Download KEY Final Exam Review - Iowa State University

Final Exam Review: 1
Supplemental Instruction
Iowa State University
Leader: Olivia
Course: Chem 178
Instructor: Bonaccorsi
Date:
1.What is the rate-determining step and why is it important?
Rate Determining step is the slowest step in the elementary steps. It looks the most like the rate
law.
The following data were measured for the reaction of BF3(g)+NH3(g) →F3BNH3(g):
Experiment
[BF3](M)
[NH3](M)
Initial Rate (M/s)
1
0.250
0.250
0.2130
2
0.250
0.125
0.1065
3
0.200
0.100
0.0682
4
0.350
0.100
0.1193
5
0.175
0.100
0.0596
a. What is the rate law for the reaction? k[BF3][NH3] seen by exp 1&2;4&5
b. What is the overall order of the reaction? 2
c. Calculate the Rate constant with proper units. Using exp 1
k=(0.2130)M/s/(0.250M)(0.250M)=3.41M-1s-1 could use any of the five to calculate
this. kave=3.408M-1s-1
d. What is the rate when [BF3]=0.100M and [NH3]=0.500M? rate=3.408M-1s1*(0.100M)*(0.500M)=0.170M/s
2a. Write the rate law for a reaction between A, B, and C that is the first order in A, zero order in
B, and second order in C. k=[A][C]2
b. What happens to the rate when [A] doubles and all other concentrations stay the same? They
are proportional therefore rate would double
c. What happens to the rate when C is tripled and all other concentrations stay the same? Rate
increases by a factor of 32 or 9
d. By what factor does the rate change if all three are tripled? [A]=3 [B]=0 [C]=2 so… 3*32=27
e. By what factor does the rate change if all three are cut in half? (1/2)*(1/2)2=1/8
3. Write the mass action expression for the reaction C(s) + CO2 (g)  2CO (g)
K= [CO]2/[CO2]
4. Find the Kc value for the overall reaction given:
1) CaO (s) + H2 (g)  Ca (s) + H2O (g)
K1=67
2) CaO (s) + CO (g)  Ca (s) + CO2 (g)
K2=490
overall: H2 (g) + CO2 (g)  CO (g) + H2O (g)
Kc = K1/K2 = [H2O][CO]/[H2][CO2] = 67/490 = 0.14
5. Label if the following will favor products or reactants
K>1 products
1060 Hixson-Lied Student Success Center  515-294-6624  [email protected]  http://www.si.iastate.edu
K<1 reactants
K>Q products
K<Q reactants
CH16
6. For Nitrous Acid, HNO2 the Ka = 4.5*10-4. What is the Kb value for the nitrite ion, NO2-?
Kb=Kw/Ka
= (1.0 x 10^-14)/(4.5 x 10^-4) =
2.2 x 10^-11.
7. Calculate the pH of 0.1M HNO3 at 298 K.
HNO3 is a strong acid
pH = -log(0.1) = 1
8. Calculate the pH of 0.1 M HNO2 at 298 K. Ka=4.5x10-4
HNO2
H2O
I
0.1
C
-x
E
0.1-x
-
H3O
10-7
+x
10-7+x
NO2
+x
+x
4.5*10-4 = x*(10-7+x)/(0.1-x) = x2/0.1
x = 6.49*10-3 M
pH = -log (6.49*10-3 M) = 2.19
9. Initially 0.035 moles of SO2, 0.5 moles of SO2Cl2, and 0.08 moles of Cl2 are combined in a 5liter flask. What is the value of Q and which direction will the reaction proceed to establish
equilibrium?
SO2Cl2 (g) ⇄ SO2 (g) + Cl2 (g)
Kc = 0.078
SO2Cl2
SO2
Cl2
0.5/5
0.35/5
0.08/5
0.1M
0.007M
0.016
Q = [(0.016)(0.007)]/(0.1) = 1.12*10-3
K>Q moves towards products
10. Aspirin has a pKa of 3.4. What is the ratio of A¯ to HA in:
(a) the blood (pH = 7.4)
(b) the stomach (pH = 1.4)
General comment about the solutions: You have to find the ratio between A¯ and HA
so the concentrations are not needed
Solution to part (a):
pH = pKa + log [A¯] / [HA]
7.4 = 3.4 + log [A¯]/ [HA]
7.4 - 3.4 = 4 = log [A¯]/ [HA]
104 = 10000 = [A¯]/ [HA]
Solution to part (b):
1.4 = 3.4 + log [A¯]/ [HA]
1.4 - 3.4 = - 2.0 = log [A¯]/ [HA]
10-2 = 0.01 = [A¯]/ [HA]
11. What is the hydroxide ion concentration in a solution that has a pOH of 5.70?
5.70 = - log [OH-]
-5.70 = log[OH-]
[OH ] = 10-5.70 = 2.00 x 10-6 M
12. 32.00 g of sodium hydroxide were dissolved in 250.0 mL of solution to prepare
the titrant. 25.00 mL of sulfuric acid were titrated with above titrant. It took 16.00 mL
of sodium hydroxide solution to titrate to the end point. What is the molarity of the
sulfuric acid?
