Final
ChemEXCEL Chemistry II 2014
Name:
Good luck everyone! You can do it!
Constants / Conversions
1 cal = 4.184 J
101.33 J = 1 L atm
H2O specific heat = 1 cal g–1 °C–1
R = 0.08206 L atm mol–1 K–1
R = 8.31 J mol–1 K–1
Equations
PV = nRT
q = mC∆T
w = –P∆V
∆H = ∆U – w
∆G = ∆H – T∆S
KP = KC(RT)∆n gas
∆G = –RTln(KC)
1
Final
ChemEXCEL Chemistry II 2014
1. Balance the following equations
a. S8 + Ag  Ag2S
S8 16Ag8Ag2S
5 pts
b. NaClO3  NaCl + O2
5 pts
2NaClO32NaCl 3O2
c. Combustion of caffeine, C8H10N4O2.
Assume that all the Nitrogen in caffeine is converted to N2 during combustion.
C8H10N4O2 9.5O25H2O 8CO2 2N2
5 pts
d. KNO3 + C12H22O11  N2 + CO2 + H2O + K2CO3
48KNO3 5C12H22O1124N2 36CO2 55H2O 24K2CO3
5 pts
e. HIO3 + FeI2 + HCl  FeCl3 + ICl + H2O
5HIO3 4FeI2 25HCl4FeCl3 13ICl 15H2O
2
5 pts
Final
ChemEXCEL Chemistry II 2014
2. A 1.000 g sample of red phosphorus (P) reacts with oxygen gas to give 2.291 g of a phosphorus
oxide (PnOm). What is the empirical formula and molecular formula of the product phosphorus
oxide given the molar mass is approximately 284 g mol–1.
Byconservationofmass,the2.291gofproductmusthave1gofP,sotherestofitshouldbeO.
molesO
2.291g
1.000gP molesP 1.000gP
1 mol O
0.08068molO
16.00gO
1molP
0.03229molP
30.97gP
10 pts
Therefore,theformulaisP0.03229O0.08068
WhichsimplifiestoP1O2.5,sotheempiricalformulaisP2O5
ThemolarmassofP2O5is141.94,thereforethemolecularformulaisP4O10
3. Dennis creates four questions for his class to solve. Determine whether or not the questions
provide enough information to solve the problem. If yes, solve it. If no, explain what additional
information is needed.
a. The change in internal energy of a certain reaction is 500 J. What is the change in enthalpy
of that same reaction?
5 pts
Can’tdoit,wedon’tknowifthereisworkornotforthereaction.
b. What is KC for the following reaction if at equilibrium there is 0.00100 M hemoglobin
(aq), 0.00250 M O2 (aq), and 0.0100 M Hemoglobin•4O2 (aq)?
Hemoglobin (aq) + 4O2 (aq) ⇌ Hemoglobin•4O2 (aq)
KC
Hemoglobin•4O2
Hemoglobin O2 4
0.0100
0.00100 0.00250
3
4
2.56
10 5 pts
Final
ChemEXCEL Chemistry II 2014
c. A mad scientist is feeling frisky and fills a bomb calorimeter with silicone instead of water.
The scientist runs a reaction in the calorimeter which is known to have qrxn = 1059 J. The
silicone increases from 298 K to 305 K. What is the specific heat of the silicone?
Can’tdothisunlessweknowthemassofthesilicone.Wecancalculatetheheatcapacityofthis
particularcalorimeterthough…
5 pts
d. A chemistry lab owns 1.00 L of a 0.500 M-solution of NaCl (aq). How does a sane scientist
in that lab make 100 mL of a 0.010 M solution of NaCl?
M1 V1
M2 V2
0.500MNaCl V1 0.010MNaCldesired 0.100Ldesired
5 pts
V1 0.00200L
Take0.00200Lofthe0.500Msolutionandadd0.098LofH2Otoittogetthedesiredsolution.
Ialsoacceptedsolutionswhichadded49Ltothe1L0.500Mandthentook100mLfromit,
eventhoughthat’smorewastefulandmessesupyourstocksolution.
4. Limestone is etched away slowly by H2O and CO2 in the following reaction:
CaCO3 (s) + H2O (l) + CO2 (aq) ⇌ Ca(HCO3)2 (aq)
KC = 2.3 × 10–4 at 298 K
A 1 kg block of CaCO3 is submerged in a closed system containing 3.00 L of H2O, which also has
5 g of CO2 dissolved in it. Assume that the container is completely full so that there is no
equilibrium between CO2 (aq) and CO2 (g).
a. What is the concentration of Ca(HCO3)2 when the system reaches equilibrium?
5g
KC
Ca HCO3
CO2
x 0.000026,so Ca HCO3
2
⇌
CO2
CO2
.
x CO2
2
5g
Ca HCO3 2
xmol
3.00L
1molCO2
44.01gCO2
3.00L
xmol
x
2.3
10 10 pts
is0.0000087M
Itturnsoutifyoudidn’tdotheICEtableyouwouldgetthesameanswer…butthatisbecausex
hasasmallvalue.Thiswon’talwaysbethecase,soshowingtheworkmatters.
4
Final
ChemEXCEL Chemistry II 2014
b. What will happen to the equilibrium if more CaCO3 (s) is added to the container? Why?
Nothingbecausetheequilibriumequationdoesn’tcontainCaCO3 s .
5 pts
c. What will happen to the equilibrium if CO2 (g) is added to the container? Why?
5 pts
IfsomeoftheCO2dissolvesinthewater,thentheequilibriumwillshifttowardtheproducts
becausetheequilibriumequationcontainsCO2 aq butnotCO2 g .
