The law of mass action (or law of equilibrium) states that at a given temperature, there is a constant relationship between the concentrations of products and reactants at equilibrium.
A certain amount of time must elapse before chemical equilibrium is established. Initially, the concentration of the reactants is at its maximum, while the products are practically non-existent. However, as time passes, the concentration of reactants decreases while that of products increases until equilibrium is reached. Once equilibrium is achieved, the forward and reverse reaction rates are equal. However, the concentrations of reactants and products are not necessarily equal. Based on this information, scientists have developed a constant that describes the relationship between the concentrations of substances at equilibrium, the equilibrium constant |K_{c}|.
- Equilibrium Constant Expression
- Equilibrium Constant Interpretation
- Effect of Temperature on the Equilibrium Constant
- Calculations of Equilibrium Concentrations
In the case where a chemical reaction involves ions in solution, the writing of the equilibrium constant is changed. In fact, the ionic balance in solutions is established between the concentrations of the different ions after the dissociation of a chemical compound. This equilibrium is observed in water, in acidic and basic substances, as well as in solid ionic compounds dissolved in solution. Variations of the equilibrium constant will then be used. To be able to study these different equilibrium constants, it is necessary to gain a deeper knowledge of acid and base properties.
The expression of the equilibrium constant is based on the equation of the chemical reaction involved. As each chemical reaction is different, the expression of each equilibrium will also vary. However, the mathematical expression of the equilibrium constant as a function of the concentrations can be generalized as follows:
Consider the general equation |aA + bB \rightleftharpoons cC + dD|
||K_{c}=\displaystyle \frac{\left[C\right]^{c}\cdot\left[D\right]^{d}}{\left[A\right]^{a}\cdot\left[B\right]^{b}}||
where
- |K_{c}| represents the equilibrium constant as a function of the concentrations
- |[C]| and |[D]| represent the molar concentrations of the products (mol/L)
- |[A]| and |[B]| represent the molar concentrations of the reactants (mol/L)
- exponents (lower case letters) refer to the coefficients in the balanced chemical equation
In all chemical reactions, the equilibrium constant is set at a given temperature. A change in temperature modifies the numerical value of the constant, but does not modify the expression.
The numerical value of |K_{c}| tells us about the quantities in moles present at equilibrium. However, in the case where a reaction only involves substances in a gaseous phase, the equilibrium constant can also be calculated based on the partial pressures of different substances:
Consider the general equation |aA + bB \rightleftharpoons cC + dD|
||K_{p}=\displaystyle \frac{\left[P_{pC}\right]^{c}\cdot\left[P_{pD}\right]^{d}}{\left[P_{pA}\right]^{a}\cdot\left[P_{pB}\right]^{b}}||
where
- |K_{p}| represents the equilibrium constant as a function of partial pressures
- |[P_{pC}]| and |[P_{pD}]| represent the partial pressures of the products (kPa)
- |[P_{pA}]| and |[P_{pB}]| represent the partial pressures of the reactants (kPa)
- exponents (lower case letters) refer to the coefficients in the balanced chemical equation
The calculation of the equilibrium constant |(K_{c})| is carried out only with molar concentrations at equilibrium of gases or substances in aqueous solution.
Substances in the pure state, whether they are solids or liquids, are not taken into account, since their concentration corresponds to their density and, consequently, they are constant values.
2) No unit of measurement is assigned to the equilibrium constant, as it would change depending on the type of system.
The equilibrium constant is established for the direction of the reaction considered. Thus, it will not be the same for the forward reaction as for the reverse reaction. In fact, since the products and the reactants are no longer the same substances, the numerators and denominators of the expression of the constant will be reversed. Thus, to determine the value of the equilibrium constant of the reverse reaction, simply calculate the mathematical inverse of the equilibrium constant of the forward reaction:
|K_{Crev}=\displaystyle \frac{1}{K_{Cdir}}|
where
|K_{Crev}| represents the equilibrium constant of the reverse reaction
|K_{Cfor}| represents the equilibrium constant of the forward reaction
In summary, in order to establish and calculate the equilibrium constant of a chemical reaction, it is necessary to:
- use the concentrations (or partial pressures) at equilibrium;
- place the products in the numerator;
- place the reactants in the denominator;
- use the coefficients of the balanced chemical equation as exponents.
The value of the equilibrium constant can be used to predict the direction in which the equilibrium will be achieved. Since this is a ratio of the concentration of the products in the numerator to the concentration of the reactants in the denominator, it can be determined which direction of the reaction will be favoured.
If the equilibrium constant is greater than 1, the numerator is greater than the denominator. Thus, this indicates a higher concentration of products compared to the reactants. It can, therefore, be established that the reaction favouring the products, namely the forward reaction, is preferred. Conversely, if the equilibrium constant is less than 1, the denominator of the ratio is greater than the numerator. In this case, the reactants are dominant relative to the products. It can be stated that the reverse reaction is then favoured. Finally, if the constant is approximately equal to 1, the system does not favour any reaction direction.
A value of |K_c > 1| means that there is a greater concentration of products than reactants. The forward reaction will be favoured.
A value of |K_c < 1| represents a concentration of reactants greater than that of the products. The reverse reaction will be favoured.
A value of |K_c \cong 1| means that there are as many reactants as there are products. No reaction is favoured.
According to Le Chatelier's principle, a change in the concentrations or in the pressures of various substances temporarily disturbs the equilibrium. However, since the equilibrium constant establishes a ratio between the concentrations or pressures at equilibrium, this relationship is always constant. Thus, changes in concentration, pressure, or volume have no impact on the value of the equilibrium constant.
Only temperature can modify the value of the equilibrium constant |K_{c}| of a given reaction. According to Le Chatelier's principle, an increase in temperature promotes an endothermic reaction, whereas a decrease in temperature promotes an exothermic reaction. The new equilibrium that is established does so in different proportions from those of the initial equilibrium. Thus, this new concentration ratio modifies the value of the equilibrium constant of the system. This is why the temperature at which a system is found must always be specified when giving its equilibrium constant.
| Type of reaction | Temperature change | Favoured reaction | Changing the value of the equilibrium constant |
| Exothermic(ΔH < 0) Reactants → Products + energy |
Increase | Reverse (←) | Decrease |
| Decrease | Forward (→) | Increase | |
| Endothermic(ΔH > 0) Reactants + energy → Products |
Increase | Forward (→) | Increase |
| Decrease | Reverse (←) | Decrease |
Pour valider ta compréhension à propos de la constante d'équilibre de façon interactive, consulte la MiniRécup suivante :
