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NAME:_________________________
Fall 2008
INSTRUCTIONS:
Section:_____
Student Number:________________
Chemistry 1000 Midterm #2C
____/ 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
Q8
/7
/2
/4
/2
/4
/7
/7
/ 11
Total
/ 44
NAME:_________________________
1.
(a)
Section:_____
Student Number:________________
Write a balanced chemical equation for each of the following reactions.
Include all states of matter.
Sodium is burned in air.
2 Na(s) + O2(g) → Na2O2(s)
[7 marks]
[1 mark]
(b)
Magnesium is burned in air.
2 Mg(s) + O2(g) → 2 MgO(s)
[1 mark]
(c)
A chunk of calcium is dropped into a flask of water.
Ca(s) + 2 H2O(l) → Ca(OH)2(s) + H2(g)
[1 mark]
(d)
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)
or
or
or
(e)
or
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)
2.
(a)
Name the products of reactions 1(a) and 1(b).
sodium peroxide
(b)
magnesium oxide
[2 marks]
NAME:_________________________
3.
(a)
Section:_____
Student Number:________________
Lithium metal is produced according to the same method as sodium metal (replacing all
sodium reagents with the lithium equivalent).
[4 marks]
Given this information, write a balanced chemical equation (including states of matter)
describing the industrial process used to make lithium metal.
[1 mark]
2 LiCl(l) → 2 Li(l) + Cl2(g)
(b)
This same method cannot also be used to make potassium metal. Why not?
[1 mark]
This method cannot be used to make potassium because K(l) is soluble in KCl(l).
(c)
What is different about the industrial production of potassium (compared to sodium or
lithium)? As part of your answer, write a balanced chemical equation (including states of
matter) describing the industrial process used to make potassium metal.
[2 marks]
KCl(l) + Na(l) → K(g) + NaCl(l)
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).
4.
What is “hard water” and how does a household water softener “soften” it?
[2 marks]
Hard water is water containing cations with charges greater than +1. The most common
cations in hard water are Ca2+ and Mg2+.
A household water softener uses an ion exchange column to exchange the Ca2+ and Mg2+
cations in the water for “softer” Na+ cations.
NAME:_________________________
5.
(a)
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 lithium 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 reacts violently with water; lithium reacts slowly with water. Set
up a beaker of water behind a blast shield. One at a time, drop a small piece
of each metal into the water. Whichever metal gives a more violent reaction
is potassium.
Method B:
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.
Lithium will be less dense than potassium.
Method B is less practical than method A as it requires fairly large samples
to get a measurable volume (and we don’t give you large samples of
potassium for safety reasons!).
(b)
You’re given two vials, each containing a white salt. You’re told that one vial contains
beryllium oxide (BeO) and the other contains magnesium oxide (MgO). How do you
determine which salt is which? Briefly, explain how your method will tell you which is
which.
Method A:
BeO is amphoteric; MgO is basic. If you attempt to dissolve each salt in a
solution of concentrated NaOH(aq), only the BeO will react with the NaOH
and dissolve.
(c)
You’re given two vials, each containing a white salt. You’re told that one vial contains
lithium carbonate (Li2CO3) and the other contains lithium nitrate (LiNO3). 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
lithium carbonate will react with the acid to produce bubbles of carbon
dioxide. As the carbonate reacts, the salt will appear to dissolve. The lithium
nitrate 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 lithium carbonate will decompose to give lithium oxide and carbon
dioxide gas, so whichever sample loses mass was lithium carbonate.
(Lithium nitrate does not decompose upon heating.)
Method B is not very practical as we don’t have a hot enough oven in the lab.
NAME:_________________________
Section:_____
Student Number:________________
6.
The reaction below occurs when sulfuric acid is added to a solution of strontium nitrate.
[7 marks]
H2SO4(aq)
+
Sr(NO3)2(aq)
SrSO4(s)
+
2 HNO3(aq)
(a)
Balance the reaction equation above.
(b)
A solution is prepared by dissolving 9.15 g of solid Sr(NO3)2 in water. To this solution was
then added 65.00 mL of 4.75 M H2SO4. Determine the maximum mass of strontium 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
nSr( NO3 )2
65 .00 mL
1L
1000 mL
9.15 g
1mol
211 .6298 g
4.75 mol
1L
0.309 mol
0.0432 mol
Sr(NO3)2 is the limiting reagent.
(c)
nSrSO4
nSr ( NO3 )2 0.0432 mol
mSrSO4
0.0432 mol
183 .6776 g
1mol
7.94 g
Suggest one reason why Sr(NO3)2 is soluble in water but SrSO4 isn’t.
[1 mark]
SrSO4 forms a stronger lattice because the charge of the sulfate ion (SO42-) is more negative
than the charge of the nitrate ion (NO3-).
NAME:_________________________
7.
Section:_____
Student Number:________________
Sketch a Born-Haber cycle for AlCl3. 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”)
Al3+(g)
+
3 Cl-(g)
I3(Al)
positive enthalpy change
Al2+(g)
I2(Al)
positive enthalpy change
3
Al+(g)
EAH(Cl)
LFH
negative enthalpy change
(AlCl3)
negative enthalpy change
I1(Al)
positive enthalpy change
Al(g)
+
3/2
sublH(Al)
positive enthalpy change
Al(s)
3 Cl(g)
+
BDH(Cl2)
positive enthalpy change
3/2 Cl2(g)
AlCl3(s)
fH (AlCl3)
negative enthalpy change
NAME:_________________________
Section:_____
Student Number:________________
8.
The unit cell for BaO 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 BaO unit cell.
x
(c)
2r
2rO2
Ba2
2(149 pm) 2(140 pm) 578 pm
Calculate the density of BaO.
[7 marks]
Approach: d = m/V so find m and V (for unit cell) then divide to get d
mcell
4mBa2
4mO 2
mcell
613 .306
g
mol
x
578 pm
x3
Vcell
d
m
V
[1 mark]
g
g
g
) 4(15 .9994
) 613 .306
mol
mol
mol
1mol
g
1.01842 10 21
23
6.02214 10 unitcells
unitcell
4(137 .327
1m
100 cm
12
10 pm
1m
(5.78 10 8 cm)3
1.01842 10 21 g
1.93 10 22 cm3
5.78 10 8 cm
1.93 10
5.27
g
cm3
22
cm3
NAME:_________________________
Section:_____
Student Number:________________