IfyousaidnothinghappensbecausenoequilibriumbetweengasanddissolvedCO2 that’sfine.
d. What will happen to the equilibrium 6.00 L of water are used instead of 3.00 L? Why?
5 pts
Theequilibriumwon’tchangebecausethevolumetermscanceloutintheequationforKC.
e. Is this reaction a spontaneous process at 298 K? Hint: check the equations on the first
page.
5 pts
∆G
RTln K
.08206L atm mol
K
298 K
ln 0.00023
204L atm mol
Since∆Gispositive,theprocessisnonspontaneous.
f.
Assume that the reaction is not under equilibrium and goes to completion. What would
be the limiting reactant?
molesCO
5g
1 mol CO
48.01 g CO
molesCaCO 1000gCaCO molesH O 3000gH O
ThereforeCO2isthelimitingreactant.
5
0.104molCO 1molCaCO
100.09gCaCO
9.99molCaCO 1molH O
166.57molH O
18.01gH O
5 pts
Final
ChemEXCEL Chemistry II 2014
5. The gas diborane (B2H6) was investigated by NASA as a possible rocket fuel, but rejected.
a. The external fuel tank of a space shuttle has a volume of 1.5 million L. How many moles
of diborane are needed to fill the tank to a pressure of 2 atm at 298 K?
n
PV
RT
2 atm 1500000 L
122680mol
0.08206 Latm mol‐1 K ‐1 298 K
5 pts
b. The overall reaction for diborane as rocket fuel is B2H6 (g)  2B (s) + 3H2 (g)
How many kilograms of B (s) are produced if all the moles of B2H6 calculated in part 5a
are converted to B (s) and H2 (g)?
5 pts
122680molB H
2 mol B
1molB H
6
10.81 g B
1molB
1 kg B
1000gB
2652kgB
Final
ChemEXCEL Chemistry II 2014
c. How much pressure-volume work in kJ is done if all the moles of B2H6 calculated in part
5a are converted to B (s) and H2 (g)? Assume now that the reaction is not in the fuel tank,
but is under an external pressure of 1 atm at 298 K.
10 pts
AtadifferentpressuresoweneedtorecalculatevolumethatB2H6takesup.
V
nRT
P
122680mol
0.08206 Latm mol‐1 K ‐1
1atm
298 K
3000002LB H Lawofcombiningvolumes!SovolumeofH2producedis9000006LH2.
3"L"H
1"L"B H
3000002LB H
w
∆
1atm
9000006
9000006LH 3000002 6000004Latm 607980kJ
d. What is ∆H° of the reaction in kJ if 1 mol of B2H6 is converted to B (s) and H2 (g), given
the following information?
B2H6(g) + 3O2(g)  B2O3(s) + 3H2O(g)
ΔH°= −2035 kJ
2B2O3(s)  4B(s) + 3O2(g)
ΔH°= 2546 kJ
ΔH°= 1452 kJ
6H2O(g)  6H2(g) + 3O2(g)
5 pts
B2H6 g 3O2 g B2O3 s 3H2O g ΔH° 2035kJ
½ 2B2O3 s 4B s 3O2 g
ΔH° 2546kJ ½
½ 6H2O g 6H2 g 3O2 g B2H6 g 6H2 g 4B s 7
ΔH° 1452kJ ½
ΔH° 36kJ
Final
ChemEXCEL Chemistry II 2014
e. What is ∆H° of the reaction in kJ if all the moles of B2H6 calculated in part 5a are converted
to B (s) and H2 (g)?
Foronemol,∆H ‐36kJ.
5 pts
For122680mols,∆H ‐36 122680 ‐4416480kJ
f.
What volume of 298 K water in L would be needed in a massive bomb calorimeter to
absorb all of the heat calculated in part 5e if you don’t want the temperature of the
calorimeter to rise higher than 50 °C? Assume that only the water in the calorimeter
absorbs heat and that the density of water is 1 g mL–1.
10 pts
AlotofyouguysthoughtImeantthat∆T 50,Icanseewhyit’sconfusingsoIacceptedthose
answerstoo.
Fromparteweknowthatqrxn ‐4416480kJ
–q
q
4416480kJ
Sinceassumeonlythewaterisabsorbingheat,qcalorimeter qwater
m
V
m
–q
C∆T
4416480kJ
4.184J
g
density
50
1000J
1kJ
25
42222562gwater
That’sonly1.7%ofanOlympicswimmingpool!?
8
42222562gwater
1mL
1g
1L
1000mL
42222L
Final
ChemEXCEL Chemistry II 2014
6. Two chemists, James and Bond, walk into a bar. They each order a Vesper, which is composed of
66.6% Gordon’s, 22.22% vodka, and 11.12% Lina Killet, and some ice. James demands his be
shaken for 5 sec while Bond asks for his to be stirred for 5 sec.
Who will have the more entropic drink, and why? Hint: there is no wrong answer if you provide
a reasonable justification.
5 pts
Justificationforequal:Ifyouwereabletostirthedrinkwithsameamountof“energy”as
someoneshakingthedrink,theywouldbesimilar.
Justificationforshakenbeingmoreentropic:Usuallypeopleareabletoshakeharderthanthey
areabletostir,soshakendrinkswouldmoreentropicwithmoremixing/brokenchunksofice.
Justificationforstirredbeingmoreentropic:Maybesomepeoplestirmoreenergeticallythan
theywouldshake,sothenstirredwouldgetbettermixing/brokenchunksofice.
7. What is the difference between Gibb’s free energy and entropy?
10 pts
Gibb’saccountsfortheentropyoftheuniverse soitisameasureofspontaneity,whileentropyS
accountsforjusttheamountofdisorderofthesystem.
8. What is your favorite element?
5 pts
9