Solution:
1) Determine molarity of NaOH:
MV = grams divided by molar mass
(x) (0.2500 L) = 32.00 g / 40.00 g/mol
x = 3.200 M
2) Determine moles NaOH in 16.00 mL of 3.200 M solution:
moles NaOH ⇒ (3.200 mol/L) (0.01600 L) = 0.05120 mol
3) Determine moles of H2SO4 that react:
2NaOH + H2SO4 ---> Na2SO4 + 2H2O
The molar ratio between NaOH and H2SO4 is 2:1
Therefore, 0.05120 moles of NaOH will neutralize 0.02560 moles of H2SO4
4) Calculate molarity of H2SO4 solution:
0.02560 mol / 0.02500 L = 1.024 M
13. H2S + NO3¯ ---> S8 + NO
Solution:
1) The unbalanced half-reactions:
H2S ---> S8
NO3¯ ---> NO
2) balance each half-reaction:
8H2S ---> S8 + 16H+ + 16e¯
3e¯ + 4H+ + NO3¯ ---> NO + 2H2O
3) Make the number of electrons equal:
24H2S ---> 3S8 + 48H+ + 48e¯ <--- multiplied by a factor of 3
48e¯ + 64H+ + 16NO3¯ ---> 16NO + 32H2O <--- multiplied by a factor of 16
Note that 16 and 3 have no common factors except 1, so both 16 and 3 had to be
used to obtain the lowest common multiple of 48 for the number of electrons.
4) Add:
24H2S + 16H+ + 16NO3¯ ---> 3S8 + 16NO + 32H2O
Comment: removing a factor of 8 does look tempting, doesn't it? However, the
three in front of the S8 (or the five in the next example) makes it impossible. Also,
note that duplicates of 48 electrons and 48 hydrogen ions were removed.
14. In the reaction 2 NO2(g) _ N2O4(g) the initial concentration of NO2 was 0.250 M and N2O4
was 0.000 M. At equilibrium, the concentration of N2O4 was measured as 0.0133 M.
(a) Calculate the equilibrium concentration of NO2
(b) Calculate
Kc.
15. What is the pH and chemical reaction of a solution containing 0.25 M HCO3- and 0.34 M
Na2CO3? Ka(HCO3-)= 5.6x10-11
HCO3- + H2O equil CO32- + H3O+
0.25
0.34
pH = pKa +log (A-/HA)= -log (5.6x10-11) + log (0.34/0.25) = 10.39
CH19
16. Is the following reaction spontaneous at 25 C under the following conditions:
N2 (g) + O2 (g)  2NO (g)
Nitrogen and oxygen have a pressure of 1 atm and nitrogen oxide has a pressure of 4 atm at some
point during the reaction. (delta)Go=173.1 kJ.
Delta G = Delta G* + RTlnQ
Q = P2NO /(PN2*PO2) = 42/(1)(1) = 16
Delta G = 173.1 + 8.314*10^-3 (298)ln(16)
Delta G = 180 kJ
Not Spontaneous.
17. For each pair, which molecule has a greater entropy?
a. CO2 (s) or CO2 (l)
b. NH3 (l) or NH3 (g)
c. Pure magnesium at 0K or 200K
CH20
18. Calculate the cell potential for a cell operating with the following reaction at 25o C, in which
[MnO4 -] = 0.010 M, [Br- ] = 0.010 M, [Mn2+] = 0.15M and [H+ ] = 1.0 M.
2 MnO4 - (aq) + 10 Br- (aq) + 16 H+ (aq)  2Mn2+(aq) + 5Br2(l) + 8H2O(l)
19. Consider the reaction 3CH4 (g)  C3H8 (g) + 2H2 (g)
a. Using ΔG0 of C3H8 = -23.47kJ and ΔG0 of H2 = 0 and ΔG0 of CH4 = -50.8,
calculate, at 298K, ΔG0
b. Calculate delta G at 298K if the reaction mixture consists of 40.0 atm of CH4,
0.0100atm of C3H8, and 0.0180 atm of H2.
20. For a certain chemical reaction, delta H knot = -35.4 kJ and detla S knot = -85.5 J/K
a. Is the reaction endothermic or exothermic?
b. Does the reaction lead to an increase or decrease in the randomness or disorder of the system?
c. Calculate delta G knot for the reaction at 28 degrees C
d. Is the reaction spontaneous at that temperature under standard conditions?