Tuesday, November 17, 2020

Free NECO Chemistry Obj & Essay Expo 2020 - ZAZZAFA TV



Free NECO Chemistry Obj & Essay Expo 2020 - ZAZZAFA TV

A. 2020 NECO CHEMISTRY (OBJECTIVES) ANSWERS:


1-10: BBDDADEBCA

11-20: BDEABDBCEE

21-30: DEEECCCCEC

31-40: ABDAADCAAB

41-50: EDEDAEADDD
51-60: DBEBABBADA


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B. 2020 NECO CHEMISTRY (ESSAY) ANSWERS: 

(1a)
(i)Diamond
(ii)Graphite

(1aii)
(i)Coal
(ii)Coke

(1aiii)
(i)Elements with similar properties recur at regular intervals or periods
(ii)The properties of the elements are a periodic function of their atomic numbers

(1b)
(i)Periodicity is the variation of the properties of elements in a regular pattern both down the groups and across the periods.
(ii)1 mole contains 6.023 x 10^23 atoms
0.5 moles of oxygen contains 0.5 x 6.023 x 10^23 atoms
= 3.0115 x 10^23 atoms

(1ci)
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.

(1cii)
No. of moles, N = Q/Fe = 20300/96500x4 = 20300/386000 = 0.05259
volume of oxygen librated at s.t.p = 0.05259 x 22.4
=0.1178dm^3

(1di)
Cathodic half cell reaction: 4H+(aq) + 4^e- ---> 2H2(g)
Anodic half cell reaction: 4OH-(aq) ---> 2H2O(i) + O2(q) + 4^e-

(1dii)
Tetraoxosulphate(iv)acid ---> Electrolyte
Ester ---> Non-electrolyte
Phenol ---> Non-electrolyte

(1ei)
Mercury

(1eii)
(i)Number of electrons = 16
(ii)M-16 = 18 (no. of neutrons)
M=18+16 = 34
Mass number = 34
(iii)Sulphur





======================================================

(2ai)
The basicity of an acid is the number of replaceable hydrogen ions N+ in one molecule of the acid.
(2aii)
(i) H3p04 ---> basicity = 3
(ii) HC00H ---> basicity = 1
(iii) H2S04 ---> basicity = 2

(2bi)
-Temperature of the reacting system
-Pressure of the reacting system
-Concentration of the reacting system

(2bii)
-Nature of reactants
-Concentration/Pressure of reactants
-Surface area of reactants

(2ci)
In a tabular form
Under S/N
I, II

Under Indicator:
Methyl orange, Phenolphthalein

Under colouric acid:
Pink, Colourless

Under colour of endpoint:
Orange, colourless

Under colour in base:
Yellow, pink

Under suitable for:
Strong acid and weak base,
Weak acid and strong base

(2cii)
I. Nitrogen
1S, 2S, <-2P->
↑↓ ↑↓ ↑↑↑

III. Fluorine
1S, 2S, <-2P->
↑↓ ↑↓ ↑↓↑↓↑

II. Aluminium
1S, 2S, 2P, 3S, 3P
↑↓ ↑↓ ↑↓↑↓↑↓ ↑↓ ↑


(2di)
(i) Heat is evolved liberating hydrogen gas
(ii) Decolourization of purple KMn04 with deposit of sulphur
(iii) Black residue of carbon due to dehydration

(2dii)
(i) Glucose is used as an immediate source of energy for sick people and sportsmen
(ii) It is used in the manufacture of sweets.

======================================================

(3ai)
Cl2(g) + 2NaOH(aq) + Nacl + H2O(l)
[Cold dilute]

3Cl2(aq) + 6NaOH(aq) + 5Nacl(aq) + 3H2O(l)
[Hot Conc.]

(3aii)
Green to Brown

(3aiii)
DIAGRAM

(3bi)
(i)Hard water tastes better due to dissolved minerals in it
(ii)Hard water helps snails and crabs to make their shells

(3bii)
(i)Exhaust of power plants and vehicles
(ii)Gases from chemical warfare

(3biii)
(i)Starch ---> Iodine
Fats and oil ---> Sudan III

(3ci)
(i)Standard NaOH cannot be accurately prepared because it is deliquescent
(ii)Standard H2SO4 cannot be accurately prepared because it is hydroscopic. Both absorb moisture from the atmosphere.

(3cii)
(i)2,2-dibromo-3,3dimethyl-butane
(ii)2-methyl-Hexane
(iii)Pentan-3-one
(iv)Butan-2-one

(3d)
(i)Boyl’s law states that the volume of a given mass of gas is inversely proportional to its pressure, provided the temperature remains constant.
(ii)V1/T1 = V2/T2 i.e V2 = V1T2/T1
= 400 x (273+150)/(273+30)
Volume at 50°c = 400x323/303 = 426.4cm^3

=======================================================

(4ai)
(I) Water is necessary for corrosion to take place whereas burning is different in the presence of water.
(II) Boiling takes place at a specific temperature called boiling point whereas evaporation takes place at all temperatures.

(4bi)
A concentrated solution is one containing large amount of the solute dissolved in water.

(4bii)
(i) The positions of the ions in the electrochemical series.
(ii) The concentration of the ions in the electrolyte.
(iii) The nature of the electrode.

(4ci)
R.M.M = 2[Al] + 3[(S) + 4(O)]
= 2(27) + 3[32 + 4(16)]
= 54 + 3[96]
=342

(4cii)
Aluminium tetraoxosulphate(vi).

(4ciii)
Propane -1,2,3 -triol and salt.

(4di)
(i) Soapless detergents do not form scum with hard water, soaps do.
(ii) Soaps are biodegradable; certain soapless detergents are not and hence cause pollution of water ways.

(4dii)
OH and COOH

(4diii)
v1p1=v2p2 ie v2 = v1p1/p2
= 300×760/800 = 285cm³

(4div)
Change = +13 - 10 = +3

(4dv)
Aluminium ion




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