background [prelab assignment (15 points) due at the begining of lab]

what is Beer's Law ?  calibration curve (relate to Beer's law; hint:  vitamin C lab) ? 

fill-in the below ICE table in regards to the below reaction

	[Fe3+]	[SCN-]	[FeSCN2+]
[inital]	5.0 mM	10.0 mM	0
[change]
[equilibrium]			2.0 mM

and calculate the value of K_c based on the information in the above ICE table.

msds on nitric acid, HNO₃

view screen cast on the use of the colorimeter prior to the lab

purpose

• determine the relationship between [FeSCN²⁺] versus absorption, i.e. generate a calibration curve
• determine the K_c of the reaction:  

• determine the relationship between initial [Fe³⁺ ]  versus equilbrium [FeSCN²⁺]

materials (per 2 groups @ a table)

colorimeter; cuvette; 100 mL volumeric flask

100 mL, 200.0 mM iron (III) nitrate; 50 mL, 2.0 mM potassium thiocyanate

2 mM iron (III) nitrate (dilute 200 mM solution)

150 mL, 0.1 M HNO₃ - use as the solvent

methods

student design; delegate your work - do calculations on how to prepare the below solutions for tables 1 - 3 prior to the lab

table 1.  concentration of __ in 100 mL mixture for calibration curve.  Use 200 mM Fe(NO₃ )₃ , 2 mM KSCN, and 0.1 M HNO₃ to prepare these solutions. A group makes a solution, label it, and share with your classmates @ all tables. 

std	[Fe3+ ]  (mM)	[SCN- ]  (µM)	[FeSCN2+]	absorbance
1	40	200
2	40	150
3	40	100
4	40	50
zero	40	0

table 2.  concentration of  __ in 10 mL mixture - to determine K_c .  Use 2 mM Fe(NO₃ )₃ (dilute 200 mM solution to make 2 mM solution), 2 mM KSCN, and 0.1 M HNO₃ to prepare these solutions and share with your classmates @ the same table. 

[Fe3+ ]  (mM)	[SCN- ]  (mM)	absorbance
1.0	0.1
1.0	0.5
1.0	1.0

table 3.  concentration of  __ in 10 mL mixture  -  to determine the effect of [Fe³⁺ ] on [FeSCN²⁺ ].  You'll have to determine how to prepare these solutions, and share with your classmates @ the same table.

[Fe3+ ]  (mM)	[SCN- ]  (mM)	absorbance
0.1	0.1
0.2	0.1
0.5	0.1
1.0	0.1
2.0	0.1

 

use of colorimeter (vodcast; assume only FeSCN²⁺ absorbs light)

1. turn-on Xplorer data collection device
2. insert colorimeter into Xplorer device
3. open lid, then insert small glass vial (cuvette) filled with "zero" (see table 1, above) into colorimeter
4. close lid; should display %T ~ 100 or Abs ~ 0 for all wavelengths;  press "+" on Xplorer - to cycle thru various setting (red, green, blue, & orange, where reading % T or Abs
5. if %T not ~ 100% or ABS not ~ 0 , then press green button on colorimeter / wait till lights goes off  else goto next step
6. replace cuvette with samples; press "+" on Xplorer - to cycle thru various setting; record data (i.e. absorbance at different wavelengths); repeat with other samples
7. DO NOT PRESS ANY OTHER BUTTON DURING YOUR MEASUREMENT

data analysis

calibration curve;  ICE table

Content of lab report (30 points; no lab report revision)

data analysis

calibration curve - based on data in table 1

determine K_cbased on the ICE table & data in table 2; show all calculations

summary tables / graph: 

• K_c as a function of the initial [Fe³⁺] & [SCN^- ];  (based on table 2)
• make / label a graph of experinental data (curve 1):  initial [Fe³⁺]  versus equilbrium [FeSCN²⁺] (based on table 3)

statistics / conclusions: 

• is the value of K_c truly a constant (i.e. is the value of the Kc the same under all (different) initial conditions) ? use / refer to the appropriate statistics to support your conclusion.
• describe / rationalize the relationship between initial [Fe³⁺]  versus equilbrium [FeSCN²⁺]

resources:

site 1;  site 2