NAME:_________________________
Fall 2008
INSTRUCTIONS:
Section:_____
Student Number:________________
Chemistry 1000 Midterm #2A
____/ 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
/7
/2
/6
/4
/ 11
/7
/7
Total
/ 44
NAME:_________________________
1.
(a)
Section:_____
Student Number:________________
Write a balanced chemical equation for each of the following reactions.
Include all states of matter.
Potassium is burned in air.
K(s) + O2(g) → KO2(s)
[7 marks]
[1 mark]
(b)
A chunk of aluminium is dropped into a flask of chlorine gas.
2 Al(s) + 3 Cl2(g) → 2 AlCl3(s)
[1 mark]
(c)
Magnesium reacts with nitrogen.
3 Mg(s) + N2(g) → Mg3N2(s)
[1 mark]
(d)
Aluminium oxide is added to a test tube containing concentrated aqueous sodium hydroxide
solution.
[2 marks]
Al2O3(s) + 3 H2O(l) + 2 NaOH(aq) → 2 Na[Al(OH)4](aq)
Al2O3(s) + 3 H2O(l) + 2 OH-(aq) → 2 [Al(OH)4]-(aq)
or
(e)
or
or
or
Aluminium metal is added to a test tube containing concentrated aqueous hydrochloric acid
solution.
[2 marks]
2 Al(s) + 6 HCl(aq) → 2 AlCl3(aq) + 3 H2(g)
2 Al(s) + 6 HCl(aq) + 6 H2O(l) → 2 [Al(OH2)6]Cl3(aq) + 3 H2(g)
2 Al(s) + 6 H+(aq) → 2 Al3+(aq) + 3 H2(g)
2 Al(s) + 6 H+(aq) + 6 H2O(l) → 2 [Al(OH2)6]3+(aq) + 3 H2(g)
2.
(a)
Name the products of reactions 1(a) and 1(b).
potassium superoxide
(b)
aluminium chloride
[2 marks]
NAME:_________________________
Section:_____
Student Number:________________
3.
There is a significant difference between the industrial methods used to make sodium metal
and potassium metal.
[6 marks]
(a)
What is the main difference, and what is the main reason for this difference?
[2 marks]
Sodium is made by electrolysis of molten NaCl. The sodium produced is insoluble in the
NaCl(l) and is therefore easily separated.
This method cannot be used to make potassium because K(l) is soluble in KCl(l).
Instead, potassium is made by chemical reduction of the K+ in KCl(l) using sodium metal as
the reducing agent. This reaction is done at a temperature at which the potassium metal
produced evaporates out of the Na/KCl/NaCl mixture (allowing isolation of the potassium).
(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 potassium metal.
[1 mark]
KCl(l) + Na(l) → K(g) + NaCl(l)
(d)
Now, consider the production of sodium hydroxide instead. Would it be reasonable to
attempt to prepare potassium hydroxide using the sodium hydroxide method (but
replacing all sodium reagents with the potassium equivalent)? Or would you expect the
same problem to arise that prevents us from making potassium using the sodium method?
Briefly, explain your answer.
[2 marks]
It would be reasonable to prepare KOH using the NaOH method. The potassium solubility
problem 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. They would not be written in
the net ionic equation describing this process:
2 Cl-(aq) + 2 H2O(l) → 2 OH-(aq) + H2(g) + Cl2(g)
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:_________________________
4.
(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 solid sample of metal. You’re told that one vial
contains aluminium and the other contains potassium. How do you determine which metal
is which? Briefly, explain how your method will tell you which is which.
Method A:
Potassium is a lot softer than aluminium. It should be possible to cut a small
sample of potassium into pieces using a spatula. This should not be possible
for aluminium.
Method B:
Potassium reacts violently with water; aluminium 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 aluminium. If there is
a violent reaction, it was potassium. Repeat the procedure with the other
metal to confirm that only one reacts.
Method C:
Weigh a sample of each metal then determine its volume by displacement of
an inert liquid such as oil. Use the mass and volume of each piece of metal to
determine its density. As an alkali metal, potassium will be less dense than
aluminium.
Method C is less practical than methods A and B.
You’re given two vials, each containing a salt. You’re told that one vial contains calcium
carbonate (CaCO3) and the other contains calcium sulfate (CaSO4). How do you determine
which salt is which? Briefly, explain how your method will tell you which is which.
