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Passage Students Conducted an Experiment to Determine the Changes in Enthalpy

Question 233

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Passage
Students conducted an experiment to determine the changes in enthalpy ΔH°, entropy ΔS°, and Gibbs free energy ΔG° resulting from the dissociation of acetic acid (CH3COOH) in water.  The students first determined the acid dissociation constant Ka for the reaction by measuring the pH of a 1 M acetic acid solution at various temperatures over a narrow range of less than 10 °C.To perform the pH measurements, the acetic acid solution was initially cooled to 0 °C using an ice bath and then warmed with gentle heating to room temperature with continuous stirring.  The pH was recorded at intervals of 1 °C.  From the pH data, the value of Ka at each temperature T was calculated.  The experimental results over the selected temperature range are shown in Table 1.Table 1  Student Data for the pH and Ka of Acetic Acid with Increasing Temperature
Passage Students conducted an experiment to determine the changes in enthalpy ΔH°, entropy ΔS°, and Gibbs free energy ΔG° resulting from the dissociation of acetic acid (CH<sub>3</sub>COOH) in water.  The students first determined the acid dissociation constant K<sub>a</sub> for the reaction by measuring the pH of a 1 M acetic acid solution at various temperatures over a narrow range of less than 10 °C.To perform the pH measurements, the acetic acid solution was initially cooled to 0 °C using an ice bath and then warmed with gentle heating to room temperature with continuous stirring.  The pH was recorded at intervals of 1 °C.  From the pH data, the value of K<sub>a</sub> at each temperature T was calculated.  The experimental results over the selected temperature range are shown in Table 1.<strong>Table 1</strong>  Student Data for the pH and K<sub>a</sub> of Acetic Acid with Increasing Temperature To evaluate ΔH° and ΔS°, the students constructed a van 't Hoff plot (Figure 1) , which is a graph of the natural log of the dissociation constant (ln K<sub>a</sub>) vs. the reciprocal absolute temperature (1/T) . <strong>Figure 1</strong>  Linear van 't Hoff plot relating the dissociation constant K<sub>a</sub> of acetic acid to temperature TThe relationship between K<sub>a</sub>, T, ΔH°, and ΔS° in the van 't Hoff plot was then described using a linear fit of the data following the form of Equation 1, in which R is the gas constant (8.3 J∙mol<sup>−1</sup>∙K<sup>−1</sup>) . Adapted from: C. Rezsnyak Determination of Thermodynamic Values (S°, H°, and G°) from the Dissociation of a Weak Acid World Journal of Chemical Education. ©2017 Science and Education Publishing. -During the dissociation of acetic acid at the temperatures tested in Table 1, the molecular disorder within the reaction vessel: A) increases, and the energy of the reactants is greater than the energy of the products. B) increases, and the energy of the reactants is less than the energy of the products. C) decreases, and the energy of the reactants is greater than the energy of the products. D) decreases, and the energy of the reactants is less than the energy of the products. To evaluate ΔH° and ΔS°, the students constructed a van 't Hoff plot (Figure 1) , which is a graph of the natural log of the dissociation constant (ln Ka) vs. the reciprocal absolute temperature (1/T) .
Passage Students conducted an experiment to determine the changes in enthalpy ΔH°, entropy ΔS°, and Gibbs free energy ΔG° resulting from the dissociation of acetic acid (CH<sub>3</sub>COOH) in water.  The students first determined the acid dissociation constant K<sub>a</sub> for the reaction by measuring the pH of a 1 M acetic acid solution at various temperatures over a narrow range of less than 10 °C.To perform the pH measurements, the acetic acid solution was initially cooled to 0 °C using an ice bath and then warmed with gentle heating to room temperature with continuous stirring.  The pH was recorded at intervals of 1 °C.  From the pH data, the value of K<sub>a</sub> at each temperature T was calculated.  The experimental results over the selected temperature range are shown in Table 1.