Is it true that solids and liquids are not included in the expression of the equilibrium constant?

Question

Avery Adams · Accepted Answer

Yes, that is correct.

The key point to keep in mind when dealing with a heterogeneous equilibrium—which, as you may know, entails substances in multiple phases—is that the equilibrium's position remains unaffected by variations in a solid's concentration. The same holds true for a pure liquid, though it should be noted that the solutions in question need to be initially fairly diluted. Regarding the equilibrium reaction you have given #"NH"_4"Cl"_text((s]) rightleftharpoons "NH"_text(3(g]) + "HCl"_text((g])# adding more ammonium chloride, #"NH"_4"Cl"#, will not affect the position of the equilibrium. This is why the concentration of the solid is not included in the expressions for the two equilibrium constants #K_c# and #K_p#, which for this equilibrium will take the form #K_c = overbrace(["NH"_3] * ["HCl"])^(color(purple)("concentrations"))" "# and #" "K_p = overbrace((NH_3) * (HCl))^(color(green)("partial pressures"))# Now, without getting too technical, the reason solids do not disturb chemical equilibria can be attributed to their molecular activity. Chemical activity, to put it simply, is a measurement of the difference between a substance's concentration under particular conditions and its concentration under standard conditions. So, if a substance has the same concentration (you'll sometimes see this referred to as molar density) at some given conditions and at standard conditions, then its chemical activity is equal to #1#. Solids will never have a concentration that is too different from zero, barring abrupt variations in temperature and pressure. In your example, the partial pressures and product concentrations at a given temperature are independent of the ammonium chloride content. The position of the equilibrium does not change once the equilibrium is established because ammonium chloride's chemical activity, or concentration, is unaffected by additions or withdrawals.

Peyton Adams · Answer

undefined Yes, that statement is generally true. In the expression of the equilibrium constant, solids and liquids are typically not included. This is because the concentrations of solids and liquids do not change significantly during a reaction, so their presence does not affect the equilibrium constant. The equilibrium constant expression only includes the concentrations of aqueous ions and gases involved in the reaction.

Ellie Andrews · Answer

undefined Yes, it is true. Solids and liquids are not included in the expression of the equilibrium constant, as their concentrations remain constant during a reaction. The equilibrium constant expression only includes the concentrations of gases and aqueous ions, as they can vary during the reaction and affect the equilibrium position.