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Author Topic: 2018 waec theory and OBJ chemistry answers  (Read 138 times)

Emeka

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2018 waec theory and OBJ chemistry answers
« on: April 10, 2018, 03:46:26 pm »
CHEMISTRY OBJ:
1-10: ABDCBCABAC
11-20: DACBBACCAA
21-30: CBCDCAAACB
31-40: AACACBABAA
41-50: DBBDDBDCDB

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1ai)
Fermentation is the process in which a substance breaks down into a simpler substance. Microorganisms like yeast and bacteria usually play a role in the fermentation process, creating beer, wine, bread, yogurt and other foods.

(1aii)
Zymase

(1b)
-The strength of an acid
-The PH of the solution

(1c)
Fe+H2SO4->FESO4 + H2
H2SO4=10cm^3
Concentration=1mol/dm^3
Amount=Vol * Concentration
=10/100*1
=0.01mol
1mol=56g
0.01mol=56*0.01
=0.56g
Mass of unreacted=5-0.056
=4.44g

===========================
(2ai)
Collision theory states that the rate of a reaction depends on the rate of collision of the reactant molecules. Hence effective collision determines the rate of the reaction because it is the collision that leads to the formation of a product.

(2aii)
When the rate of collision increases, the rate at which molecules collide with each other will also increase thereby making the kinetic energy of the molecules to also increase. The temperature will also increase because temperature is a measure of average kinetic energy.

(2bi)
Draw your diagram

(2bii)
C2H5OH + 3O2 ---->2CO2 + 3H2O
1mole of ethanol = 3moles of O2
2.5moles of ethanol will require 3 × 2.5/1 = 7.5moles of O2.
but 1 mole of gas = 22.4dm3
7.5moles = 7.5 × 22.4 = 168dm3 of O2.

(2Ci)
Esterification is the formation of an Ester by the reaction between Alkanol and an acid.

(2cii) Two uses of alkanols
(i) They are used as solvents for cellulose
(ii) They are uses in making perfumes and cosmetics.
(iii) They are used for quick drying of paints and nail varnishes.

(2ciii)
Sodium Ethanoate

(2di)
Tin

(2dii )
This is because the galvanized plate is corrosion-resistant. It has a protective coating which prevents further oxidation of the metal.

(3ai)
H H H H H
| | | | |
H-C-C-C-C-C-C=C-H Heptene
| | | | | |
H H H H H H

(3aii)
The sixth member of Allene C7H14 =
7(molecular mass of carbon) + 14(molecular mass of H)
= 7(12) + 14(1)
= 84 + 14 = 98gmol-¹

(3aiii)
-Cracking is the breaking down of higher molecules into smaller molecules while reforming is the change in the functional group or activity of the compound.
-In cracking more than one product or component is formed. But in reforming one product can still be gotten.

(3bi)
Enthalpy of Neutralization change is the heat change which offers when one mole of H+ from an acid reacts with one mole of OH- from an alkali to form one mole of water.

(3bii)
Weigh about 100cm³ of dil HCL(aq) and that of KOH(aq) of equal volume and put each into a glass calorimeter. The temperature of the two Solutions are taken. The mean volume of the temperature of the acid and base taken. Then quickly transfer the alkali to the acid, turn and take the final temperature of the solution. Record the mass of the mixture. Find the temperature difference.
Total heat covered = mass × specific heat capacity × temperature change.

(3ci)
I - copper
II - silver

(3cii)
Because silver is below copper in the electrochemical series.

(3ciii)
Oxidation - anode
Reduction - Cathode
3civ)
Cu(s)+2Ag+ -->Cu²+ + 2Ag

(3d)
(i) Brown hydrated Iron(iii)oxide (FeO3 . xH2O)
(ii) 4Fe(s) + 3O2(g)+2xH2O ->2Fe2O3. xH2O(s)
(iii) Haematite


(4ai)
DRAW THE DIAGRAM

(4aii)
Na2SO3(aq) +2HCl(aq) ->2NaCl(aq)+H2O(l)+SO2(g)

(4aiii)
Dry the gas evolved by passing it through H2SO4 and collect it by downward delivery

(4aiv)
Because it is denser than air

(4bi)
It states that there is a mixture of gases which do not react chemically together,then the total pressure exerted by the mixture is the sum of the partial pressure of the individual gases that make up the mixture

(4bii)
T1=12C=285K
P1=690mmHg
V1=30cm^3
T2=273K
P2=760mmHg
V2=?
Using general gas law
V1P1/T1=V2P2/T2
V2=V1P1T2/T1P2
=(30*690*273)/(285*760)
V2=26cm^3

(4ci)
Cl2(g)+H20(l)->HCl(aq)+HOcl(aq)
2HOCl(sunlight)->2HCl(aq+O2(g)

(4cii)
Litmus turns red because of the presence of acid HCl

(4di)
Chlorine->Bromine->iodine (increasing boiling point)

(4dii)
Because water has two lone pairs of electrons as compared to ammonia which has one





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