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## AP®︎/College Chemistry

### Course: AP®︎/College Chemistry>Unit 9

Lesson 1: Entropy

# Absolute entropy and entropy change

The standard molar entropy of a substance is the absolute entropy of 1 mole of the substance in the standard state. For any chemical reaction, the standard entropy change is the sum of the standard molar entropies of the products minus the sum of the standard molar entropies of the reactants. Created by Jay.

## Want to join the conversation?

• I wondered if some of these factors influencing entropy (moles, volume, state) are more dominant than others, e.g:
If I had a reaction, which results in a change of state from solid to gasseous, but a decrease in moles:
X(s) + Y(s) --> XY(g) |↓n ↑states
Would we predict an increase or decrease in entropy? • How do you measure W in S = K*ln(W) when calculating the standard molar entropy of a substance?
(1 vote) • Since W is the number of energetically equivalent ways to arrange the particles of a system we can view this as a combinatorics problem. We can use the combination formula: n!/(r!*(n-r)!), where n is the number of particles and r is the number of ways to place those particles in the container.

It should be noted that even for relatively small molecules, W becomes very large for molar quantities. For example the standard molar entropy of H2 is 130.7 J/(mol*K). Using Boltzmann’s equation where k Boltzmann’s constant set at, 1.380649 x 10^(-23) J/K, we can calculate the number of microstates (technically microstates per mole). In which case W is calculated as e^(9.466 x 10^(24)), an extremely large number which we would expect dealing with a mole amount of particles.

Hope that helps.
(1 vote)