It depends on the ionic strength and temperature and is independent of the concentrations of reactants and products in a solution. The equilibrium constant K c is calculated using molarity and coefficients:.
The equilibrium constant is a dimensionless quantity has no units. Although the calculation is usually written for two reactants and two products, it works for any numbers of participants in the reaction.
The calculation and interpretation of the equilibrium constant depends on whether the chemical reaction involves homogeneous equilibrium or heterogeneous equilibrium. For any given temperature, there is only one value for the equilibrium constant. K c only changes if the temperature at which the reaction occurs changes. You can make some predictions about the chemical reaction based on whether the equilibrium constant is large or small.
If the value for K c is very large, then the equilibrium favors the reaction to the right, and there are more products than reactants. The reaction may be said to be "complete" or "quantitative. If the value for the equilibrium constant is small, then the equilibrium favors the reaction to the left, and there are more reactants than products.
If the value of K c approaches zero, the reaction may be considered not to occur. If the values for the equilibrium constant for the forward and reverse reaction are nearly the same, then the reaction is about as likely to proceed in one direction, and the other and the amounts of reactants and products will be nearly equal.
This type of reaction is considered to be reversible. For the equilibrium between copper and silver ions:. The equilibrium constant expression is written as:.
Note the solid copper and silver were omitted from the expression. Also, note the coefficient for the silver ion becomes an exponent in the equilibrium constant calculation. The value of the equilibrium constant, K, for a given reaction is dependent on temperature.
Please do not block ads on this website. Imagine a reaction in which the gaseous reactants A g and B g react in a sealed vessel to produce the gaseous products C g at a constant temperature. The result is an equilibrium reaction which can be represented by the following chemical equation:.
The mass-action expression for this reaction is:. At constant temperature, the equilibrium concentrations of A g , B g and C g do not appear to change because the rate of the forward reaction is the same as the rate of the reverse reaction. Play the game now! Remember that chemical reactions themselves either absorb energy endothermic reactions or release energy exothermic reactions.
When the equilibrium mixture is heated, by Le Chatelier's Principle , the equilibrium position will shift to the right to consume some of the additional heat. This means that when the reaction mixture reaches equilibrium again, the concentration of each reactant will have decreased by an amount x , while the concentration of the product will have increased by an amount x.
We can summarise these changes in concentration of each species in a R. Let's compare the relative values of the equilibrium constant before and after heating the reaction mixture:. If a reaction is endothermic, the value of the equilibrium constant increases when the reaction mixture is heated. Endothermic reaction: increasing temperature, increases the value of K.
Compare what happens to the value of the equilibrium constant when an exothermic reaction is heated. When the equilibrium mixture is heated, by Le Chatelier's Principle , the equilibrium position will shift to the left to consume some of the additional heat.
This means that when the reaction mixture reaches equilibrium again, the concentration of each reactant will have increased by an amount x , while the concentration of the product will have decreased by an amount x. We can represent these changes in a R. The most important consideration for a heterogeneous mixture is that solids and pure liquids and solvents have an activity that has a fixed value of 1.
From a mathematical perspective, with the activities of solids and liquids and solvents equal one, these substances do not affect the overall K or Q value. This convention is extremely important to remember, especially in dealing with heterogeneous solutions. In this case, since solids and liquids have a fixed value of 1, the numerical value of the expression is independent of the amounts of A and B.
If the product of the reaction is a solvent, the numerator equals one, which is illustrated in the following reaction:. The equilibrium constant expression must be manipulated if a reaction is reversed or split into elementary steps. When the reaction is reversed, the equilibrium constant expression is inverted. The new expression would be written as:. When there are multiple steps in the reaction, each with its own K in a scenario similar to Hess's law problems , then the successive K values for each step are multiplied together to calculate the overall K.
Because the concentration of reactants and products are not dimensionless i. For gases that do not follow the ideal gas laws, using activities will accurately determine the equilibrium constant that changes when concentration or pressure varies. Homogeneous Reactions A homogeneous reaction is one where the states of matter of the products and reactions are all the same the word "homo" means "same".
Heterogeneous Reactions A heterogeneous reaction is one in which one or more states within the reaction differ the Greek word "heteros" means "different". 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.
Equilibrium Constant of Activities The thermodynamically correct equilibrium constant expression relates the activities of all of the species present in the reaction. Equilibrium Constant of Concentration To avoid the use of activities, and to simplify experimental measurements, the equilibrium constant of concentration approximates the activities of solutes and gases in dilute solutions with their respective molarities.
Equilibrium Constant of Pressure Gaseous reaction equilibria are often expressed in terms of partial pressures. Heterogeneous Mixture The most important consideration for a heterogeneous mixture is that solids and pure liquids and solvents have an activity that has a fixed value of 1.
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