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Passage Students Conducted an Experiment to Determine the Changes in Enthalpy

Question 105

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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. -Based on the van 't Hoff plot shown in Figure 1, the dissociation of acetic acid is: A) endothermic, because the y-intercept of the trend line is positive, and therefore ΔS° is positive. B) endothermic, because the slope of the trend line is negative, and therefore ΔH° is positive. C) exothermic, because the slope of the trend line is positive, and therefore ΔH° is negative. D) exothermic, because the y-intercept of the trend line is negative, and therefore ΔS° is negative. 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. -Based on the van 't Hoff plot shown in Figure 1, the dissociation of acetic acid is: A) endothermic, because the y-intercept of the trend line is positive, and therefore ΔS° is positive. B) endothermic, because the slope of the trend line is negative, and therefore ΔH° is positive. C) exothermic, because the slope of the trend line is positive, and therefore ΔH° is negative. D) exothermic, because the y-intercept of the trend line is negative, and therefore ΔS° is negative. 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. -Based on the van 't Hoff plot shown in Figure 1, the dissociation of acetic acid is: A) endothermic, because the y-intercept of the trend line is positive, and therefore ΔS° is positive. B) endothermic, because the slope of the trend line is negative, and therefore ΔH° is positive. C) exothermic, because the slope of the trend line is positive, and therefore ΔH° is negative. D) exothermic, because the y-intercept of the trend line is negative, and therefore ΔS° is negative. 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.
-Based on the van 't Hoff plot shown in Figure 1, the dissociation of acetic acid is:


A) endothermic, because the y-intercept of the trend line is positive, and therefore ΔS° is positive.
B) endothermic, because the slope of the trend line is negative, and therefore ΔH° is positive.
C) exothermic, because the slope of the trend line is positive, and therefore ΔH° is negative.
D) exothermic, because the y-intercept of the trend line is negative, and therefore ΔS° is negative.

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