Nitrogen dioxide, NO2, an air pollutant, dissolves in...

Nitrogen dioxide, $\mathrm{NO}_{2}$, an air pollutant, dissolves in rainwater to form a dilute solution of nitric acid. The equation for the reaction is $$ 3 \mathrm{NO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow 2 \mathrm{HNO}_{3}(l)+\mathrm{NO}(g) $$ Calculate $\Delta S^{\circ}$ for this reaction in J $\mathrm{K}^{-1}$.

Nitrogen dioxide, $\mathrm{NO}_{2}$, an air pollutant, dissolves in rainwater to form a dilute solution of nitric acid. The equation for the reaction is
$$
3 \mathrm{NO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow 2 \mathrm{HNO}_{3}(l)+\mathrm{NO}(g)
$$
Calculate $\Delta S^{\circ}$ for this reaction in J $\mathrm{K}^{-1}$.

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Key Concepts

Standard Entropy

Standard entropy is a thermodynamic property that quantifies the degree of randomness or disorder of a substance under standard conditions (usually 298 K and 1 atm pressure). It is expressed in units of J K?¹ mol?¹ and used as a baseline to calculate changes in entropy for chemical reactions, helping to predict the spontaneity and feasibility of processes.

Entropy Change (?S°) Calculation

The standard entropy change for a reaction is computed by using the standard molar entropy values of the reactants and products. It is determined through the equation ?S° = ?(n × S°_products) ? ?(n × S°_reactants), where n represents the stoichiometric coefficients. This method involves summing the entropy contributions from all species involved in the reaction, weighted by their moles, to assess the overall change in disorder.

Impact of Physical States on Entropy

The physical state of a substance—whether it is a gas, liquid, or solid—has a significant impact on its entropy. Gases typically have much higher entropy due to the greater freedom of movement and larger number of accessible microstates, while liquids have lower entropy, and solids even lower still. Recognizing these differences is important when calculating the total entropy change in a reaction, especially when reactants and products are in different phases.

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