# 6.R: Determination of Kc for a Complex Ion Formation (Lab Report)


Name: ____________________________ Lab Partner: ________________________

Date: ________________________ Lab Section: __________________

## Part A: Initial concentrations of $$\ce{Fe^{3+}}$$ and $$\ce{SCN^{-}}$$ in Unknown Mixtures

Experimental Data

Tube Reagent Volumes (mL) Initial Concentrations (M)
-- 2.00 x 10-3 M $$\ce{Fe(NO3)3}$$ 2.00 x 10-3 M $$\ce{KSCN}$$ Water $$\ce{Fe^{3+}}$$ $$\ce{SCN^{-}}$$
1 5.00 5.00 0.00
2 5.00 4.00 1.00
3 5.00 3.00 2.00
4 5.00 2.00 3.00
5 5.00 1.00 4.00
• Show a sample dilution calculation for [$$\ce{Fe^{3+}}]$$ initial in Tube #1 only

## Part B and C: The Standard $$\ce{FeSCN^{2+}}$$ Solution (Visual Method)

Given that 10.00 mL of 0.200 M $$\ce{Fe(NO3)3}$$, 2.00 mL of 0.00200 M $$\ce{KSCN}$$, and 8.00 mL of water

• Equilibrium $$[\ce{FeSCN^{2+}}]$$ in Standard Solution: ______________ M

Note that since $$[\ce{Fe^{3+}}] \gg [\ce{SCN^{-}}]$$ in the Standard Solution, the reaction is forced to completion, thus causing all the $$\ce{SCN^{-}}$$ to convert to $$\ce{FeSCN^{2+}}$$.

• Show the stoichiometry and dilution calculations used to obtain this value.

Equilibrium Concentrations of $$\ce{FeSCN^{2+}}$$ in Mixtures:

Tube Solution Depths (mm) $$[\ce{FeSCN^{2+}}]_{equil}$$ (M)
-- Mixtures Standard --
1
2
3
4
5
• Show a sample calculation for $$[\ce{FeSCN^{2+}}]_{equil}$$ in Tube #1 only.

## Part D: Spectrophotometric Method

Calibration Curve Data

Tube Absorbance $$[\ce{FeSCN^{2+}}]$$ (M)
Blank
1
2
3
4
• Show a sample dilution calculation for $$[\ce{FeSCN^{2+}}]$$ in Tube #1 and 2 only.

Note that since $$[\ce{Fe^{3+}}] \gg [\ce{SCN^{-}}]$$ in the Standard Solution, the reaction is forced to completion, thus causing all the added $$\ce{SCN^{-}}$$ to be converted to $$\ce{FeSCN^{2+}}$$.

• Plot Absorbance vs.$$[\ce{FeSCN^{2+}}]$$ for the standard solutions. Obtain an equation for the line. This can be used to determine the $$[\ce{FeSCN^{2+}}]$$ in the table below. Attach your plot to this report.

Test mixtures

Mixture Absorbance $$[\ce{FeSCN^{2+}}]$$ (M)
1
2
3
4
5
• Show a sample calculation for $$[\ce{FeSCN^{2+}}]$$ in mixture 1.
• Linear Equation (A vs. $$[\ce{FeSCN^{2+}}]$$) of Calibration Curve: ___________________

## Calculations and Analysis

The reaction that is assumed to occur in this experiment is:

$\ce{Fe^{3+} (aq) + SCN^{-} (aq) <=> FeSCN^{2+} (aq)} \label{eq1}$

• Write the equilibrium constant expression for the reaction.
• Create a Reaction Table (or ICE table), as in Table 1, to demonstrate how the values below are calculated. Use the data for Mixture #1 only. Start with the known values for the initial concentrations of each species and the final value of $$[\ce{FeSCN^{2+}}]$$ from the data table on the previous page. Show how you find the value of the stoichiometric change in reaction concentrations that occurs, and the resulting equilibrium concentrations of the reactants.
• Show a sample calculation for the value of $$K_{c}$$ using the data for Tube #1.

Using the same method you outlined above, complete the table for all the equilibrium concentrations and value of $$K_{c}$$:

Tube Equilibrium Concentrations (M) $$K_{c}$$
-- $$\ce{Fe^{3+}}$$ $$\ce{SCN^{-}}$$ $$\ce{FeSCN^{2+}}$$ --
1
2
3
4
• Average value of $$K_{c}$$ ________________ (Use reasonable number of significant digits, based on the distribution of your $$K_{c}$$ values.)

## Optional Analysis: Is an alternative reaction stoichiometry supported?

Suppose that instead of forming $$\ce{FeSCN^{2+}}$$, the reaction between $$\ce{Fe^{3+}}$$ and $$\ce{SCN^{-}}$$ resulted in the formation of $$\ce{Fe(SCN)_2^{+}}$$. The reaction analogous to Equation \ref{eq1} would be:

$\ce{Fe^{3+} (aq) + 2SCN^{-} (aq) <=> Fe(SCN)2^{+} (aq)} \label{6}$

i.e., two moles of $$\ce{SCN^{-}}$$ displace two moles of $$\ce{H2O}$$ in $$\ce{Fe(H2O)6^{2+}}$$ to make $$\ce{Fe(SCN)2(H2O)4^{+}}$$.

• Write the equilibrium expression for this reaction.
• Create a Reaction Table for Mixture #1 only (or ICE table), as in Table 1, to clearly show how all the values below were obtained. Then show the calculation for the value of $$K_{c}$$ for Tube 1. Pay special attention to the stoichiometry in this system. Begin by assuming that the equilibrium value for $$[\ce{Fe(SCN)_2^{+}}]$$ is be equal to $$\frac{1}{2} [\ce{FeSCN^{+2}}]$$ at equilibrium obtained previously. (This is because the moles of $$\ce{SCN^{-}}$$ assumed to be equal to the moles of complex ion product, $$\ce{FeSCN^{2+}}$$, in our standard solutions. In the alternate reaction, moles of product, $$\ce{Fe(SCN)_2^{+}}$$, equal ½ the number of moles of $$\ce{SCN^{-}}$$ added in the standard solutions.)

Using the method you outlined above, complete the table for all the equilibrium concentrations and values of $$K_{c}$$

Tube Equilibrium Concentrations (M) $$K_{c}$$
-- $$\ce{Fe^{3+}}$$ $$\ce{SCN^{-}}$$ $$\ce{Fe(SCN)_2{+}}$$ --
1
2
3
4
5
• Average value of $$K_{c}$$ ________________ (Use a reasonable number of significant digits, based on the distribution of your $$K_{c}$$ values.)
• Based on the calculated values of $$K_{c}$$ for each reaction stoichiometry, which reaction is the valid one?
• Briefly explain your conclusion. Compare the percent difference between the average value and the individual measurements. What does this tell you about the two possible stoichiometries? Do these reactions give consistent values of $$K_{c}$$ for different initial reaction conditions?

6.R: Determination of Kc for a Complex Ion Formation (Lab Report) is shared under a not declared license and was authored, remixed, and/or curated by Santa Monica College.