SPONTANOUS REACTION ENTHALPY AND ENTROPY

GEORGIA MILITARY COLLEGE

NATURAL SCIENCE DEPARTMENT

ONLINE CAMPUS

LABORATORY 5 – THERMOCHEMISTRY: ENTHALPY AND ENTROPY OF A SPONTANOUS REACTION

NAME

STUDENT NUMBER

CLASS

PROFESSOR’S TITLE AND NAME

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 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). However, the change in free energy (?G) depends on the entropy and enthalpy changes taking place in the system as shown below:

?G = ?H - T ?S

Enthalpy (symbolized as ?H) is a measure of a system’s heat content. A positive ?H value shows that the system is endothermic (gaining heat), while a negative value shows an exothermic system (one that loses heat to the surroundings). On the other hand, entropy (symbolized as ?S) is a measure of the degree of disorder in a system (King, 2023). A positive ?S value shows an increase in the amount of disorder in a system, while a negative value shows a decrease in the amount of disorder (King, 2023). ?G is always negative for a spontaneous reaction because of the release of free energy. However, depending on the nature of the reaction, ?H and ?S can be either negative or positive, while T, which is measured in Kelvin, is always positive. Thus, ?G can take on one of four possible outcomes as shown in table 1 below:

Table 1: Enthalpy and Entropy vs Spontaneous Process

?H

?S

?G

Negative

Positive

Always Negative (spontaneous) at all T

Positive

Positive

Positive (non-spontaneous) at lower T, negative (spontaneous) at higher T

Negative

Negative

Positive (non-spontaneous) at higher T, negative (spontaneous) at lower T

Positive

Negative

Always positive (non-spontaneous)

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

Based on the above context, 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 that:

i) The reaction between X and Y is exothermic

ii) The ?S (entropy) of the reaction mixture is negative

Materials and Methods

Preparing the Lab

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 the supplies needed for the lab are available.

Performing the Lab

iv) Select ‘Empty 1000mL Erlenmeyer flask’, 10mL pipette, and 0.2L foam cup from the ‘Glassware’ tab in the stockroom.

v) In the ‘Solutions’ tab, select ‘Reagent X’ and move the flask containing the 0.1M Reagent X to the workbench.

vi) Similarly, select ‘0.1M Reagent Y’ and move it to the workbench.

vii) Using the pipette, transfer 10mL of Reagent X to the foam cup and record the temperature and volume. Transfer 10mL 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.

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, in 5 to 10 sentences, what happens to the reaction mixture’s molecules when energy transfer occurs in an exothermic process.

xii) Describe, in 5 to 10 sentences, how exothermic processes affect the system’s entropy.

xiii) Determine whether ?S is positive or negative in an exothermic reaction.

xiv) Use the data collected during the experiment to calculate ?H for the reaction assuming that specific heat capacity of the reaction mixture equals 4.18J/g0C, and density of the solution is 1.00g/mL (q=mCs ?T)

Data and Discussion

Defining Enthalpy

The initial temperature of Reagent X was 250C, while the final temperature was 27.290c. This represents a change (?H) of 2.290C. The system was releasing thermal energy into the surrounding during the chemical reaction, causing an increase in the temperature of the reaction mixture. Thus, the system is exothermic, implying that the ?H is negative.

Defining Entropy

Entropy is the degree of disturbance in a system. The synthesis of the two reagents X and Y increases the volume of the solution from 10mL to 20mL. The increase in volume increases the number of particles available in the system, which increases the amount of disorder, thus increasing the system’s entropy. The ?S is positive – an increase in the degree of disorder in a system is represented by a ?S greater than zero. To determine whether the reaction between X and Y was simultaneous, one would use the relationship:

?G = ?H - T ?S

A reaction is spontaneous if ?G is negative. Since ?H is negative, while both T and ?S are positive, then ?G will be negative, satisfying the condition of spontaneity. As table 1 presented earlier shows, ?G will always be negative when ?H is negative and ?S is positive, regardless of the temperature (T). The negative ?G implies that the reaction between X and Y was spontaneous.

Discussion

A crucial component of thermodynamics is understanding how the transfer of thermal energy affects the system as well as the surroundings during a chemical process. An exothermic reaction is characterized by the release of energy from the system to the immediate external environment (King, 2023). In most cases, this energy is released either in the form of light or heat. Since energy is moving out of the system as it converts reactants to products, the products in an exothermic reaction will often have lower energy than the reactants (King, 2023). This implies that the molecules of the products will demonstrate reduced energy and move around less than those of the reactants (King, 2023). Consequently, the molecules of the products will form stronger bonds than those broken in the reactants.

There is a strong relationship between exothermic processes and a system’s entropy. An increase in temperature during a chemical reaction increases the amount of disorder in the system (King, 2023). This is because temperature increases the energy of the molecules in the reactants, causing them to move faster and collide more randomly with each other. If a reaction is exothermic, the system releases heat into the surrounding, and this causes a temperature decrease within the system, and consequently, a decrease in disorder (King, 2023). The decrease in disorder reduces the system’s entropy (King, 2023). At the same time, the entropy of the surrounding environment increases due to the increase in temperature caused by the exothermic reaction (King, 2023).