The ratio of the product concentrations to reactant concentrations is calculated. Because the concentrations are measured at equilibrium, the equilibrium constant remains the same for a given reaction independent of initial concentrations. This knowledge allowed scientists to derive a model expression that can serve as a "template" for any reaction. This basic "template" form of an equilibrium constant expression is examined here.
The thermodynamically correct equilibrium constant expression relates the activities of all of the species present in the reaction. Although the concept of activity is too advanced for a typical General Chemistry course, it is essential that the explanation of the derivation of the equilibrium constant expression starts with activities so that no misconceptions occur.
For the hypothetical reaction:. 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. However, the activities of solids, pure liquids, and solvents are not approximated with their molarities. Instead these activities are defined to have a value equal to 1 one. Here, the letters inside the brackets represent the concentration in molarity of each substance.
Notice the mathematical product of the chemical products raised to the powers of their respective coefficients is the numerator of the ratio and the mathematical product of the reactants raised to the powers of their respective coefficients is the denominator. This is the case for every equilibrium constant. A ratio of molarities of products over reactants is usually used when most of the species involved are dissolved in water.
A ratio of concentrations can also be used for reactions involving gases if the volume of the container is known. Gaseous reaction equilibria are often expressed in terms of partial pressures. The equilibrium constant of pressure gives the ratio of pressure of products over reactants for a reaction that is at equilibrium again, the pressures of all species are raised to the powers of their respective coefficients.
It is used to determine which way the reaction will proceed at any given point in time. Reactants are on the left and products are on the right.
We have made an implicit assumption… that reactions react to completely transform reactants to products. A state in which the concentrations of reactant and product are no longer changing, for a reversible reaction. A state in which the rate of the forward reaction is equal to the rate of the reverse reaction; reactant is being consumed to form product at the same rate at which product is being degraded to form reactant. Individual molecules are constantly being converted from one form to another, but no overall change in the concentrations of any species is observed.
What do we mean by dynamic? They are just balancing each other out. This reaction is exothermic so heat can be considered a product. This means that the forward reaction and the reverse reaction are still taking place, but at the same rate so the concentrations of each don't change. Add a comment. Active Oldest Votes. Improve this answer. ManishEarth ManishEarth Jan Jan Anas Qaisrani Anas Qaisrani 1.
That's why they are called forward and reverse. They also always occur in the same place - the reaction vessel. I think what you are trying to say is that the rate of the forward and reverse reaction is the same but this has already been comprehensively covered in other answers so this doesn't add anything new. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password.
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