Thermochemistry Lab: Enthalpy and Entropy of a Spontaneous Reaction

Introduction

As a roaring bonfire rages, it releases heat (energy) to the surrounding environment while also producing gaseous products such as water vapor and carbon dioxide. When it releases heat to the environment, the system's enthalpy decreases while that of the surrounding environment increases. At the same time, the gaseous products increase the number of particles in the system, causing disorder and a subsequent increase in entropy. A combustion reaction thus serves as an example of a spontaneous reaction. A spontaneous reaction is a chemical reaction characterized by the release of free energy (King, 2023). The change in free energy (ΔG) depends on the entropy and enthalpy changes taking place in the system as shown below:

ΔG = ΔH − TΔS

Enthalpy (symbolized as ΔH) is a measure of a system's heat content. A positive ΔH value indicates that the system is endothermic (gaining heat), while a negative value indicates an exothermic system (one that loses heat to the surroundings). Entropy (symbolized as ΔS) is a measure of the degree of disorder in a system (King, 2023). A positive ΔS value indicates an increase in disorder, while a negative value indicates a decrease in disorder (King, 2023). ΔG is always negative for a spontaneous reaction because of the release of free energy. However, depending on the nature of the reaction, ΔH and ΔS can be either negative or positive, while T, which is measured in Kelvin, is always positive. Thus, ΔG can take on one of four possible outcomes as shown in Table 1 below:

Table 1: Enthalpy and Entropy vs. Spontaneous Process

According to the Second Law of Thermodynamics, a chemical reaction is spontaneous if it causes an increase in entropy — that is, an increase in the amount of disorder in the system (King, 2023). Generally, solids are less disordered than liquids because of their regular structure. Liquids, in turn, are less disordered than gases because gaseous particles are constantly in a state of random motion (King, 2023).

This laboratory seeks to realize the following objectives:

(i) To enhance understanding of enthalpy, entropy, and spontaneity in chemical reactions.
(ii) To explore the Second Law of Thermodynamics by investigating the process by which energy is transferred between a system and the immediate external environment.

The general hypotheses established at the start of the lab were:
(i) The reaction between Reagent X and Reagent Y is exothermic.
(ii) The ΔS (entropy) of the reaction mixture is negative.

Materials and Methods

(i) Select "Virtual Lab" on the course home page to load the lab environment.
(ii) From the lab environment, choose "File" and proceed to "Load an Assignment."
(iii) Choose the category labeled "Thermochemistry" and select the assignment titled "Camping Part 1." At this point, all supplies needed for the lab are available.

(iv) Select an empty 1000 mL Erlenmeyer flask, a 10 mL pipette, and a 0.2 L foam cup from the "Glassware" tab in the stockroom.
(v) In the "Solutions" tab, select "Reagent X" and move the flask containing the 0.1 M Reagent X to the workbench.
(vi) Similarly, select "0.1 M Reagent Y" and move it to the workbench.
(vii) Using the pipette, transfer 10 mL of Reagent X to the foam cup and record the temperature and volume. Transfer 10 mL of Reagent Y to the foam cup and record the temperature and volume of the reaction mixture.
(viii) Close the workbench and proceed to data analysis.

(ix) From observing the reaction mixture, determine whether the system lost or gained energy, and hence whether the process is exothermic or endothermic.
(x) Based on the reaction below, determine whether the synthesis of Reagents X and Y would decrease or increase the entropy of the system and hence whether the reaction was spontaneous:

Reagent X + Reagent Y → Product XY

(xi) Describe what happens to the reaction mixture's molecules when energy transfer occurs in an exothermic process.
(xii) Describe how exothermic processes affect the system's entropy.
(xiii) Determine whether ΔS is positive or negative in an exothermic reaction.
(xiv) Use the data collected during the experiment to calculate ΔH for the reaction, assuming that the specific heat capacity of the reaction mixture equals 4.18 J/g°C and the density of the solution is 1.00 g/mL (q = mCsΔT).

Defining Enthalpy

The initial temperature of Reagent X was 25°C, while the final temperature was 27.29°C. This represents a change (ΔT) of 2.29°C. The system was releasing thermal energy into the surroundings during the chemical reaction, causing an increase in the temperature of the reaction mixture. The system is therefore exothermic, which implies that ΔH is negative.

Defining Entropy and Spontaneity

Entropy is the degree of disorder in a system. The synthesis of the two reagents, X and Y, increases the volume of the solution from 10 mL to 20 mL. This increase in volume raises the number of particles present in the system, which in turn increases the amount of disorder and thus increases the system's entropy. The ΔS is positive — an increase in the degree of disorder is represented by a ΔS greater than zero.

To determine whether the reaction between X and Y was spontaneous, one uses the relationship:

ΔG = ΔH − TΔS

A reaction is spontaneous if ΔG is negative. Since ΔH is negative while both T and ΔS are positive, ΔG will be negative, satisfying the condition for spontaneity. As Table 1 shows, ΔG will always be negative when ΔH is negative and ΔS is positive, regardless of temperature. The negative ΔG confirms that the reaction between X and Y was spontaneous.

Conclusion

This laboratory sought to enhance understanding of enthalpy, entropy, and spontaneity in chemical reactions and to explore the Second Law of Thermodynamics by investigating the process of thermal energy transfer in chemical reactions. Enthalpy (ΔH) is a measure of a system's heat content, while entropy (ΔS) is a measure of the degree of disorder in a system. The experiment involved observing the temperature change resulting from combining 10 mL of Reagent X with 10 mL of Reagent Y. The lab findings indicate that the reaction was exothermic, as it involved a loss of heat from the system to the surrounding environment. Consequently, the loss of heat reduced disorder in the system, leading to a reduction in the system's entropy. The calculated heat of reaction (q = −191.44 J) further confirms the exothermic nature of the process. These findings demonstrate that in an exothermic reaction the system's entropy (ΔS) is negative, while in an endothermic reaction it is positive.

References

King, G. C. (2023). Physics of matter. John Wiley & Sons.