Question
Elements of group I and II are prepared
by the electrolysis of their molten salts. Sodium is manufactured
from molten sodium chloride and magnesium from magnesium
chloride.
(a) Give the balanced equations,
including state symbols, for the reaction
occurring at each electrode during the electrolysis of molten
sodium chloride.
(i) Release of 1 mole of product in its standard state at
anode.
(ii) Gain of all electrons from anode to
form product at cathode.
(b) Give an overall reaction for the
process.
(c) In a
certain electrolysis cell for the manufacturing of sodium, 6500
dm3 of the product was
collected at anode. Using your answer from (b) calculate the
amount, in tonnes, of the product formed at cathode.
(d) A white powder contains the three
oxides MgO, Al2O3 and SiO2. Three
samples were tested each containing only one of these
oxides. Read the following description of
tests then answer the questions that follow.
Adding
aqueous HCl to a sample of white power produces a colorless
solution. To this resultant solution dilute sodium hydroxide was
added dropwise, a white precipitate was seen which dissolves in
excess of the reagent. A second sample of white powder produces a white
precipitate on adding little water and then in excess. Adding
aqueous HCl to this second sample of white powder also gave no
reaction. Adding aqueous HCl to the third
sample of white power produces a colorless solution which gave a
white precipitate on further adding NaOH(aq) dropwise
until in excess
(i) Which two oxides
might be present in the first sample of white powder?
(ii) Which of these
samples first, second or third was MgO?
(iii) Using your
answer from (d)(i) give the
exact formulae of the two species present
in colorless solution in the first sample stating the pH associated
with each species and the total pH of the colorless solution as a
result of both these species. Mg(aq)2+ and Al(aq)3+ 6.5, 3 ,
2.95
Formula…………………………… pH……………
Total pH……………
Formula…………………………... pH……………
(iv) Identify the
oxide which was actually present in the first sample. Also give two
reactions resulting in the formation of colorless solution and the
addition of dilute NaOH.
………………………………………………………………………………………………………………………………….…….
• Balanced
equations for the two reactions occurring when the oxide you have
chosen in (iii) produced:
a)
Colorless
solution………………………………………………………………………………………………………..
b)
Reaction with
NaOH(aq)………………………………………………………………………………………….……..
(v) By considering
your reactions a and b what “word” is used for this type
of oxide.
………………………………………………………………………………………………………………………………….……
(vi) Identify the
oxide present in the second sample and explain why no reaction took
place here.
……………………………………………………………………………………………………………………………………..…
(vii) Give the
formula of the white precipitate formed in the third sample. Also
state the pH of this solution.
………………………………………………………………………………………………………………………………………..
Question
Elements of group I and II are prepared by the
electrolysis of their molten salts. Sodium is manufactured from
molten sodium chloride and magnesium from magnesium
chloride.
(a) Give the balanced equations, including state
symbols, for the reaction occurring at each
electrode during the electrolysis of molten sodium
chloride.
(i) Release of 1 mole of product in its
standard state at anode.
2Cl-(g)
Cl2(g) + 2e-
(ii) Gain of all electrons from anode to form product at
cathode.
2Na+(l)
+ 2e-
2Na(s)
(b) Give an overall reaction for the process.
2Cl-(g) + 2Na+(l)
2Na(s) +
Cl2(g)
(c) In a certain electrolysis cell for
the manufacturing of sodium, 6500 dm3
of the product was collected at anode. Using your answer from (b)
calculate the amount, in tonnes, of the product formed at
cathode.
moles of Cl2 =6500/24= 270.8 or
271
Cl2: Na from (b)
1: 2
moles of Na= 271.2 x 2 =
542
mass of Na= 542x23 = 12466g = 12.5
tonnes
d) A white powder contains the three oxides MgO,
Al2O3 and SiO2. Three samples were
tested each containing only one of these oxides. Read
the following description of tests then answer the questions that
follow.
Adding aqueous HCl to a sample of white power produces
a colorless solution. To this resultant solution dilute sodium
hydroxide was added dropwise, a white precipitate was seen which
dissolves in excess of the reagent. A
second sample of white powder produces a white
precipitate on adding little water and then in excess. Adding
aqueous HCl to this second sample of white powder also gave no
reaction. Adding aqueous HCl to the third sample
of white power produces a colorless solution which gave a white
precipitate on further adding NaOH(aq) dropwise until in
excess.
(i) Which two oxides might be present in the first sample
of white powder?
MgO and
Al2O3
(ii) Which of these samples first, second or third was
MgO?
third
(iii) Using your answer from (d)(i) give
the exact formulae of the two species present in
colorless solution in the first sample stating the pH associated
with each species and the total pH of the colorless solution as a
result of both these species. Mg(aq)2+ and Al(aq)3+ 6.5, 3 ,
2.95
Formula: [Mg(OH2)6]2+
pH
6.5
Total pH
2.95-2.98 (must be less than
3.0)
Formula: [Al
(OH2)6]3+ pH
3.0
(iv) Identify the oxide which was actually present in the
first sample. Also give two reactions resulting in the formation of
colorless solution and the addition of dilute
NaOH.
Al2O3
• Balanced equations for the two reactions
occurring when the oxide you have chosen in (iii) produced:
a)
Colorless solution
Al2O3 + 6HCl
2AlCl3 +
3H2O
b)
Reaction with NaOH(aq) Al(OH)3(s) + OH-(aq)
AL(OH)4-(aq)
(v) By considering your reactions a and
b what “word” is used for this type of
oxide.
amphoteric
(vi) Identify the oxide present in the second sample and
explain why no reaction took place here.
SiO2. Since it is an
acidic oxide and does not react with HCl and is insoluble in water
forming a precipiatate.
(vii) Give the formula of the white precipitate formed in
the third sample. Also state the pH of this solution.
Mg(OH)2 (s). pH would be
9
