Thursday, 26 July 2012

UPS III Kimi-'A'! PDT KMS_hehdi


CONFIDENTIAL                                                                                                 DK 035                                                                                                                                     
                                                                                                                              Chemistry
                                                                                                                              Semester III                                                                                                                                          
                                                                                                                              2012/2013
                                                                                                                              1 hour                                                                                                                                                              





KOLEJ  MATRIKULASI SELANGOR
KEMENTERIAN  PELAJARAN MALAYSIA

                               PRA UPS III

SEMESTER III
TWO YEAR PROGRAMME
SESSION 2012/2013



CHEMISTRY 
DK 035



Answers all FOUR (4) questions


TABLE OF RELATIVE ATOMIC MASSES

Element
Symbol
Proton number
Relative atomic mass
Aluminum
Al
13
26.98

Antimony
Sb
51
121.76

Argon
Ar
18
39.95

Arsenic
As
33
74.92

Barium
Ba
56
137.33

Beryllium
Be
4
9.01

Bismuth
Bi
83
208.98

Boron
B
5
10.81

Bromine
Br
35
79.90

Cadmium
Cd
48
112.41

Calcium
Ca
20
40.08

Carbon
C
6
12.01

Cerium
Ce
58
140.12

Cesium
Cs
55
132.91

Chlorine
Cl
17
35.45

Chromium
Cr
24
52.00

Cobalt
Co
27
58.93

Copper
Cu
29
63.55

Fluorine
F
9
19.00

Gold
Au
79
197.00

Helium
He
2
4.00

Hydrogen
H
1
1.01

Iodine
I
53
126.90

Iron
Fe
26
55.85

Krypton
Kr
36
83.80

Lead
Pb
82
207.20

Lithium
Li
3
6.94

Magnesium
Mg
12
24.31

Manganese
Mn
25
54.94

Mercury
Hg
80
200.59

Neon
Ne
10
20.18

Nickel
Ni
28
58.69

Nitrogen
N
7
14.01

Oxygen
O
8
16.00

Phosphorus
P
15
30.97

Platinum
Pt
78
195.08

Potassium
K
19
39.10

Protactinium
Pa
91
231.04

Radium
Ra
88
226.03

Radon
Rn
86
222.02

Rubidium
Rb
37
85.47

Scandium
Sc
21
44.96

Selenium
Se
34
78.96

Silicon
Si
14
28.09

Silver
Ag
47
107.87

Sodium
Na
11
22.99

Strontium
Sr
38
87.62

Sulphur
S
16
32.07

Tin
Sn
50
118.71

Tungsten
W
74
183.84

Uranium
U
92
238.03

Zinc
Zn
30
65.39


LIST OF SELECTED CONSTANT VALUES

Ionization constant for water at 25°C         Kw  = 1.0 ´ 10-14 mol2 dm-6

Molar volume of gases                          Vm       = 22.4 dm3 mol-1 at STP
                                                                        = 24 dm3 mol-1 at room condition (25 °C, 1 atm)

Speed of light in a vacuum                       c        = 3.0 ´ 108 m s-1

Specific heat of water                                       = 4.18 kJ kg-1 K-1
                                                                        = 4.18 J g-1 K-1
                                                                        = 4.18 J g-1 °C-1

Avogadro’s number                   NA                      = 6.02´1023 mol-1

Faraday constant                        F                   = 96500 C mol-1

Planck constant                          h                   = 6.63´10-34 J s
                                                                      = 6.63´10-34 Kg m2 s-1

Rydberg constant                                RH           = 1.097 ´ 107 m-1
                                                                        = 2.18 ´ 10-18 J

Ideal gas constant                              R          = 8.314 m3 Pa mol-1 K-1
                                                                         = 8.314 J mol-1 K-1
                                                                         = 0.08206 L atm mol-1 K-1
                                                                         = 62.36 L mmHg mol-1 K-1

Density of water at 25°C                                = 1 g cm-3

Freezing point of water                                   = 0.00 °C
                                                                      = 273.15 K

Vapour pressure of water at 25°C      Pwater  = 23.76 torr


1.                                                      UNIT AND CONVERSION FACTOR

VOLUME                   1 L       =          1 dm3
1mL     =          1 cm3  

ENERGY                   1 J        =          1 kg m2 s-= 1 N m = 1 ´ 107 erg
        1 calorie  =          4.184 J
                                  1eV molecule-1 =          96.7 kJ mol-1

PRESSURE                 1 atm  = 760 mmHg = 760 torr =101 325 Pa =101 325 N m-2

TEMPERATURE          0°C   =             273.15 K

OTHERS        1 faraday (F)   =          96 500 C
      1 newton (N)   =          1 kg m s-2

Answers all FOUR (4) questions

1          a.         Define
                        i.          Open system                                                                             [ 1 mark ]
                        ii.         Differentiate between endothermic and exothermic reactions.                                
                                                                                                                                      [4 marks]



b.         A 0.1375 g sample of solid magnesium is burned in a constant-volume bomb calorimeter that has a heat capacity of 1769 J oC-1. The calorimeter contains exactly 300 g of water, and the temperature increases by 1.126 oC. Calculate the heat of combustion for Magnesium in kJ mol-1.
[ Specific Heat Capacity of water = 4.18 J g-1 oC-1
                                                                                                            [ 5 marks ]




2            a)       By using data below, construct an appropriate energy cycle diagram to show
                        the dissolution process of sodium hydroxide, NaOH and calculate enthalpy of
                        hydration of OH-.

