C, 97.20C and 1170C. explain this trend
answer
(a) Write the structural formula of:
(i) Methanol (ii)Methanoic acid (b) Write the equation for the reaction between methanoic acid and aqueous sodium hydroxide (c) (i) Name the product formed when methanol reacts with methanoic acid (ii) State one condition necessary for the reaction in (c) (i) above to take place (d) (i) Describe one chemical test that can be used to distinguish between hexane and hexane (ii) State one use of hexane (iii) Hydrogen reacts with hexane to form hexane. Calculate the volume or hydrogen gas required to convert 42g of hexane to hexane at S.T.P ( C=12.0, H=1.0, molar gas volume at S.T.P is = 22.4 litres) Explain how a sample of CH3CH2CH2OH, could be distinguished from a sample of CH3COOH by means of a chemical reaction (2mks)
expected response
Add solid hydrogen carbonate; CH3COOH produces effervescence; while CH3CH2CH2OH does not (Accept any other carbonate that behaves)
expected response
The ionic end lowers the surface tensions of water , facilitating mixing while the nonionic end (non-polar end) mixes with grease, dislodging it from the fabric.
The structures below represents a portion of a polymer
(a) Give the name of the polymer (b) Give one industrial use of the polymer
answer
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