Waiting
Login processing...

Trial ends in Request Full Access Tell Your Colleague About Jove

17.5: Standard Entropy Change for a Reaction
TABLE OF
CONTENTS

JoVE Core
Chemistry

A subscription to JoVE is required to view this content.

Education
Standard Entropy Change for a Reaction
 
TRANSCRIPT

17.5: Standard Entropy Change for a Reaction

Entropy is a state function, so the standard entropy change for a chemical reaction (ΔS°rxn) can be calculated from the difference in standard entropy between the products and the reactants.

Eq1

where np and nr represent the stoichiometric coefficients in the balanced equation of the products and reactants, respectively.

For example, ΔS°rxn for the following reaction at room temperature

Eq2

is computed as follows:

Eq3

A partial listing of standard entropies is provided in the table.

Substance   S° (J/mol·K)  
C (s, graphite) 5.740
  C (s, diamond)   2.38
CO (g) 197.7
CO2 (g) 213.8
CH4 (g) 186.3
C2H4 (g) 219.5
C2H6 (g) 229.5
CH3OH (l) 126.8
 C2H5OH (l 160.7
H2 (g) 130.57
H (g) 114.6
H2O (g) 188.71
H2O (l) 69.91
HCI (g) 186.8
H2S (g) 205.7
O2 (g) 205.03

Determination of ΔS°

Consider the condensation of water, in which 1 mole of gaseous H2O changes into 1 mole of liquid H2O.

Eq4

The standard entropy changes for the reaction, ΔS°rxn is calculated using the standard molar entropies and stoichiometric coefficients.

Eq5

The value for ΔS°rxn is negative, as expected for this phase transition (condensation).
As a second example, consider the combustion of methanol, CH3OH:

Eq6

The same procedure is followed to calculate the standard entropy change of the reaction:

Eq7

This text is adapted from Openstax, Chemistry 2e, Chapter 16.2: The Second and Third Law of Thermodynamics.

Tags

Standard Entropy Change Entropy Measurement Calorimeter State Function Standard Molar Entropies Stoichiometric Coefficients Combustion Of Ethylene Standard Conditions Carbon Dioxide Gas Water Ethylene Gas Oxygen Gas Reference Table

Get cutting-edge science videos from JoVE sent straight to your inbox every month.

Waiting X
Simple Hit Counter