Find the equilibrium constant.
1. The equilibrium constant for the reaction,!
Fe3+ + SCN- ⇋ FeSCN2+ (at a specific temperature),
can be determined by first preparing a standard solution of FeSCN2+ and comparing its absorbance of light to an equilibrium system of unknown concentrations.
i.) Standard preparation:
25.0 mL of 0.500 M Fe(NO3)3 + 5.00 mL of 0.00500 M KSCN, Absorbance (A) = 0.700
☝ (excess!)
Calculate the equilibrium concentration of FeSCN2+ for this standard:
ii.) Unknown equilibrium system:
30.0 mL of 0.00400 M Fe(NO3)3 + 20.0 mL of 0.00400 M KSCN, Absorbance (A) = 0.400
******My answer: 221
2.) ! !
!
A student investigated the same equilibrium system as above, but at a new temperature, by first preparing a standard solution:
36.0 mL of 0.300 M Fe(NO3)3 + 4.00 mL of 0.00300 M KSCN, !Astd = 0.525
And then an equilibrium system of unknown concentrations:
25.0 mL of 0.00300 M Fe(NO3)3 + 15.0 mL of 0.00300 M KSCN ; ! A = 0.442
!
!
!
! Calculate the equilibrium constant for this new temperature (to 3 sig figs).
* Ignore the exclamation points.
****My answer: 179
To find the equilibrium constant, you need to use the absorbance values and the concentrations of the species involved in the reaction. The equilibrium constant (K) is determined using the formula:
K = (Absorbance of equilibrium system) / (Absorbance of standard system)
Let's calculate the equilibrium constant for each of the given scenarios:
1. Standard Preparation:
From the given information:
Volume of Fe(NO3)3 solution = 25.0 mL = 0.0250 L
Volume of KSCN solution = 5.00 mL = 0.00500 L
Concentration of Fe(NO3)3 = 0.500 M
Concentration of KSCN = 0.00500 M
Absorbance of standard system (Astd) = 0.700
To calculate the equilibrium concentration of FeSCN2+ in the standard system, we can use the equation:
Astd = ε*l*c
where ε is the molar absorptivity, l is the path length of the cuvette (assumed to be constant), and c is the concentration of FeSCN2+.
Assuming ε*l = 1, we can rearrange the equation as follows:
c = Astd / 1 = Astd
Therefore, the equilibrium concentration of FeSCN2+ in the standard system is 0.700.
2. Unknown Equilibrium System:
From the given information:
Volume of Fe(NO3)3 solution = 30.0 mL = 0.0300 L
Volume of KSCN solution = 20.0 mL = 0.0200 L
Concentration of Fe(NO3)3 = 0.00400 M
Concentration of KSCN = 0.00400 M
Absorbance of equilibrium system (A) = 0.400
Using the same equation as before:
c = A / 1 = A
Therefore, the equilibrium concentration of FeSCN2+ in the unknown equilibrium system is 0.400.
Finally, we can calculate the equilibrium constant for each scenario:
1. Equilibrium constant at the specific temperature:
K = A / Astd = 0.400 / 0.700 = 0.571
2. Equilibrium constant at the new temperature:
Given the absorbance values and concentrations, we can apply the same formula:
K = A / Astd = 0.442 / 0.525 = 0.841 (rounded to 3 significant figures)
Therefore, the equilibrium constant for the new temperature is approximately 0.841.