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    Standard equilibrium constant

    • Last Update: 2021-06-18
    • Source: Internet
    • Author: User
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    2.
    Standard equilibrium constant

    Before defining the standard equilibrium constant, you need to define the relative concentration and relative partial pressure
    .

    The substance concentration divided by its standard concentration c Θ (1mol·dm -3 ) is the relative concentration
    .
    It can be seen that the relative concentration is the multiple of the concentration of the substance relative to its standard concentration


    .


    It can be seen that the relative partial pressure is the multiple of the partial pressure relative to the standard pressure
    .
    Obviously, there is no relative concentration of gas-phase substances, because the standard state of gas-phase substances is 100kPa, which has nothing to do with the concentration


    .


    The relative concentration and relative partial pressure are obviously quantities of dimension 1
    .
    When the chemical reaction reaches equilibrium, the relative concentration or relative partial pressure of each substance no longer changes


    .


    For reversible reactions in solution

    The relative concentration of each substance at equilibrium can be expressed as

    Then the definition formula of the standard equilibrium constant K Θ is

    Obviously, for the reaction in solution, the standard equilibrium constant K Θ and the empirical equilibrium constant Kc have the following relationship

    And for the gas phase reaction

    The relative partial pressure of each substance at equilibrium can be expressed as

    The standard equilibrium constant of the gas phase reaction can only be expressed by the relative partial pressure, then the definition of K° is

    For gas phase reactions, the relationship between the standard equilibrium constant K Θ and the empirical equilibrium constants Kp and Kc is

    [Example 4-1] At 100℃, the reaction

    The equilibrium constant Kc=0.
    21mol·dm -3 , the standard equilibrium constant K Θ of the calculated reaction will be the empirical equilibrium constant Kp


    .


    Solve the difference of the stoichiometric number △ v =1
    .
    According to the relationship between K Θ and Kc

    Get

    According to the relationship between K c and K p

    Kp=K c (RT) △v

    Get

    For the water present in a large amount in pure solid phase, pure liquid phase and dilute solution, it can be considered that their mole fraction x i =1, which can be defined as a relative physical quantity similar to relative concentration and relative partial pressure, that is, to divide it by the standard state x Θ =1, the relative amount is 1 as a result
    .
    Therefore, their concentration is not written into the equilibrium constant expression


    .


    The Kc of the reaction in the solution and its K Θ are numerically equal, because the standard concentration c Θ =1mol·dm -3 , but the dimensions of the two are generally different
    .
    For gas phase reactions, K Θ must be expressed in terms of relative partial pressure, and K Θ and Kp are generally not equal in terms of numerical values ​​or physical units


    .


     

     

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