Entropy ( $S$ )

A measure of the distribution of available energy among the particles.

Simply put: Entropy is a measure of disorder. The Universe naturally tends towards chaos (increasing entropy).

Ordered. Low Entropy.

More movement. Medium Entropy.

Chaotic. High Entropy!

Deep Think Concept

Look at the change in moles of GAS ( $\Delta n_{gas}$ ).

Increase in gas moles $\rightarrow$ Entropy increases ( $\Delta S > 0$ , Positive).
$CaCO_3(s) \rightarrow CaO(s) + CO_2(g)$ (0 gas $\rightarrow$ 1 gas)
Decrease in gas moles $\rightarrow$ Entropy decreases ( $\Delta S < 0$ , Negative).
$N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)$ (4 gas $\rightarrow$ 2 gas)

\Delta S^\theta = \sum S^\theta(Products) - \sum S^\theta(Reactants)

Units: $J K^{-1} mol^{-1}$ (Watch out! This is usually in J while $\Delta H$ is in kJ).

Putting it into Practice

Paper 2 Style

Predict the sign of the entropy change ( $\Delta S$ ) for the following reaction:

$NH_3(g) + HCl(g) \rightarrow NH_4Cl(s)$

[2 Marks]

Calculate the standard entropy change ( $\Delta S^\theta$ ) for the reaction: $N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)$

$S^\theta(N_2) = 191 \: J K^{-1} mol^{-1}$

$S^\theta(H_2) = 131 \: J K^{-1} mol^{-1}$

$S^\theta(NH_3) = 192 \: J K^{-1} mol^{-1}$