NAME:_________________________
Fall 2008
INSTRUCTIONS:
Section:_____
Student Number:________________
Chemistry 1000 Midterm #2B
____/ 44 marks
1) Please read over the test carefully before beginning. You should have 6
pages of questions, a blank page that can be used if you run out of space
on any question, and 2 pages of data/formula/periodic table sheet.
2) If you use the “overflow” page, indicate this next to the question and
clearly number your work on the “overflow” page.
3) If your work is not legible, it will be given a mark of zero.
4) Marks will be deducted for incorrect information added to an otherwise
correct answer.
5) Marks will be deducted for improper use of significant figures and for
missing or incorrect units.
6) Show your work for all calculations. Answers without supporting
calculations will not be given full credit.
7) You may use a calculator.
8) You have 90 minutes to complete this test.
Confidentiality Agreement:
I agree not to discuss (or in any other way divulge) the contents of this exam until after 8pm
Mountain Time on Wednesday, November 5th, 2008. I understand that, if I were to break this
agreement, I would be choosing to commit academic misconduct and that is a serious offense which
will be punished. The minimum punishment would be a mark of 0/44 on this exam and removal of
the “overwrite midterm mark with final exam mark” option for my grade in this course; the
maximum punishment would include expulsion from this university.
Signature: ___________________________
Course: CHEM 1000 (General Chemistry I)
Semester: Fall 2008
The University of Lethbridge
Date: _____________________________
Question Breakdown
Q1
Q2
Q3
Q4
Q5
Q6
Q7
/4
/7
/6
/2
/7
/7
/ 11
Total
/ 44
NAME:_________________________
1.
(a)
(b)
(c)
Section:_____
Student Number:________________
Answer any two of the questions below. Only your first two answers will be marked.
Assume that you have access to any materials/equipment you’ve used in the Chemistry 1000
lab. Your answer may not violate any safety regulations!
[4 marks]
You’re given two vials, each containing a white salt. You’re told that one vial contains
barium chloride and the other contains potassium chloride. How do you determine which
salt is which? Briefly, explain how your method will tell you which is which.
Method A:
Dissolve each salt in water then perform a flame test on the resulting
solutions. The BaCl2 solution will give a green flame. The KCl solution will
give a purple flame.
Method B:
Dissolve each salt in water. Add sulfuric acid (H2SO4) to each solution. The
one that forms a precipitate (BaSO4) contained BaCl2. There are other
solutions that could be used instead of H2SO4 – anything that would give an
insoluble barium salt through a double replacement reaction.
You’re given two vials, each containing a metal. You’re told that one vial contains
aluminium and the other contains gallium. How do you determine which metal is which?
Briefly, explain how your method will tell you which is which.
Method A:
Gently heat both vials. The gallium will melt at ~30 C. Aluminium does
not melt until much hotter (660 C).
Method B:
Weigh a sample of each metal then determine its volume by displacement of
a liquid. Use the mass and volume of each piece of metal to determine its
density. The density of gallium is greater than the density of aluminium.
(Density generally increases from top to bottom within a group.)
This method requires a relatively large sample of each metal so may be less
practical than Method A.
You’re given two vials, each containing a solid sample of metal. You’re told that one vial
contains calcium and the other contains zinc. How do you determine which metal is which?
Briefly, explain how your method will tell you which is which.
Method A:
Calcium reacts violently with water; zinc does not react at all with water. Set
up a beaker of water behind a blast shield. Drop a small piece of one metal
into the water. If there is no reaction, it was zinc. If there is a violent
reaction, it was calcium. Repeat the procedure with the other metal to
confirm that only one reacts.
Method B:
Weigh a sample of each metal then determine its volume by displacement of
a an inert liquid such as oil. Use the mass and volume of each piece of metal
to determine its density. The density of zinc is greater than the density of
calcium.
This method requires a relatively large sample of each metal so may be less
practical than Method A.
NAME:_________________________
2.
(a)
Section:_____
Student Number:________________
There is a significant difference between the industrial methods used to make sodium metal
and sodium hydroxide.
