WAEC Chemistry Obj And Theory/Essay Expo Answer – 2017 May/June

Verified WAEC Chemistry Essay/Theory and OBJ Questions and Answer Solution – May/June 2017 Expo Runs Real and correct

1ai)

an acid base indicator is a weak acid or weak base that exhibits a colour change as the concentration of hydrogen (H^+) or hydroxide (H^-) ions changes in an aqueous solution

1aii)

methyl-orange

1b)

i)oxidizing agent

ii)mixed anhydride

1c)

enthalpy of combustion is the heat evolved when one mole of a substance is completely burns in oxygen gas under standard conditions

1cii)

enthapy of combustion is always negetive since it involvs a rise in temperature and heat of reactants is greater than the heat of products

1d)

primary cell is a cell that cannot be re charged while secondary cell is a cell that can be recharged

1dii)

primary cell — daniell cell

secondary cell — lead accumulator

1e)

mole is the unit of measurement in the International System of Units (SI) for amount of substance. The unit is defined as the amount of a chemical substance which contains as many representative particles, e.g., atoms, molecules, ions, electrons, or photons, as there are atoms in 12 grams of carbon-12 (12C), the isotope of carbon with relative atomic mass

1f)

n= V=40cm^3

c= 0.40moldm^3

n=cv/1000

0.40*40/1000 = 0.016mol

1g)

i)carbon or graphite

ii)platinum

1h)

i)induced dipole-induced dipole

interaction or dispersion force

ii)dipole-dipole interaction

1i)

i)carbon(ii)oxide and water or CO and H20

1j)

2bFe^3+-15^2,2s^2,2p^6,3s^2,3p^6,3d

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3ai)

Thermal cracking

3aii)

C4H8

3aiii)

(I) H H. H

| | |

H-C-C-C=C

| | | |

H H H H

and

(II) H H

| |

H-C-C=C-C-H

| | | |

H H H H

3aiv)

I-normal butene

II -But-2-ene

Ni

3av)

C4H8 +H2 ——-> C4H10

250c

3b)

in other words the oil is stained red. This is a aspecifit test for fats

if suden (iii) is added to fats a distinct red colouration is observed

3c)

Given 2502+02+2H20—>2H2SO4

1mole produce 2mole

1.3mole produce 3mole

moles of H2SO4=3mole

molar mass of H2SO4=(1+2)+32+(4*16)

=2+32+64

=98gmol-^1

mole=mass/molar mass

mass and H2SO4=Mole*molar mass

=3*98

=294g

3di)

DH2<DH3<DH1

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2a)

i)periodic law states that properties of elements are the periodic function of their atomic number

ii)periodic property of element is the variation of the properties of element in a regular pattern both down the group and acrose the period

2aii)

i)ionization energy

ii)electron affinity

iii)electropositivity

2aiii)

ionization energy increase from left to right acrose a period because the nuclear charge increase progressively while the screening effect remain constant

-atomic reduis decreases from left to right acrose the period because nuclear charge effect out weighs screening effect,which remains constant because the number of shell in a period is constant

-electron affinity increase from left to right across a period because an additional electron will experience a greater repulsion than altraction from the positively charged nucleus

2b)

the relative atomic mass of an element is the mass of one atom of the element compared with the mass of an atom of the carbon -12 isotope whos mass is exactly 12

2bci)

isotopy

2cii)

the isotopes have difference number of neutron

2ciii)

R.A.M=(75/100*35)+(25/100*37)

=2634.25

2di)

I CO2 by downward delivery

II HCL by downward delivery

III H2 by upward delivery

2dii)

I because it is denser than air

II because it is denser than air

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5ai)

worm the unknown substance with concentrated H2SO4 in a test tube. if trioxonitrate(v) ion’s are/is present, some trioxonitrate(v) acid will be condensed as oily drops on the upper part of the tube. On further heating, the acid decomposes yielding brown fums of NO2 gas.

5iia)

Sodium chloride is used for:

i)preserving food

ii)glazing pottery

5bi)

C(s) + O2(g) =====> Co2(g)

Co2(g) + C(s) =====> 2Co(g)

Fe2O3(s) + 3Co(g) =====> 2Fe(l) + 3Co2(g)

The limestone present decomposes as follows:

CaCo3(s) =====> CaO(s) + Co2(g)

Which is available to remove silicon(iv) oxide impurity

SiO2(s) + CaO(s) =====> CaSiO3(s)

5bii)

Faraday’s first law of electrolysis states that, the mass (m) of an element discharged during an electrolysis is directly proportional to the quantity of electricity (Q) passing through it.

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