∆H (kj mol-1)
Enthalpy of hydration of Na+
-410
Enthalpy of solution of NaOH
-44.5
Lattice energy of NaOH
-900
 [5 marks]








b)   The Born-Haber cycle for magnesium chloride, MgCl2 is as shown below:
Enthalpy            
                           Mg2+ (g) + 2e + 2Cl (g)
 Bond dissociation           DH4 = +243 kJ                                                     DH5 = –368 kJ                                                               
       energy                                                                         
                                          C                                                        Mg2+ (g) + 2Cl- (g)
Second Ionization          
energy of Mg                 DH3 = +1450 kJ
                                      
                        Mg+ (g) + e + Cl2 (g)                                                             DH6 = U kJ
                                        DH2 = +736 kJ
                          Mg (g) + Cl2 (g)
                                        DH1 = +151 kJ
                         Mg (s) + Cl2 (g)
                                                   DH7 = –645 kJ
                                                                
                                                                  MgCl2 (s)

Name the enthalpies labeled ∆H2 and ∆H5.  Write the equation for reaction C. Calculate the lattice energy for MgCl2.                                                                                          
                                                                                                                                [5 marks]

  

3.         a)         A galvanic cell consist of a silver cathode immersed in a 0.15 M solution of 
AgNO3 and a platinum anode that has gaseous hydrogen bubbling over it at a pressure of 1.0 atm. The hydrogen ion concentration of the solution is 0.02 M.

Based on the data given, draw and labeled the cell diagram to represent the above galvanic cell.
                                                                                                            [ 6 Marks ]


            b)         An electrochemical cell is set up using copper and silver half-cells.
                                                Cu(s)| Cu2+(aq) || Ag+(aq) | Ag(s)
                       
i.                    What is the overall cell reaction?
ii.                  Calculate the cell potential under standard condition
iii.                The cell potential is 0.45V when the concentrations of copper(II) ion and silver ion are x M and 0.70 M respectively.
What is the value for x?
                                                                                                [ 4 Marks ]

E0cell Cu2+ Cu(s) = 0.34 V              Ag+(aq) | Ag(s) = 0.80 V



4          a)    Predict the products of the electrolysis of concentrated aqueous solution of sodium chloride in a cell using graphite electrodes. Give reasons.
                                                                                                      [4 marks]




            b)   In electrolysis of molten chromium(III) chloride, CrCl3 using graphite electrodes, calculate the time (in minutes) required to obtain 3.24 g of chromium using a current of  10.0 A.          
                                                                                                                                      [6 marks]


   
End of question paper


NO
PART
ANSWERS

1

(a)    i

Open system is a system that allowed the exchange of mass and energy from the system to its surrounding
 
  ii

Endothermic Reaction
Exothermic Reaction
∆Hreactant < ∆Hproducts
∆Hreactant > ∆Hproducts

∆H is positive

∆H is negative
Heat is absorbed from surrounding
Heat is released to surrounding

∆Hproduct
 






∆Hreactant
 


Progress of reaction
 

∆H= -ve
 


∆Hreactant
 

∆Hproduct
 

Progress of reaction
 

b

     q   =   mwcw∆T   +   Cc∆T      @
          = (300g)(4.18 Jg-1oC-1)(1.126oC)  +  (1769 JoC-1)(1.126oC)
          = 3403.898 J @ 3.4039 kJ

0.1375 g
 



24.3 gmol-1
 
   nMg  =                  

          =   5.6584   x   10-3 mole
              5.6584  x  10-3 mole Mg    ≡    3.4039 kJ
                                 1.0 mole Mg    ≡     601.57 kJ

             The enthalpy of  Mg is 601.57 kJ/mol

                                                                                           


2
(a)








∆Hsoln = -∆Hlattice + ∆Hhyd of Na+  + ∆Hhyd of OH-
@
 -  44.5  = -(-900) + (-410)  + ∆Hhyd of OH-
                           
∆Hhyd of OH- = -534.5 kj mol-1



(b)

∆H2 = First Ionisation Energy of Mg and
∆H5 = Electron affinity of Cl

Equation for reaction C :
Mg2+ (g) + 2e + Cl2 (g)

DH7 = DH1  +DH2  +DH3  +DH4  +DH5  +DH6
                                               @
–645 kJ = +151 kJ + +736 kJ  +1450 kJ +243 kJ + (–368 kJ) + DH6

DH6 = -2857 kJmol-1 (lattice energy of MgCl2)

















3






a













bi.

  Cu(s)     +     2Ag+(aq)                  Cu2+(aq)      +      2Ag(s)

bii.

   Eocell     =     Eocathode  -  Eoanode
                =     + 0.80 V – (+0.34 V)
                =     + 0.46 V


biii.

[ Cu2+]
 

                  
                  
                    
              
 


n
 
   Ecell      =      Eocell -   0.0592 log      [ Cu2+]

                                           @

    (x)
 


2
 


(0.70)2
 
   Ecell      =      Eocell -   0.0592 log

         x     =      1.1 M









4

(a)

At cathode:

Hydrogen gas is produced
@
2H2O(l)  + 2e-  à  H2(g) + 2OH-(aq) 
Reason:
Eo for water molecules is more positive
@
H2O is easier to be reduced than Na+

At anode:
Chlorine gas is produced.
@
2Cl- (aq)  à  Cl2(g)  +  2e-
Reason:
Because of high concentration of Cl-

                                                                                                                                                             


(b)

Cr3+(l)  +  3e-  à Cr(s)
Number of mole of Cr =   = 0.0623 mol  

1 mol Cr  º 3 mole of e- º 3 F º 3 x 96500 C

0.0623 mol   Cr  º 0.0623 mol   x 3 x 96500 C   @
                           =  18035.85 C

                    Q = I x t
    18035.85 C = 10.0 t
                    t  = 30 minutes






End of Answers

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