Investigating Fe + SCN Equilibrium

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In summary: Since the Keq = [Fe(SCN)]/[Fe] [SCN] and [Fe(SCN)] =[ SCN] the results should be 1/[Fe]...right? No. Given the initial concentration of thiocyanate is equal to [SCNi], let x equal to the amount of [SCN] converted to [Fe(SCN)]. The concentration of thiocyanate in the equilibrium expression is therefore [SCNi-x] and the concentration of [Fe(SCN)] you measure is simply 'x'. Substitute these values into your equilibrium equation and solve for x. x
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Whalstib
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Homework Statement


Hi,

I've run into a problem and fear I'm just approaching from the wrong angle.

From Ebbing Experiments in General Chemistry 9e 14B if you have it to reference..

It's the classic Fe + SCN > Fe(SCN)

In our experiment we are using SCN as the limiting reactant and overwhelming amts of Fe so that the initial [SCN] should equal the final [Fe(SCN)] at equalibrium. We are using a spectophotometer to measure absorbance at 450nm

Homework Equations


Our Keq is obtained by: [Fe(SCN)]/[Fe] [SCN] resulting in m=∑xy/∑x^2 ; K = 2.704 10^4
Our fist 5 series we varied [SCN] keeping [Fe] constant.

6-10 we varied [SCN] and keeping [Fe] to a different constant.

11-15 we varied [SCN] and keeping [Fe] to yet a different constant.

The Attempt at a Solution



So we have 3 constant [Fe].

Since the Keq = [Fe(SCN)]/[Fe] [SCN] and [Fe(SCN)] =[ SCN] the results should be 1/[Fe]...right?

[Fe] in 5-10 = 3.57E-4 ... inv= 2.8E-3 so K = 2.8E-3
[Fe] in 5-10 = 7.14E-4 ... inv= 1.4E-3 so K = 1.4E-3

Using Beer's Law A=kc I have from for example #5 A=.216 (@450nm) K = 2.8E-3
Solving for c [Fe(SCN)]? gives me 605 moles!

Subsequent equations yield equally baffling results (to me baffling!)Via plotting data from the first 5 experiments in Excel and crunching number in a linear regression analysis formula:

Since m=∑xy/∑x^2; K = 2.704 10^4 so my k values are reasonable but the 605 moles is not! ...(is it?)

What am I not seeing here?

Perhaps I'm not grasping what the "c" in A=kc is for as I already know the Keq for [FE(SCN)] and in fact ALL my concentrations...and A's and K's...

Thanks
Whalstib
 
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  • #2
Whalstib said:

Homework Statement


Hi,

I've run into a problem and fear I'm just approaching from the wrong angle.

From Ebbing Experiments in General Chemistry 9e 14B if you have it to reference..

It's the classic Fe + SCN > Fe(SCN)

In our experiment we are using SCN as the limiting reactant and overwhelming amts of Fe so that the initial [SCN] should equal the final [Fe(SCN)] at equalibrium. We are using a spectophotometer to measure absorbance at 450nm




Homework Equations


Our Keq is obtained by: [Fe(SCN)]/[Fe] [SCN] resulting in m=∑xy/∑x^2 ; K = 2.704 10^4



Our fist 5 series we varied [SCN] keeping [Fe] constant.

6-10 we varied [SCN] and keeping [Fe] to a different constant.

11-15 we varied [SCN] and keeping [Fe] to yet a different constant.




The Attempt at a Solution



So we have 3 constant [Fe].

Since the Keq = [Fe(SCN)]/[Fe] [SCN] and [Fe(SCN)] =[ SCN] the results should be 1/[Fe]...right?

No. Given the initial concentration of thiocyanate is equal to [SCNi], let x equal to the amount of [SCN] converted to [Fe(SCN)]. The concentration of thiocyanate in the equilibrium expression is therefore [SCNi-x] and the concentration of [Fe(SCN)] you measure is simply 'x'. Substitute these values into your equilibrium equation and solve for x.
 

1. What is the purpose of investigating the Fe + SCN equilibrium?

The purpose of investigating the Fe + SCN equilibrium is to understand the factors that affect the equilibrium constant and the rate of the reaction. This information can be used to optimize reaction conditions and determine the products and concentrations at equilibrium.

2. How is the equilibrium constant (Kc) for the Fe + SCN equilibrium determined?

The equilibrium constant (Kc) for the Fe + SCN equilibrium is determined by measuring the initial concentrations of reactants and products and the concentrations at equilibrium. The equilibrium constant is then calculated using the equation Kc = [Products]/[Reactants].

3. What factors affect the equilibrium constant for the Fe + SCN equilibrium?

The equilibrium constant for the Fe + SCN equilibrium is affected by temperature, pressure, and the concentrations of reactants and products. Changes in any of these factors can shift the equilibrium and change the value of Kc.

4. How does Le Chatelier's principle apply to the Fe + SCN equilibrium?

Le Chatelier's principle states that when a system at equilibrium is subjected to a stress, the system will shift to counteract that stress and reach a new equilibrium. In the Fe + SCN equilibrium, changes in temperature, pressure, or concentrations can shift the equilibrium to favor the formation of either the reactants or products, depending on the direction of the stress.

5. How can the rate of the Fe + SCN reaction be increased?

The rate of the Fe + SCN reaction can be increased by increasing the temperature, using a catalyst, or increasing the surface area of the reactants. These factors can increase the frequency of collisions between reactant particles and therefore increase the rate of the reaction.

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