WAEC Chemistry Answers 2020:
PAPER 2 [Essay]
Answer any FOUR questions.
Write your answers on the answer booklet provided.
1. (a) (i) What is Faraday’s first law of electrolysis?
(i) Faraday’s first law of electrolysis states that the mass(m) of an element discharged during electrolysis is directly proportional to the quantity of electricity(Q) passing through it.
(ii) -Strong electrolytes conduct large currents while Weak electrolytes do not conduct current readily.
– Strong electrolytes ionize completely while weak electrolytes ionize only slightly.
(b) Ethyne burns in air to give a smoky and luminous flame (complete combustion)
(c) (i) If an organic compound contains carbon atoms joined by double or triple covalent bonds. The compound is said to be an unsaturated hydrocarbon.
(ii) CH3COOH(aq) + CH3OH(aq) <–> CH3COOCH3(l) + H2O(l)
(iii) Methyl ethanoate
(d) By removing the main product continuously
(e) Number of moles of Zn dust = Mass/molars mass = 3.75/65 =0.0077
Reacting mole ratio of Zn to H2 is 1:1
: No of moles of H2 produced =0.0677 moles
1 mole = 6.02×10²³ molecules
0.0577moles of hydrogen produces
=6.02×10²³ x0.0577
=0.347×10²³ molecules
Or 3.47×10²² molecules.
(f) Flooding
(g) (i) Reaction C
(ii) Zinc (from + 2 to + 4)
(h) (i) – Each spectral line is caused by one electron
– Electron can exist only in circular orbit of definite quantum energy.
(ii) It could not account for the spectrum at note complicated atoms
(i) (i)Temperature of the reacting system
(ii)Pressure of the reacting system
(iii)Concentration of the reacting system
(j) (i) Both liberate carbon(iv) oxide
(ii) One produce a chloride salt , the other produce an ethanoate salt.
2. (a) (i) What is the structure of the atom as proposed by Rutherford?
(ii) Distinguish between the atomic number and the mass number of an element.
(iii) Explain briefly why the relative atomic mass of chlorine is not a whole number.
(b) (i) What is meant by first ionization energy?
(ii) List three properties of electrovalent compounds
(iii) Consider the following pairs of elements:
9F and 17CL;
12Mg and 20Ca.
Explain briefly why the elements in each pair have similar chemical properties.
(c) Explain briefly the following terms using an appropriate example in each case
(i) homologous series;
(ii) heterolytic fission.
(d) State the indicator(s) which could be used to determine the end-point of the following titrations:
(i) dilute hydrochloric acid against sodium hydroxide solution;
(ii) dilute hydrochloric acid against ammonium hydroxide solution;
(iii) ethanoic acid against sodium hydroxide solution.
(e) A solid chloride E which sublimed on heating reacted with an alkali F to give a choking gas G. G turned moist red litmus paper blue. Identify E,F and G.
ANS: (a) (i) The atom has small/ tiny positively charged centre /nucleus with electrons surrounding the space around the centre.
(ii) Atomic number of an element is the number of protons/electrons in an atom of the element while mass number is the sum of the protons and neutrons in the atom of the element.
(iii) Chlorine atom is made up of a mixture of isotopes and the relative atomic mass of chlorine is the average of its isotopic masses.
(b) (i) Is the (minimum) energy required to remove one mole of an electron from one mole of gaseous atom (to form one mole gaseous charged ion
(ii) High melting /boiling point;
Ability to conduct electricity in the molten state or in solution;
Solid at room temperature;
Soluble in water or polar solvents /insoluble in non-polar solvents.
(iii) Atoms of the elements in each pair have the same number of electrons in their outer-most shell therefore similar chemical properties.
(c) (i) Is a family of organic compounds:
– where successive members differ by –CH2 of molar mass of 14;
– with similar chemical properties;
– which conform to the same general formula;
– which show gradation of physical properties;
– which have the same general method of preparation. e.g alkanes, alkenes , alkanols.
(ii) Is a process in which a (covalent) bond is broken in such a way that the electron pair is completely transferred to one of the atoms (resulting in the formation of ions)
H ÷ CI → H+ + Cl-/ HCl ® H+ + Cl-
(d)(i) Methyl orange/ methyl red/ phenolphthalein;
(ii) Methyl orange/ methyl red;
(iii) Phenolphthalein.
(e) E – NH4Cl
F – NaOH, KOH, or Ca (OH)2, Li OH, CsOH, Ba(oH)2, Mg(OH)2
G – NH3.
3. (a) (i) Define saturated solution.
(ii) The solubility of KN03 at 20°C was 3.00 mol dm-3 If 67.0g of KN03 was added to 250cm3 of water and stirred at 20°C, determine whether the solution formed was saturated or not at that temperature.
(b) (i) Distinguish between dative bond and covalent bond.
(ii) Explain why sugar and common salt do not conduct electricity in the solid state.
(iii) State the type of intermolecular forces present in
I. hydrogen fluoride;
II. argon.
(iv) Consider the compounds with the following structures:
S – H —-N and 0 – H —–N
In which of the compounds is the hydrogen bond stronger? Give reason for your answer.