This online Equilibrium Constant Calculator finds the equilibrium constant of concentration for a given chemical reaction. For each reactant and product of the reaction you need to specify the respective stoichiometric coefficient and concentration (molarity). To add or delete a reactant or product you may click the “+” symbol or the “–” symbol respectively.
Equilibrium Constant Expression
Most chemical reactions occurring in a closed system are reversible. A reversible reaction is a reaction that can proceed in either the forward or backward direction.
The double half-arrow sign, used when writing equations for reversible reactions, reminds us that these reactions can go either forward, creating products, or backward, creating reactants.
All reactions tend towards a state of chemical equilibrium, i.e. a state in which both the forward and reverse processes proceed at the same rate. When the forward and reverse rates become equal, the concentrations of reactants and products in the resulting state of equilibrium stop changing. It is worth noting that even though the concentrations in equilibrium are constant, the reaction is still going on. This is why such a state is also called dynamic equilibrium.
Based on the concentrations of all substances in equilibrium, we can define a quantity called the equilibrium constant \(K_{c}\). This equilibrium constant describes the molar concentrations in mol/L at equilibrium for a specific temperature. The equilibrium constant indicates whether the reaction leads to higher concentrations of products or reactants in equilibrium. We can also use \(K_{c}\) to determine if the reaction is already at equilibrium.
Consider a balanced reversible chemical reaction in its most general form:
$$aA+bB+…\rightleftharpoons cC+dD+…$$
where
\(A, B, …\) – are the chemical reaction reactants,
\(a, b, …\) – are the stoichiometric coefficients of the reactants,
\(C, D, …\) – are the chemical reaction products,
\(c, d, …\) – are the stoichiometric coefficients of the products.
If we know the molar concentrations for each of the substances involved in the reaction, then the value of \(K_{c}\) is defined by the following formula:
$$K_{c}=\frac{[C]^{c}\times[D]^{d}\times …}{[A]^{a}\times[B]^{b}\times …},$$
where the value in square brackets […] denotes the molar concentration of the corresponding substance.
As we can see, the equilibrium constant is defined as the ratio between the amount of products and the amount of reactants, which is used to determine chemical behavior. When calculating the equilibrium constant, it is important to remember that \(K_{c}\) is constant for a specific reaction and at a specific temperature.
The unit of the equilibrium constant is [Mole/L]N, where N = sum of stoichiometric coefficients of products – sum of stoichiometric coefficients of reactants. But it is often written without units.
The value of \(K_{c}\) can give us some information about the concentrations of reactants and products at equilibrium:
• The case when \(K_{c}\) > 1 indicates that equilibrium favors the products.
• The case when \(K_{c}\) < 1 indicates that equilibrium favors the reactants.
• The case when \(K_{c}\) = 1 indicates that the reaction mixture contains similar amounts of both products and reactants at equilibrium.
For reactions that are not in equilibrium, we can write a similar expression called the reaction quotient \(Q\) which is, obviously, equal to \(K_{c}\) at equilibrium. \(K_{c}\) and \(Q\) can be used to determine if a reaction is at equilibrium and to estimate whether a reaction favors products or reactants at equilibrium.
If any of the reactants or products are gases, we can similarly express the equilibrium constant not in terms of concentrations, but in terms of the partial pressure of the gases. The equilibrium constant thus obtained is usually referred to as \(K_{p}\) to distinguish it from the equilibrium constant \(K_{c}\), which uses molar concentrations. Note, that this Equilibrium Constant Calculator calculates only the constant \(K_{c}\).
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