Practice Report #5 "Spectrophotometric Determination of The Free Energy of Gibbs Reaction Standard."

Practice report #5
“SPECTROPHOTOMETRIC DETERMINATION OF THE FREE

ENERGY OF GIBBS REACTION STANDARD.”
►Objective.
Determine on the basis of the experimental data provided the Gibbs free energy,
the equilibrium constant (K), ∆H and ∆S of the ferrotiocyanate ion formation
reaction II.
► Fundament:
The spectrophotometer is an electronic instrument that measures intensities of light
absorbed after passing through a sample solution.
Reaction systems containing a single colored component can be studied by
spectrophotometry. The ratio of
absorbance to concentration for the color
component is measured and used to
determine its concentration in the reaction
system of interest. The concentrations of
the colorless components can be
calculated using the balanced chemical
equation and the measured concentration
of the color component.
Each chemical reaction is associated with
an equilibrium constant K, which reflects the ratio of product concentrations to reactant
concentrations in chemical equilibrium, in chemical systems that are not in equilibrium,
forward and reverse reactions occur at different rates even the concentrations of
reactants and products satisfy the equilibrium expression.
The equilibrium concentrations of the reagents can be calculated by subtracting the
equilibrium concentration of the product from the initial concentrations of the reactants.
Gibbs free energy is: the energy released by a system to perform useful work at
constant pressure. This is represented by the
symbol G and considers both changes in such a
way that:
∆G = ∆H – T∆S
The variation of free energy ∆G, is a state function
and has units of energy. Thus, if useful work is
released in a chemical reaction regardless of what
happens in the universe, ∆G is negative and
therefore it will be a spontaneous reaction, since it considers the dispersion of energy
∆H = - dispersion of matter ∆S = + in the system.
► Development and experimentation.
Use a wavelength of 560 nm on the

spectrophotometer (wait 20 min for the
equipment to warm up before the first
START measurement).
Measure the absorbance and Prepare Fe(NO3)3 0.2 M with a

temperature of each an of the concentration of 25 mL, NaSCN 0.002
solutions. M of [25 mL] and H2O.
Evaluate concentrations of the

Evaluate B using Eq.(4.21).
FeSCN+2 ion using Eq.(4.22).
Calculate the K for each solution. Using the stoichiometry of the reaction and
They must be the same; If not, report the concentration of the ferrothiocyanate ion,
them all. calculate [Fe(NO3)3] and NaSCN at
equilibrium for each solution.
Evaluate ΔG° with equation (4.11) for Report the average value as well as
each value for K. the typical deviation.
►Resultados y Observaciones.
►Conclusion.
END
► References.
►Pretrabajo.
Leslie Rocha
1. ¿Qué dice la Ley de Lambert Beer?
La absorbancia de una solución es directamente proporcional a la concentración
de la especie absorbente y así se puede medir su concentración.
2. ¿Qué es el avance de reacción?
En sistemas cerrados y en el equilibrio químico modifican la temperatura a
volumen constante.
3. ¿Cómo calcula las concentraciones de reactivos y productos en el equilibrio
para cualquier reacción química?
A través de la ecuación:
aFeSC N+2
Ka=
aFe a¿¿ ¿
+1
4. Anote el valor reportado para el ΔGº de la reacción de formación del complejo

ferrotiocianato +2
Fabián Romero.
1. ¿Qué dice la Ley de Lambert Beer?
R: A medida que la luz atraviesa un medio que la absorbe, la cantidad de luz
absorbida en cualquier volumen corresponde a la intensidad de luz que incide,
luego se multiplica por el coeficiente de la absorción.
2. ¿Qué es el avance de reacción?

R: Es el cociente entre el cambio total en el número de moles de una especie y su
coeficiente estequiométrico.
3. ¿Cómo calcula las concentraciones de reactivos y productos en el equilibrio

para cualquier reacción química?
R: Con la formula general de la constante:
K=¿ ¿
4. Anote el valor reportado para el ΔGº de la reacción de formación del complejo

ferrotiocianato +2
R: ΔGº [FeSCN+2] = -38495.10 J/mol

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Practice report #5

“SPECTROPHOTOMETRIC DETERMINATION OF THE FREE

► Development and experimentation.

Use a wavelength of 560 nm on the

Measure the absorbance and Prepare Fe(NO3)3 0.2 M with a

Evaluate concentrations of the

4. Anote el valor reportado para el ΔGº de la reacción de formación del complejo

2. ¿Qué es el avance de reacción?

3. ¿Cómo calcula las concentraciones de reactivos y productos en el equilibrio

4. Anote el valor reportado para el ΔGº de la reacción de formación del complejo

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