[7 marks]
What is the main difference, and what is the main reason for this difference?
[2 marks]
Sodium is made by electrolysis of liquid NaCl. The sodium produced (Na(l)) is insoluble in
the NaCl(l) and is therefore easily separated.
Sodium hydroxide with made by electrolysis of aqueous NaCl instead.
Water (specifically, H+ ions in the water) is more easily reduced than the Na+ cations. As a
result, H2(g) is produced instead of Na(l) when NaCl is electrolyzed in the presence of water.
(b)
Write a balanced chemical equation (including states of matter) describing the industrial
process used to make sodium metal.
[1 mark]
2 NaCl(l) → 2 Na(l) + Cl2(g)
(c)
Write a balanced chemical equation (including states of matter) describing the industrial
process used to make sodium hydroxide.
[2 marks]
2 Cl-(aq) + 2 H2O(l) → 2 OH-(aq) + H2(g) + Cl2(g)
or
2 NaCl(aq) + 2 H2O(l) → 2 NaOH(aq) + H2(g) + Cl2(g)
(d)
We know that it is not possible to make potassium metal using the same method used to
make sodium metal. Should this limitation also apply to the production of potassium
hydroxide? In other words, would it be reasonable to attempt to prepare potassium
hydroxide using the sodium hydroxide method (but replacing all sodium reagents with
the potassium equivalent)? Briefly, explain your answer.
[2 marks]
It would be reasonable to prepare KOH using the NaOH method. The problem whereby
liquid potassium is soluble in liquid potassium chloride will not arise as the product is not a
pure alkali metal but an ionic compound. More importantly, the alkali metal cation is a
spectator ion (not directly involved in the reaction).
Sodium hydroxide is made by electrolysis of an aqueous solution of sodium chloride. In this
reaction, the sodium cations are neither oxidized nor reduced. (See the first answer to 2(c)).
The reaction at the anode is the same for production of NaOH as for production of Na.
Chloride anions are oxidized to chlorine gas:
2 Cl- → Cl2 + 2eThe reaction at the cathode is not the same for these processes, though. In the production of
NaOH, it is water (or H+ in the water) that is reduced to produce hydrogen gas:
2 H+ + 2 e- → H2
This results in a shift in equilibrium to produce more hydroxide ions (H2O ↔ H+ + OH-).
Since its only role would be as counterion (balancing the hydroxide’s negative charge),
replacing Na+ with K+ should not affect the overall process.
NAME:_________________________
3.
(a)
Section:_____
Student Number:________________
Write a balanced chemical equation for each of the following reactions.
Include all states of matter.
Cesium is burned in air.
Cs(s) + O2(g) → CsO2(s)
[6 marks]
[1 mark]
(b)
A chunk of sodium is dropped into a flask of liquid bromine.
2 Na(s) + Br2(l) → 2 NaBr(s)
(c)
Aluminium metal is added to a test tube containing concentrated aqueous sodium hydroxide
solution.
[2 marks]
2 Al(s) + 6 H2O(l) + 2 NaOH(aq) → 2 Na[Al(OH)4](aq) + 3 H2(g)
2 Al(s) + 6 H2O(l) + 2 OH-(aq) → 2 [Al(OH)4]-(aq) + 3 H2(g)
or
(d)
[1 mark]
or
or
or
Aluminium oxide is added to a test tube containing concentrated aqueous hydrochloric acid
solution.
[2 marks]
Al2O3(s) + 6 HCl(aq) → 2 AlCl3(aq) + 3 H2O(l)
Al2O3(s) + 6 HCl(aq) + 6 H2O(l) → 2 [Al(OH2)6]Cl3(aq) + 3 H2O(l)
Al2O3(s) + 6 H+(aq) → 2 Al3+(aq) + 3 H2O(l)
Al2O3(s) + 6 H+(aq) + 6 H2O(l) → 2 [Al(OH2)6]3+(aq) + 3 H2O(l)
4.
(a)
Name the products of reactions 3(a) and 3(b).
cesium superoxide
(b)
sodium bromide
[2 marks]
NAME:_________________________
Section:_____
Student Number:________________
5.
