Physical chemistry

Chemical equilibrium

Chemical equilibrium is a dynamic state in which forward and reverse rates are equal; the equilibrium constant K relates composition to temperature.

For aA+bB ⇌ cC+dD, the reaction quotient Q has the same form as Kc but uses instantaneous concentrations. At equilibrium Q=Kc; gas partial pressures define Kp.

Model and quantities

Read the relation together with assumptions about state, experimental conditions, and sign conventions. Keep units consistent and check dimensions before interpreting a result.

K_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}, \qquad \Delta G^\circ=-RT\ln K

Definition: Equilibrium constant

K is the activity quotient at equilibrium; pure solids and liquids usually have unit activity and are omitted. K changes only when temperature changes.

Quantities in the relation are defined for the reaction or system at hand. In particular, distinguish standard-state quantities from actual conditions and do not infer a mechanism from a general expression alone.

Example: Worked example

At equilibrium, A ⇌ B has [A]=0.20 M and [B]=0.80 M. For a 1:1 reaction, find Kc.

Solution

Kc=[B]/[A]=0.80/0.20=4.0; products are favored in this equilibrium composition.

Concept summary
Concept	Description	Unit / note
Key relation	Use under stated conditions	Check units and sign convention
Measured quantity	Relates a state or process	Compare data with model
Scope	Model specific conditions	Check assumptions first

The sign of $\Delta G^\circ$ indicates whether K is large or small: K>1 gives $\Delta G^\circ<0$ . This is a standard-state quantity, not proof every initial mixture proceeds forward; the actual direction uses $\Delta G=RT\ln(Q/K)$ .

In the worked example, which result follows from the given data?

Which statement is consistent with this lesson?

References

Peter Atkins, Julio de Paula (2014). Physical Chemistry