Entropy Changes in Chemical Reaction

Basic Concept

Entropy (S) is a thermodynamic function, which can be viewed as a measure of randomness or disorder, and describes the number of arrangements (position and/or energy levels) that are available to a system existing in a given state. With entropy, the second law of thermodynamics can be stated "In any spontaneous process there is always an increase in the entropy of the universe." The standard entropy (Sº) of a substance is the value of entropy of the substance at 298 K and 1 atm.

Because entropy is a state function of a system, the entropy change in chemical reactions can be calculated by taking the difference between the standard entropy values of products and those of the reactants:

ΔSº_reaction = Σn_pSº(products) - Σn_rSº(reactants)

where the first Σ to take sum of all the products in this reaction, and the second sums all the reactants. The n_p and n_r represent the moles of each product or reactant, respectively.

An example calculating the entropy change in a reaction:

CH₄(g) + 2O₂(g) = CO₂(g) + 2H₂O(l)             

From the Table of Thermodynamics Data, the Standard entropies of the substances involved in above reaction are:

Substances                Sº (J/K.mol)

CH₄(g)                         186

O₂(g)                            205

CO₂(g)                         214

H₂O(l)                            70

The entropy change of the reaction can be calculated as:       

ΔSº = [214 + 70 * 2] - [186  + 205 * 2] = -242 J/K

User Instructions

For the above example,  

1. Select Entropy Changes in Chemical Reactions link either from the front page (Table of Content) or from the Thermochemistry module. The Input and Output screen appears.

2. In the Input area, enter the problem equation; then click Balance button to balance the equation; then enter corresponding Entropy (S) value. You may consult the Selected Thermodynamic Data sub-module for reference data.