The reaction below occurs when chromic acid (H2CrO4) is added to a solution of barium
chloride.
[7 marks]
H2CrO4(aq)
+
BaCl2(aq)
BaCrO4(s)
+
2 HCl(aq)
(a)
Balance the reaction equation above.
(b)
A solution is prepared by dissolving 3.75 g of solid barium chloride in water. To this
solution was then added 75.00 mL of 1.25 M H2CrO4. Determine the maximum mass of
barium chromate (BaCrO4) that could be produced.
[5 marks]
[1 mark]
Approach: Find the number of moles of each reactant, determine limiting reagent, calculate
moles of product then calculate mass of product.
nH 2CrO4
nBaCl2
75 .00 mL
3.75 g
1L
1.25 mol
1000 mL
1L
1mol
208 .2324 g
0.0938 mol
0.0180 mol
BaCl2 is the limiting reagent.
(c)
nBaCrO4
nBaCl2
0.0180 mol
mBaCrO4
0.0180 mol
253 .3207 g
1mol
4.56 g
Suggest one reason why BaCl2 is soluble in water but BaCrO4 isn’t.
[1 mark]
BaCrO4 forms a stronger lattice because the charge of the chromate ion (CrO42-) is more
negative than the charge of the chloride ion (Cl-).
Give 0.5 mark if students comment on relative sizes of ions (i.e. that large anions like CrO42tend to form stronger lattices with large cations like Ba2+).
NAME:_________________________
6.
Section:_____
Student Number:________________
Sketch a Born-Haber cycle for ZnCl2. Clearly label the enthalpy change involved with each
step. (i.e. give the name or symbol for each enthalpy change) Also, indicate whether each
step has a positive or negative enthalpy change.
[7 marks]
Steps to a Born-Haber cycle:
- Write overall chemical equation for formation of ionic compound from its
constituent elements (“enthalpy of formation”)
- Convert each element into single gaseous atoms
- Make the appropriate ions from those atoms (“ionization energy” to make
cations; “enthalpy of electronic attraction” to make anions)
- Bring ions together to form a lattice (“enthalpy of lattice formation”)
Zn2+(g)
+
2 Cl-(g)
I2(Zn)
positive enthalpy change
2
Zn+(g)
EAH(Cl)
negative enthalpy change
LFH
(ZnCl2)
negative enthalpy change
I1(Zn)
positive enthalpy change
Zn(g)
+
BDH(Cl2)
sublH(Zn)
positive enthalpy change
Zn(s)
2 Cl(g)
+
positive enthalpy change
Cl2(g)
ZnCl2(s)
fH
(ZnCl2)
negative enthalpy change
NAME:_________________________
Section:_____
Student Number:________________
7.
The unit cell for CuCl is shown:
[11 marks]
(a)
Does this image most closely resemble the CsCl lattice, the NaCl lattice, the wurtzite lattice
or the zinc blende lattice? Justify your answer by naming the type of lattice formed by the
anions and the type of holes filled by the cations.
[3 marks]
Ionic lattice type = NaCl
Anions form a face-centered cubic (fcc) lattice.
Cations are in octahedral holes.
(b)
Calculate the length of one side of the CuCl unit cell.
x
2r
2rCl
Cu
2(96 pm) 2(181 pm) 554 pm
(c)
Calculate the density of CuCl.
Approach: d = m/V so find m and V (for unit cell) then divide to get d
mcell
4mCu
4mCl
mcell
395 .995
g
mol
x
554 pm
x3
Vcell
d
m
V
[1 mark]
g
g
g
) 4(35 .4527
) 395 .995
mol
mol
mol
1mol
g
6.57565 10 22
23
6.02214 10 unitcells
unitcell
4(63 .546
1m
100 cm
12
10 pm
1m
(5.54 10 8 cm)3
6.57565 10 22 g
1.70 10 22 cm3
5.54 10 8 cm
1.70 10
3.87
g
cm3
22
cm3
[7 marks]
NAME:_________________________
Section:_____
Student Number:________________