• dynamic equilibrium
  • ”The condition in which a forward process and its reverse are taking place simultaneously at equal rates.” (Jones, 2016)
  • “A state of balance in which opposing processes occur at the same rate” (Brown, 2012)
  • No macroscopic change occurs, but microscopic changes do. This is in contrast to static equilibrium, in which no microscopic change occurs either. (stackexchange)
  • examples:
    • vapor pressure in a closed container
    • $\mathrm{H}{}_2\mathrm{O}(\mathrm{s})\rightleftharpoons \mathrm{H}{}_2\mathrm{O}(\mathrm{l})$ (at $0^{\circ }\mathrm{C}$ and $1\mathrm{atm}$)
• related non-common terms:
  • physical equilibrium
    • ”A state in which two or more phases of a substance coexist without a tendency to change.” (Jones, 2016)
  • phase equilibrium

Chemical equilibrium

• chemical equilibrium (שיווי משקל כימי)
  • “A dynamic equilibrium between reactants and products in a chemical reaction” (Jones, 2016)
• equilibrium reaction
  • ”When the reactants in a chemical reaction are initially in a Boltzmann distribution the reaction is referred to as an equilibrium reaction.” (IUPAC)
• homogeneous equilibrium (all reactants and products are in the same physical phase)
• heterogeneous equilibrium
• equilibrium constant ($K_{\text{eq}}$) (קבוע שיווי-משקל) (of a substance in a given temperature)
  • dimensionless
  • equilibrium-constant expression
    • in terms of concentration:
      • $K_c=\frac{[\mathrm{C}{]}^c[\mathrm{D}{]}^d}{[\mathrm{A}{]}^a[\mathrm{B}{]}^b}$
    • in terms of partial pressure:
      • $K_p=\frac{(P_{\mathrm{C}}{)}^c(P_{\mathrm{D}}{)}^d}{(P_{\mathrm{A}}{)}^a(P_{\mathrm{B}}{)}^b}$
    • $K_p=K_c(RT{)}^{\Delta n}$
  • (see also dissociation constant)
  • reaction quotient ($Q$)
• equilibrium position
• todo
  • In equilibrium expressions, pure solids and liquids are omitted because their activities are unity in their standard states. In rate law, their concentrations are not variable and are incorporated into $k$; however, the rate can depend on factors such as surface area in heterogeneous systems.
• law of mass action (חוק פעולת המסות)
• reversible reaction
  • $a\mathrm{A}+b\mathrm{B}\rightleftharpoons c\mathrm{C}+d\mathrm{D}$
  • forward reaction ($\to$)
  • reverse reaction ($\leftarrow$)

examples

• Haber process

Dissociation

• dissociation (דיסוציאציה, התפרדות, פירוד)
  • “The separation of an acid or a base into ions in water.” (Timberlake, 2017)
  • “The separation of ions that occurs when an ionic solid dissolves” (Jones, 2016)
  • “a general process in which molecules (or ionic compounds such as salts, or complexes) separate or split into other things such as atoms, ions, or radicals, usually in a reversible manner” (Wikipedia)
  • dissociation constant $K_d$
    • for $\mathrm{AB}\rightleftharpoons \mathrm{A}+\mathrm{B}$: $K_d=\frac{[\mathrm{A}][\mathrm{B}]}{[\mathrm{AB}]}$
    • a type of equilibrium constant
  • todo
    • ”Weak electrolytes ionize to a limited extent, and strong electrolytes dissociate or ionize almost completely into ions” (Petrucci, 2016)
    • dissociation constant vs equilibrium constant

Le Chatelier’s principle

• Le Chatelier’s principle
  • ”If a system at equilibrium is disturbed by a change in temperature, pressure, or a component concentration, the system will shift its equilibrium position so as to counteract the effect of the disturbance” (Brown, 2012)
  • interactive simulation: https://marblescience.com/blog/le-chateliers-principle
• $a\mathrm{A}+b\mathrm{B}\rightleftharpoons c\mathrm{C}+d\mathrm{D}$
• $n_g^P$ (and $n_g^R$): total moles of gaseous products (and reactants)
• $\Delta n_g=n_g^P-n_g^R$

stress	shift
$\uparrow [\mathrm{A}]$ (or $\downarrow [\mathrm{C}]$)	$\to$
$\downarrow [\mathrm{A}]$ (or $\uparrow [\mathrm{C}]$)	$\leftarrow$
$P\uparrow$ (or $V\downarrow$)	$\min (n_g^R,n_g^P)$
$P\downarrow$ (or $V\uparrow$)	$\max (n_g^R,n_g^P)$
$T\uparrow$ (exo)	$\leftarrow$ (and $K\downarrow$)
$T\uparrow$ (endo)	$\to$ (and $K\uparrow$)
catalyst	none

• pressure/volume changes:
  • shift only if $\Delta n_g\ne 0$
  • inert gas addition:
    • at constant volume: no shift (partial pressures unchanged)
    • at constant pressure: volume increases; shifts toward the side with more gas moles
• concentration changes:
  • no shift for changes in the amount of a pure solid or pure liquid (as long as the phase remains present)
• catalyst: no shift; only speeds up attainment of equilibrium

References

• Jones, Loretta (2016). Chemical Principles. W. H. Freeman.
• Brown, Theodore L. (2012). Chemistry. Prentice Hall.
• Timberlake, Karen C. (2017). Chemistry: An Introduction to General, Organic, and Biological Chemistry. Pearson.
• Petrucci, Ralph H. (2016). General Chemistry. Pearson Education Australia.