Chemistry Study Notes Form 5 Chemistry Study Notes Form 6 Chemistry Study Notes Msomi Maktaba All Notes

ADVANCED CHEMISTRY QUESTIONS & ANSWERS – SOLVED PROBLEMS

  1 . Explain very briefly using equations where possible the extraction of copper from its commercial hall under the heading of reduction and

purification.

2.    (a)  Write down:

(i)  Four reasons which justifies the placement of hydrogen in group I of the periodic table

(ii)  Four reasons which puts hydrogen in group (vii) of periodic table

(b)   (i)  In which group do you think  hydrogen should strictly belong

(ii)  Give reasons for your answer for (i) above


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(c) Describe the action of water on hydrides of period III

(d) Compare the thermal stability of carbonates of group I and II by using a specific example show their differences.

3. Comment with help of chemical equations where necessary in the following:

(i)  Iron (ii)  chloride cannot be prepare by heating Iron filing in a steam of chlorine gas.
(ii)  Hydrochloric acid cannot be used as acidic medium during redox titration of KMnO4 against Fe SO4.
(iii) Solid Al (OHwww.edu.co.tz/library is soluble in aqueous solution of NaOH.


ANSWER:

1.   Extraction of copper

Steps:
Roasting:
Cu Fewww.edu.co.tz/library is heated in plenty supply of air
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Now silica is added to Fe to remove FeO

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Self – Reduction reaction:
In self-reduction reaction, Cwww.edu.co.tz/libraryS reacts with   Cwww.edu.co.tz/libraryO to form molter copper which later solidifies

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Purification:

Purification is done electrolytically.
At cathode
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At anode
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(c) Solution.
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The hydrides of Na and Mg form basic solution.

The hydride of sulphur – chlorine form acidic solutions due to presence of hydroxonium ion.

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The hydrides of phosphorus i.e. phosphine is amphoteric

(d) Group I:
Carbonates of group I are thermally stable due to the large size of the atoms hence it will be capable of accommodating the carbonate ion.

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3. (i) Chlorine is a strong oxidizing agent hence it will oxidize iron straight to Iron III chloride

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     (ii) Cl may react with Fe3+ (titration product) which will form FeCwww.edu.co.tz/library which is red in colour.

           This will make change in colour of  MnO4 be difficult to account for (or difficult to notice)
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1.
4.  (a) Define the following

(i) Atomic radius

(ii) Ionization energy

(b)   Contrast the action of heat on the following pairs of compounds



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(i) NaNwww.edu.co.tz/library  and Ca (Nwww.edu.co.tz/library

(ii) Nwww.edu.co.tz/library Cwww.edu.co.tz/library and PbCwww.edu.co.tz/library

(c)  Describe how you can distinguish chemically the following pairs of compounds

(i) Mg (OHwww.edu.co.tz/library and  Mg Cwww.edu.co.tz/library

(ii) NaCl  and  Al Cwww.edu.co.tz/library

(iii) Cu Swww.edu.co.tz/library and  Cu (Nwww.edu.co.tz/library

2.    5. (a) Explain the following by chemical equation. where necessary:

Solid carbonate of iron (iii) and Al have never been isolated

(i)  Lime water is used to test the presence of  Co2 gas

(ii)  When sodium hydrogen carbonate is added to copper (ii) sulphate effervescence is observed.

(iii)  Lead hydroxide ppt dissolves in excess sodium hydroxide solution.

(b) By using equation. Where necessary describe:

(i)Two methods of preparing copper (ii) chloride in the lab.



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(ii) Describe how ZnO reacts with both acid & bases.

(c) List 2 important uses of CaO in daily life

6. (a) Define

(i) Heat of reaction

(ii) Bond energy
(b) The enthalpy of formation of CCwww.edu.co.tz/library is – 135.5 KJmol and enthalpyies of atomization of graphite and chloride are 715 and 121.2KJ respectively.  Calculate the C-Cl bond energy

(c)  (i)  When www.edu.co.tz/libraryg of C, H2 and Cwww.edu.co.tz/libraryare completely burnt in excess oxygen 32, 143 and 56 KJ of heat is liberated. Calculate the standard enthalpy of formation of Cwww.edu.co.tz/library
(ii)      The heat of formation of Fe2 O3 is – 824.OKJml-1. What will DH of the reaction

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                    ANSWERS:

4.    (a)   (i) Atomic radius is half the internuclear distance between two similar atoms covalently bonded by a single bond

(ii) Ionization energy is the energy required to remove the most loosely held election from the shell of an atom.

        (b)   (i) NaNwww.edu.co.tz/library is more stable than Ca (Nwww.edu.co.tz/library because sodium is highly electropositive hence bond energy between sodium and nitration is high hence it comes very stable.

               (ii) Nwww.edu.co.tz/libraryCwww.edu.co.tz/library is more stable than PbCwww.edu.co.tz/library since sodium is highly electropositive hence there is high bond energy between sodium and

carbonate thus is becomes stable.

         (c)  (i) MgCwww.edu.co.tz/library will release Cwww.edu.co.tz/library upon heating which will turn lime water milky.

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                 (ii) AlCwww.edu.co.tz/library is a Lewis acid, hence it can react with a base to give a salt and HCl. But NaCl cannot react with a base

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                (iii) Cu (Nwww.edu.co.tz/library will form a brown ring due to formation of Nwww.edu.co.tz/library but CuSwww.edu.co.tz/library  does not.

