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 K_a 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_a 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 K_a 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_a) vs. the reciprocal absolute temperature (1/T) .
Figure 1 Linear van 't Hoff plot relating the dissociation constant K_a of acetic acid to temperature TThe relationship between K_a, 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) .
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.
-Would changes in the van 't Hoff plot be observed if the reaction rate were increased by adding a catalyst during the experiment?

Question

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 K_a 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_a 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 K_a 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_a) vs. the reciprocal absolute temperature (1/T) .

Figure 1 Linear van 't Hoff plot relating the dissociation constant K_a of acetic acid to temperature TThe relationship between K_a, 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) .

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.
-Would changes in the van 't Hoff plot be observed if the reaction rate were increased by adding a catalyst during the experiment?

Quizplus · Accepted Answer

11ecba2d_122f_0a24_a3bf_11fb0e950e2e_MD0008_00 Thermodynamic values indicate if a reaction is spontaneous or not, but they do not indicate the rate of a reaction. Enthalpy, entropy, and Gibbs free energy are state functions, which mean they describe the energetic differences between the reactants and the products in their current state. These values are independent of the chemical pathway that a reaction takes to get from reactants to products. A catalyst is a chemical species that is added to a reaction to increase the rate of the reaction by stabilizing the transition state and decreasing the activation energy of the reaction. A catalyst does not change the amount of products produced and does not change the relative energies of the reactants and products. The van 't Hoff plot for the experiment described in the passage is used to find the thermodynamic values for the dissociation of acetic acid. The results are based on the final state of the reactants and products at a given temperature, not the reaction mechanism pathway. Increasing the rate of the reaction by adding a catalyst will not affect the amount of product (H⁺) that is produced. Therefore, a catalyst will not change the pH of the solution or the thermodynamic parameters measured in the experiment. (Choice A) Although the addition of a catalyst will not change the initial concentration of acetic acid, the thermodynamic results in Figure 1 are from state functions that depend on the reaction conditions rather than on the initial amount of acetic acid or the specific reaction pathway invoked by a catalyst. (Choice C) Although it is true that the reaction rate and ΔG° are both affected by temperature, changing the reaction rate will not change the van 't Hoff plot because ΔS° and ΔH° are independent of temperature. (Choice D) Spontaneity indicates whether a reaction will proceed without the continuous input of additional energy. Spontaneity does not indicate how long the process will take. Educational objective: Thermodynamic values indicate if a reaction is spontaneous or not, but they do not indicate the rate of a reaction because thermodynamic values are state functions that are independent of the reaction pathway.

Passage Students Conducted an Experiment to Determine the Changes in Enthalpy