91Ó°ÊÓ

Chemical thermodynamics helps us understand how and why chemical reactions occur. At its core, it's about energy - tracking energy changes in reactions. Gibbs Free Energy (denoted as \(\Delta G\)) is a key part of this.

Gibbs Free Energy combines enthalpy (\(\Delta H\)) and entropy (\(\Delta S\)) to predict whether a reaction is spontaneous. It's defined by the equation:

• \[ \Delta G = \Delta H - T \Delta S \]

This equation tells us about the feasibility of reactions:

• If \(\Delta G\) is negative, the reaction is spontaneous and can proceed without added energy.
• If \(\Delta G\) is positive, the reaction isn't spontaneous and needs energy to proceed.

Calculating \(\Delta G\) involves standard free energy changes (\(\Delta G^\circ\)), which use values at standard conditions (25°C, 1 atm pressure). This helps us predict reaction behavior under these conditions.

The reaction quotient \(Q\) is a snapshot of a reaction's position at any given moment. Calculating \(Q\) involves the concentrations or partial pressures of reactants and products at a specific time.

For the reaction \(2 \text{NO}(g) + \text{O}_2(g) \longrightarrow 2 \text{NO}_2(g)\), the reaction quotient \(Q\) is calculated by:

• \[ Q = \frac{[\text{NO}_2]^2}{[\text{NO}]^2 [\text{O}_2]} \]

When \(Q\) equals the equilibrium constant \(K\), the reaction is at equilibrium. If \(Q eq K\), the reaction will shift to reach equilibrium:

• If \(Q
• If \(Q>K\), the reaction favors the reverse direction (forming reactants).

In non-standard conditions, using \(Q\) helps calculate \(\Delta G\), which guides our predictions about the reaction's direction and spontaneity.

Reactions often occur under non-standard conditions, with varying temperatures, pressures, or concentrations. These require adjustments to the standard Gibbs Free Energy, \(\Delta G^\circ\).

Non-standard Gibbs Free Energy (\(\Delta G\)) can be calculated using the expression:

• \[ \Delta G = \Delta G^\circ + RT \ln(Q) \]

Here, \(R\) is the universal gas constant, \(T\) is temperature in Kelvin, and \(Q\) is the reaction quotient.

This equation adjusts \(\Delta G^\circ\) to account for actual conditions. For our example under non-standard conditions, with specific partial pressures, \(\Delta G\) changes and tells us about the reaction's spontaneity in real life scenarios.

• A negative \(\Delta G\) indicates spontaneous reactions even at non-standard conditions.
• A positive \(\Delta G\) suggests the need for additional energy.

Understanding these adjustments allows chemists to predict and control chemical reactions effectively, even when conditions stray from the standard.