Method A:
Add each salt to a beaker containing an acidic solution such as HCl (aq). The
calcium carbonate will react with the acid to produce bubbles of carbon
dioxide. As the carbonate reacts, the salt will appear to dissolve. The
calcium sulfate will neither react with the acid nor dissolve in the water.
Method B:
Weigh a sample of each salt then heat the two samples in a *very * hot oven.
The calcium carbonate will decompose to give calcium oxide and carbon
dioxide gas, so whichever sample loses mass was calcium carbonate.
(Calcium sulfate does not decompose upon heating.)
Method B is not very practical as we don’t have a hot enough oven in the lab.
You’re given two vials, each containing a white salt. You’re told that one vial contains
magnesium hydroxide (Mg(OH)2) and the other contains sodium hydroxide (NaOH). How
do you determine which salt is which? Briefly, explain how your method will tell you which
is which.
Method A:
Add each salt to a beaker containing water. The NaOH will dissolve while
the Mg(OH)2 will not dissolve to any appreciable extent. Depending on the
amount of NaOH dissolved, the beaker containing the NaOH(aq) solution may
also be warm as salvation of NaOH is quite exothermic.
Method B:
You could attempt a flame test to determine which salt is which. Na+ ions
give an orange flame while Mg2+ ions do not give a positive flame test. This
method may not give a clear answer, though, if the flame test loop is
old/cheap as it is common for loops to contain some sodium. An equivalent
approach which might give better results would be to put each salt in a petri
dish, pour a little alcohol over each then burn the alcohol and see which
flame turned orange (like the demo in class).
NAME:_________________________
Section:_____
Student Number:________________
5.
The unit cell for CaO 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 CaO unit cell.
x
(c)
2r
2rO2
Ca2
2(100 pm) 2(140 pm)
480 pm
Calculate the density of CaO.
[7 marks]
Approach: d = m/V so find m and V (for unit cell) then divide to get d
mcell
4mCa 2
4mO 2
mcell
224 .310
g
mol
x
480 pm
x3
Vcell
d
m
V
g
g
g
) 4(15 .9994
) 224 .310
mol
mol
mol
1mol
g
3.72475 10 22
23
6.02214 10 unitcells
unitcell
4(40 .078
1m
100 cm
12
10 pm
1m
(4.80 10 8 cm)3
3.72475 10 22 g
1.11 10 22 cm3
[1 mark]
4.80 10 8 cm
1.11 10
3.37
g
cm3
22
cm3
NAME:_________________________
6.
Section:_____
Student Number:________________
Sketch a Born-Haber cycle for Li2O. 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”)
2 Li+(g)
+
O2-(g)
EA2H(O)
negative enthalpy
change
2 I1(Li)
positive enthalpy change
O-(g)
LFH
(Li2O)
negative enthalpy change
EA1H(O)
negative enthalpy change
2 Li(g)
2
+
sublH(Li)
positive enthalpy change
2 Li(s)
+
O(g)
½
BDH(O2)
positive enthalpy change
½ O2(g)
Li2O(s)
fH
(Li2O)
negative enthalpy change
NAME:_________________________
Section:_____
Student Number:________________
7.
The reaction below occurs when sulfuric acid is added to a solution of barium nitrate.
[7 marks]
H2SO4(aq)
+
Ba(NO3)2(aq)
BaSO4(s)
+
2 HNO3(aq)
(a)
Balance the reaction equation above.
(b)
A solution is prepared by dissolving 8.00 g of solid Ba(NO3)2 in water. To this solution was
then added 25.00 mL of 3.50 M H2SO4. Determine the maximum mass of barium sulfate
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 2SO4
nBa( NO3 )2
25 .00 mL
1L
1000 mL
8.00 g
1mol
261 .3368 g
3.50 mol
1L
0.0875 mol
0.0306 mol
Ba(NO3)2 is the limiting reagent.
(c)
nBaSO4
nBa( NO3 )2 0.0306 mol
mBaSO4
0.0306 mol
233 .3906 g
1mol
7.14 g
Suggest one reason why Ba(NO3)2 is soluble in water but BaSO4 isn’t.
[1 mark]
BaSO4 forms a stronger lattice because the charge of the sulfate ion (SO42-) is more negative
than the charge of the nitrate ion (NO3-).
NAME:_________________________
Section:_____
Student Number:________________