5.  (a)  (i) Fe Cwww.edu.co.tz/library and Al Cwww.edu.co.tz/library have never been isolated due to the small size of Fe and Al hence they have high polarization power. Hence they will decompose as soon as they are formed.

           (ii) Lime water is used to test the presence of Cwww.edu.co.tz/library since calcium carbonate is insoluble hence the it becomes milky.

           (iii) The reaction between sodium bicarbonate and copper (ii) sulphate, effervescence is observed due to formation of carbon dioxide

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          (iv) Lead hydroxide ppt dissolves in sodium hydroxide due to the formation of a soluble complex compound

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    (b)  (i) 1st method is by reacting copper carbonate with HCl

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                 2nd method is by reacting Cu (OHwww.edu.co.tz/library with HCl

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            (iii)  ZnO is amphoteric hence it reacts with both acids and bases



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(c)    Uses of CaO

          www.edu.co.tz/library   Manufacture of cement

          www.edu.co.tz/library   Used as a drying agent for ammonium

          www.edu.co.tz/library   Lining of furnaces

           www.edu.co.tz/library  Formation of slag in Blast furnace

          www.edu.co.tz/library   Making Cawww.edu.co.tz/library which interns is used in making methane

6.  (a)   (i) Heat of reaction is the enthalpy change when number of moles of reactants as represented by a balanced chemical equation change into products.

Bond energy is the amount of energy required to break 1mole of a bond
www.edu.co.tz/library = – www.edu.co.tz/library www.edu.co.tz/library+ www.edu.co.tz/library+ 4www.edu.co.tz/library

4 (C – Cl) = (-135.5) + 715 + 4(121.1)

C – C l = 333.725 KJ mol-1

(c)    (i) Solution:

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(ii) Solution:

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7.  Define the following terms

(a) (i) Hydrogen spectrum

Series of arrangement of various wavelength of radiation formed when hydrogen atoms are exposed under high voltage electric discharge in

a discharge tube.

(ii) Convergence Limit

Is the point at which the distance from the nucleus is so large such that electron moving beyond this point cannot go back to its ground state:

(iii) Degenerate orbitals:

 . These are orbitals with equivalent energies

. They share the same azimuthal quantum number.

(b)  (i) Solution:

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www.edu.co.tz/libraryE = www.edu.co.tz/librarywww.edu.co.tz/library
=   – 2.176 x 10-18 (www.edu.co.tz/library)

www.edu.co.tz/libraryE = 1.934 X 10 -18 J

(ii)  For shortest wavelength, www.edu.co.tz/library, = n = www.edu.co.tz/library

www.edu.co.tz/libraryE = www.edu.co.tz/library  – www.edu.co.tz/library

= -2.176 x 10-18   www.edu.co.tz/library

                    www.edu.co.tz/libraryE = 2.176 x 10-18

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   (c)    Solution
n = 2   l = 1 ml = -1 ms = –  www.edu.co.tz/library

www.edu.co.tz/library    www.edu.co.tz/library

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VSEPR – Valency shell electron pair repulsion. We use lines to indicate the bonds

8.   (a)  Dative bond

It a type covalent bond in which one atom contributes both electrons

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Van – Der Waals force is the dipole-dipole interaction between two non polar atoms.

London forces are temporary dipole dipole interactions.

Experiment:

A:  Is a solution made by dissolving 4.28g/ 500 cm3 of MIwww.edu.co.tz/library of distilled water

B:  Is 10www.edu.co.tz/library   KI solution

C:  Is dilute www.edu.co.tz/librarySwww.edu.co.tz/library

D:  Is 0.12M solution Na2 S2 O3 . SH2 O

S:  Is starch indicator solution

THEORY:

In acidic media Iodate ions (I www.edu.co.tz/library) reacts with excess Iodine ions (www.edu.co.tz/library) to liberate Iodine according to the following reaction

         Iwww.edu.co.tz/library (aq)   + 6www.edu.co.tz/library(aq) + 5www.edu.co.tz/library  →  3www.edu.co.tz/libraryO + 3www.edu.co.tz/library   ——————- (1)

The iodine l liberate with a standard solution of thiosulphate ions according to the following reaction equation.

2www.edu.co.tz/library www.edu.co.tz/library2- + www.edu.co.tz/library www.edu.co.tz/library →      www.edu.co.tz/library www.edu.co.tz/library (aq)   + 2I(aq)    ——————— (2)

Procedure:

1.   Pipette 25 cm3 of solution A into titration flask

2.   Using measuring cylinder add 5 cm3 of solution B into titrating flask containing solution A

3.   To the resultant solution in (2) above add 5 cm5 of solution C using measuring cylinder

4.   Titrate the resulting solution in (3) above with a standard solution D till the colour becomes pale yellow. Then add few drops of solution S  and continue to titrate till the blue colour just
disappears.

5.   Repeat the titration 3 times

           Results:

The volume of pipette used was 10cm3

The volume of burette used was 50cm3

Burette reading

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10cm3 of solution A requires 10.40 cm3 of solution D for complete reaction

ANSWER THE FOLLOWING

  (a)    Write down half reaction equation to show oxidation and reduction taking place in the reaction (1) and (2)

(b)   Calculate the number of moles of D used during titration

(c)    Calculate the number of moles of A used during titration

(d)   Calculate molarity of solution A’

       (e)    Calculate the R.A.M of metal M in the formula MIwww.edu.co.tz/library

           I = 127, 0 = 16

ANSWERS:

(a)    Half reactions in (1) and (2)

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        And also

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            (b)   Solution:

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                              x = 1.25 x 10-3 moles of D was used

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