Liquids and solids maintain constant active concentrations throughout chemical equilibrium calculations
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Chemical education references report that pure solids and liquids are excluded from equilibrium constant expressions because their effective concentrations remain constant during chemical equilibrium calculations.
For example, the formation of an aqueous solution of lead(II) iodide creates a heterogeneous mixture dealing with particles in both the solid and aqueous states:
\[PbI_{2 (s)} \rightleftharpoons Pb^{+2}_{(aq)} + 2I^-_{(aq)}\]
The decomposition of sodium hydrogen carbonate (baking soda) at high elevations is another example of a heterogeneous mixture, this reaction deals with molecules in both the solid and gaseous states:
\[ 2NaHCO_{3 (s)} \rightleftharpoons Na_2CO_{3 (s)} + H_2O_{ (g)} + CO_{2 (g)} \]
\[ C_{(s)} + O_{2 (g)} \rightleftharpoons CO_{2 (g)} \]
This difference between homogeneous and heterogeneous reactions is emphasized so that students remember that solids, pure liquids, and solvents are treated differently than gases and solutes when approximating the activities of the substances in equilibrium constant expressions. Writing Equilibrium Constant Expressions
The numerical value of an equilibrium constant is obtained by letting a single reaction proceed to equilibrium and then measuring the concentrations of each substance involved in that reaction. The ratio of the product concentrations to reactant concentrations is calculated.
It simply signifies the ratio between the forward and reversal rates of a chemical reaction at equilibrium. Introduction
There are a few different Equilibrium Constants such as:
- \(K_c\) (dealing with concentration)
- \(K_p\) (dealing with partial pressure)
For the sake of brevity, let us stick to the former, \(K_c\) to follow this Module. Generally, we follow this equation when dealing with the calculation of the equilibrium constant, \(K_c\):
\(K_c = \dfrac{Products}{Reactants}\)
- Where the concentrations of the Products are multiplied in the numerator (top) and the concentrations of the Reactants are multiplied in the denominator (bottom). - We separate and denote each chemicals' concentration inside of closed brackets. Important: We only include the concentrations of aqueous and gaseous substances. Do not include those of solids or liquids! The Calculation of K
- Determine whether the corresponding chemical (i.e. HCl, NaCl, etc…) is on the products or reactants side of the chemical equation. Be sure to only include those in the (aq) or (g) states. - Observe your ICE table and look at the values in the E row of the table.
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