LAB: The Determination of Keq for FeSCN2+
(30 points possible)
Pre-lab Questions: (5 pts)
1)
2a)
Ag+(aq)
2b)
+
2NH3(aq)
→
Ag(NH3)2+(aq)
I
C
.
E
3)
4)
Reference Solutions #1-5: Calculate initial [SCN-] for each reference solution (#1-5) and enter the values in Data
Table 1 where indicated. The background section explains why [SCN-]initial ≈ [FeSCN2+]eq’m for these five reference
solutions. Show a sample calculation for Ref sol’n #1 in this space.
Work: #1
Answer:
#1 [SCN-]initial =
5)
Test Solutions #6-10: Calculate initial [Fe3+] and [SCN-] for each reference solution (#6-10) and enter the values in
Data Table 2 where indicated. Show a sample calculation for Ref sol’n #6 in this space.
Work: #6 - Show for both [Fe3+] and [SCN-]
Answer:
#6 [Fe3+]initial =
#6 [SCN-]initial =
Data Tables (10 points)
Data Table 1 – Reference Solutions
Temperature:
[FeSCN2+]
Sample
Absorbance
Same as initial [SCN-], calculated in #4 of prelab
Reference solution #1
Reference solution #2
Reference solution #3
Reference solution #4
Reference solution #5
Data Table 2 – Test Solutions
Temperature:
Sample
[Fe3+]initial*
[SCN-]initial*
Calculated in #5 of prelab
Calculated in #5 of prelab
Absorbance
Test solution #6
Test solution #7
Test solution #8
Test solution #9
Test solution #10
*These are the concentrations of ions in solution immediately after mixing and before any reaction has occurred.
Data Table 3 - Results
Sample
[FeSCN2+]eq
[Fe3+]eq
[SCN-]eq
Keq
Determined from linear
trendline eq’n on graph
Test solution #6
Test solution #7
Test solution #8
Test solution #9
Test solution #10
Average Keq:_____________
Average deviation:___________
Post-lab Calculations and Analysis (13 points)
1) Attach graph (should be size of full page and either printed from Excel (with trendline and equation) or plotted on
graph paper with equation of line determined via your calculator). Use the equation of trendline to determine
equilibrium concentrations of your test solution instead of visual approximations from the graph (#2-3 below). Don’t
forget to title your graph and label each axis appropriately. (5 pts)
2-3) Record results in Data Table 3 (1 pt)
(show work for test sol’n #6 as a sample calculation by plugging #’s into eq’n from graph)
4) Sample calculation for test solution #6: (3 pts)
Complete the I-C-E chart below to find the eq’m concentrations of Fe3+(aq) and SCN-(aq) instead of using the eq’n
provided in the lab.
Fe3+(aq)
+
SCN-(aq)
→
FeSCN2+(aq)
I
C
E
.
____________
____________
6) Sample Keq calculation
for test solution #6:
(1 pt)
7) Record work under data table 3 (no need to show work)
8) Show work for
calculation of average
deviation of Keq:
(1 pt)
9) (1 pt)
10
Error #1
(1 pt)
#1 Effect? Keq ↑ or ↓ or random
#2
(1 pt)
#2 Effect? Keq ↑ or ↓ or random
#3
(1 pt)
#3 Effect? Keq ↑ or ↓ or random
Post-lab Question: (2 pts) Answer question # 4 only:
#4)