Password must contain at least one uppercase letter, a number and a specific symbol. Already have an account? Log In. What scientific concept do you need to know in order to solve this problem? Our tutors have indicated that to solve this problem you will need to apply the Gibbs Free Energy concept.
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Biochemistry Video Lessons. GOB Video Lessons. Microbiology Video Lessons. 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. 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.
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