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# 21.9: Standard Entropies Can Be Used to Calculate Entropy Changes of Chemical Reactions


Entropy is a state function, so we can calculate values for a process using any path. This allows us to calculate the entropy change of a chemical reaction using standard entropies. Specifically, we sum the entropies of the products and subtract the entropies of the reactions:

$\Delta_{rxn}S^\circ = \sum_{\text{Products}}{v_i S^\circ_i} - \sum_{\text{Reactants}}{v_i S^\circ_i} \nonumber$

Where $$v_i$$ is the stoichiometric coefficient. Let's look at the combustion of methane:

$\ce{CH_4} \left( g \right) + 2 \ce{O_2} \left( g \right) \rightarrow 2 \ce{H_2O} + \ce{CO_2}\left(g\right) \nonumber$

The standard entropies are:

 Molecule Entropy $$\left(\frac{\text{J}}{\text{mol}\cdot\text{K}}\right)$$ $$\ce{CH_4}$$ $$186.25^1$$ $$\ce{O_2}$$ $$205.15^1$$ $$\ce{H_2O}$$ $$188.84^1$$ $$\ce{CO_2}$$ $$213.79^1$$

The entropy for the combustion of methane is:

$\Delta S^\circ = \left[ 2 \left( 188.84 \right) + 1 \left( 213.70 \right) \right] - \left[ 1 \left( 186.25 \right) + 2 \left( 205.15 \right) \right] = -5.17 \: \frac{\text{J}}{\text{mol} \cdot \text{K}} \nonumber$

## References

1. Chase, M.W., Jr., NIST-JANAF Themochemical Tables, Fourth Edition, J. Phys. Chem. Ref. Data, Monograph 9, 1998, 1-1951.

21.9: Standard Entropies Can Be Used to Calculate Entropy Changes of Chemical Reactions is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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