Physical and chemical equilibrium

Physical equilibria — no new substance forms. Five patterns recur throughout the syllabus and everyday life:

• Solid $\rightleftharpoons$ Liquid: ice $\rightleftharpoons$ water at $0^\circ\text{C}$, 1 atm, rate of melting equal to rate of freezing.
• Liquid $\rightleftharpoons$ Vapour: water $\rightleftharpoons$ water vapour in a closed vessel, giving a constant vapour pressure.
• Solid $\rightleftharpoons$ Vapour: sublimation, e.g. solid iodine $\rightleftharpoons$ iodine vapour.
• Dissolution of a solid in a liquid: a saturated sugar solution with undissolved sugar at the bottom, dissolution rate equal to crystallisation rate.
• Dissolution of a gas in a liquid: a sealed carbonated drink, gas dissolving at the same rate it escapes.

Chemical equilibrium — a reversible reaction settles. Take $\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)$ in a closed vessel. As $\text{NH}_3$ accumulates, its decomposition rate rises while the formation rate (fixed by the starting concentrations, then falling) drops; the two curves cross and, past that point, both proceed at the same rate. Nothing about the amounts needs to be equal at that instant — only the rates must match.

General characteristics apply to any equilibrium, physical or chemical, at constant temperature:

where: rate$_{\text{forward}}$ is the rate of the process going one way (melting, evaporation, dissolution, or the forward reaction) and rate$_{\text{reverse}}$ is the rate of the exactly opposite process, both measured under the same fixed temperature. Because neither rate is zero, equilibrium is dynamic, not static. It follows that: (1) it is reached only in a closed system, where nothing escapes or enters; (2) macroscopic properties — mass of each phase, concentration, colour, pressure — stop changing with time; (3) the same equilibrium state is reached whether you start from pure reactants, pure products, or somewhere in between, under the same conditions; and (4) at a fixed temperature there is one definite value of the relevant constant (vapour pressure, solubility, or an equilibrium constant) — it changes only if temperature changes.

This section has no numerical equilibrium constant to derive yet — that quantitative step is the next article's job. What needs justifying here is a purely conceptual claim: why must equilibrium be dynamic, and why does it require a closed system? Both follow directly from tracking rates rather than amounts. Consider evaporation in a sealed flask from the very start. Initially there is no vapour above the liquid, so only evaporation happens: liquid molecules with enough kinetic energy escape into the empty space above, and the vapour pressure climbs from zero. As vapour accumulates, some of those molecules now strike the liquid surface and rejoin it — condensation begins. Evaporation's rate stays roughly constant, because it depends only on temperature and the exposed liquid surface, neither of which is changing. Condensation's rate, however, keeps rising as more vapour molecules become available to strike the surface. Eventually the two rates must cross and become equal — at that instant, and from then on, the vapour pressure stops changing. This is why equilibrium is inevitable in a closed system: one rate is roughly fixed while the other climbs to meet it, and once they meet, nothing forces them apart again at constant temperature. Now see why an open system can never reach this state. If the flask were open, escaping vapour never accumulates — condensation's rate stays near zero forever, evaporation keeps winning, and the liquid eventually vanishes. There is no plateau to reach because one side of the balance is permanently starved. This is exactly why the definition insists on a closed system: equilibrium is not about a process running out of steam, it is about two rates converging because both reactants (or both phases) are trapped together and forced to keep meeting each other. The same logic, unchanged, carries over to a reversible chemical reaction: as product concentration builds, the reverse reaction's rate rises from zero while the forward rate falls from its initial maximum, and the crossing point is chemical equilibrium.

Classify three sealed-container scenarios as physical or chemical equilibrium, and name the two opposing processes in each. (a) A stoppered jar of solid camphor, where after some time the space above the solid becomes saturated with camphor vapour. (b) $2\text{NO}_2(g) \rightleftharpoons \text{N}_2\text{O}_4(g)$ in a closed glass bulb, where the brown colour (from $\text{NO}_2$) stops changing in intensity. (c) A saturated $\text{NaCl}$ solution in a stoppered bottle with excess undissolved solid at the bottom.

Step 1. Check for a new substance. In (a), camphor vapour is chemically identical to solid camphor — only the state changes, so this is a physical equilibrium (sublimation). The opposing processes are solid camphor subliming to vapour, and vapour depositing back onto the solid.

Step 2. In (b), a chemical bond is broken and formed: two $\text{NO}_2$ molecules combine into one $\text{N}_2\text{O}_4$ molecule, and $\text{N}_2\text{O}_4$ can split back into two $\text{NO}_2$ molecules. Since a new substance ($\text{N}_2\text{O}_4$) forms from a different one ($\text{NO}_2$), this is a chemical equilibrium. The opposing processes are the forward combination $2\text{NO}_2 \rightarrow \text{N}_2\text{O}_4$ and the reverse dissociation $\text{N}_2\text{O}_4 \rightarrow 2\text{NO}_2$, and the colour stabilising (rather than fading to zero or deepening forever) is the experimental signal that the two rates have become equal.

Step 3. In (c), dissolved $\text{Na}^+$ and $\text{Cl}^-$ ions are the same chemical species as the solid lattice, just separated — no new substance forms, so this is a physical equilibrium (dissolution). The opposing processes are dissolution of solid $\text{NaCl}$ into ions, and crystallisation of ions back onto the solid.

Step 4. State the final classification. (a) Physical equilibrium (sublimation); (b) chemical equilibrium (dimerisation of $\text{NO}_2$); (c) physical equilibrium (dissolution) — each one dynamic, each requiring the container to stay closed for the balance to be maintained.

• Thinking equilibrium means "nothing is happening." Both forward and reverse processes continue at equal, non-zero rates; only the net, measurable change is zero.
• Believing equilibrium can be reached in an open system. An open system lets one side of the balance escape indefinitely, so the rates never converge — evaporation into open air never reaches true equilibrium.
• Confusing "equal amounts of reactant and product" with equilibrium. Equilibrium only requires equal rates; the actual amounts of the two sides at equilibrium are almost never equal to each other.
• Treating melting or boiling points as fixed numbers rather than equilibrium conditions. A melting or boiling point at a given pressure is precisely the temperature at which two phases coexist in equilibrium; change the pressure and that temperature changes too.

Ask the tutor: Ask the AI tutor to give you more sealed-vs-open scenarios to classify as physical or chemical equilibrium before you move on to the quantitative equilibrium constant.