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This question has to do with the reaction Fe3+ + SCN- ---->FeNCS2+

Assume that in a seprate lab exercise, the 3 equilibrium solutions were placed in a water bath and allowed to re-equilibrate at the higher temperature. As a consequence of heating, the Absorbance of the solution was found to decrease. Explain this observation, predict what will happen to the calculated Kc value for this reaction, and determine if the forward reaction is endothermic or exothermic.

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Patrina Schowalter
Patrina SchowalterLv2
29 Sep 2019

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Related questions

Assume that three equilibrium solutions were placed in a water bath, and allowed to re-equilibrate at the higher temperature. As a consequence of heating, the absorbance of the solution was found to decrease. Explain this observation and predict what will happen to the Kc value for the rxn and if the fwd rxn would be endothermic or exothermic.

Can you please help post lab question..

DATA AND CALCULATIONS

Absorbance

Trial 1

0.039

Trial 2

0.045

Trial 3

0.063

Trial 4

0.086

Absorbance of standard (Trial 5)

0.792

Temperature

25°C

Kc expression Kc =

[Fe3+]i

Trial 1

Trial 2

Trial 3

Trial 4

[SCN–]i

[FeSCN2+]eq

[Fe3+]eq

[SCN–]eq

Kc value

Average of Kc values

Kc = ________________________ at ___25_____°C

beaker

Fe(NO3)3
(mL)

KSCN
(mL)

H2O
(mL)

1

5

2

93

2

5

3

92

3

5

4

91

4

5

5

90

PROCESSING THE DATA
1. Write the Kc expression for the reaction in the Data and Calculation table.
2. Calculate the initial concentration of Fe3+, based on the dilution that results from addingKSCN solution and water to the original 0.0020 M Fe(NO3)3 solution. See Step 2 of theprocedure for the volume of each substance used in Trials 1-4. Calculate [Fe3+]i using theequation:

[Fe3+]i =

Fe(NO3)3 mLtotal mL X (0.0020 M)

This should be the same for all four test tubes.
3. Calculate the initial concentration of SCN–, based on its dilution by Fe(NO3)3 and water:

[SCN–]i =

KSCN mLtotal mL X (0.0020 M)

In beaker 1, [SCN–]i = (2 mL / 100 mL)(.0020 M) = .000040 M. Calculate this for the otherthree beakers.
4. [FeSCN2+]eq is calculated using the formula:

[FeSCN2+]eq =

AeqAstd

X [FeSCN2+]std

where Aeq and Astd are the absorbance values for the equilibrium and standard solutions,
respectively, and [FeSCN2+]std = (1/100)(0.0020) = 0.000020 M. Calculate [FeSCN2+]eq foreach of the four trials.
5. [Fe3+]eq: Calculate the concentration of Fe3+ at equilibrium for Trials 1-4 using the equation:

[Fe3+]eq = [Fe3+]i – [FeSCN2+]eq

6. [SCN–]eq: Calculate the concentration of SCN- at equilibrium for Trials 1-4 using theequation:

[SCN–]eq = [SCN–]i – [FeSCN2+]eq

7. Calculate Kc for Trials 1-4. Be sure to show the Kc expression and the values substituted infor each of these calculations.
8. Using your four calculated Kc values, determine an average value for Kc.

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