<strong>Table 1</strong>  Student Data for the pH and K<sub>a</sub> of Acetic Acid with Increasing Temperature To evaluate ΔH° and ΔS°, the students constructed a van 't Hoff plot (Figure 1) , which is a graph of the natural log of the dissociation constant (ln K<sub>a</sub>) vs. the reciprocal absolute temperature (1/T) . <strong>Figure 1</strong>  Linear van 't Hoff plot relating the dissociation constant K<sub>a</sub> of acetic acid to temperature TThe relationship between K<sub>a</sub>, T, ΔH°, and ΔS° in the van 't Hoff plot was then described using a linear fit of the data following the form of Equation 1, in which R is the gas constant (8.3 J∙mol<sup>−1</sup>∙K<sup>−1</sup>) . Adapted from: C. Rezsnyak Determination of Thermodynamic Values (S°, H°, and G°) from the Dissociation of a Weak Acid World Journal of Chemical Education. ©2017 Science and Education Publishing. -During the dissociation of acetic acid at the temperatures tested in Table 1, the molecular disorder within the reaction vessel: A) increases, and the energy of the reactants is greater than the energy of the products. B) increases, and the energy of the reactants is less than the energy of the products. C) decreases, and the energy of the reactants is greater than the energy of the products. D) decreases, and the energy of the reactants is less than the energy of the products. Figure 1  Linear van 't Hoff plot relating the dissociation constant Ka of acetic acid to temperature TThe relationship between Ka, T, ΔH°, and ΔS° in the van 't Hoff plot was then described using a linear fit of the data following the form of Equation 1, in which R is the gas constant (8.3 J∙mol−1∙K−1) .
Passage Students conducted an experiment to determine the changes in enthalpy ΔH°, entropy ΔS°, and Gibbs free energy ΔG° resulting from the dissociation of acetic acid (CH<sub>3</sub>COOH) in water.  The students first determined the acid dissociation constant K<sub>a</sub> for the reaction by measuring the pH of a 1 M acetic acid solution at various temperatures over a narrow range of less than 10 °C.To perform the pH measurements, the acetic acid solution was initially cooled to 0 °C using an ice bath and then warmed with gentle heating to room temperature with continuous stirring.  The pH was recorded at intervals of 1 °C.  From the pH data, the value of K<sub>a</sub> at each temperature T was calculated.  The experimental results over the selected temperature range are shown in Table 1.<strong>Table 1</strong>  Student Data for the pH and K<sub>a</sub> of Acetic Acid with Increasing Temperature To evaluate ΔH° and ΔS°, the students constructed a van 't Hoff plot (Figure 1) , which is a graph of the natural log of the dissociation constant (ln K<sub>a</sub>) vs. the reciprocal absolute temperature (1/T) . <strong>Figure 1</strong>  Linear van 't Hoff plot relating the dissociation constant K<sub>a</sub> of acetic acid to temperature TThe relationship between K<sub>a</sub>, T, ΔH°, and ΔS° in the van 't Hoff plot was then described using a linear fit of the data following the form of Equation 1, in which R is the gas constant (8.3 J∙mol<sup>−1</sup>∙K<sup>−1</sup>) . Adapted from: C. Rezsnyak Determination of Thermodynamic Values (S°, H°, and G°) from the Dissociation of a Weak Acid World Journal of Chemical Education. ©2017 Science and Education Publishing. -During the dissociation of acetic acid at the temperatures tested in Table 1, the molecular disorder within the reaction vessel: A) increases, and the energy of the reactants is greater than the energy of the products. B) increases, and the energy of the reactants is less than the energy of the products. C) decreases, and the energy of the reactants is greater than the energy of the products. D) decreases, and the energy of the reactants is less than the energy of the products. Adapted from: C. Rezsnyak "Determination of Thermodynamic Values (S°, H°, and G°) from the Dissociation of a Weak Acid" World Journal of Chemical Education. ©2017 Science and Education Publishing.
-During the dissociation of acetic acid at the temperatures tested in Table 1, the molecular disorder within the reaction vessel:


A) increases, and the energy of the reactants is greater than the energy of the products.
B) increases, and the energy of the reactants is less than the energy of the products.
C) decreases, and the energy of the reactants is greater than the energy of the products.
D) decreases, and the energy of the reactants is less than the energy of the products.

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