Reaction quotient

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In a chemical reaction with certain initial concentrations of reactants and products, it is useful to know if the reaction will shift to the right (increasing the concentrations of the products) or if it will shift to the left (increasing the concentrations of the reactants). For this purpose we define the reaction quotient for reactions of the type:

[kA + mB \leftrightarrow nC + pD]

where A, B, C, and D are four chemical species involved in this reaction and k, m, n, and p are the coefficients. And the expression for the reaction quotient is:

[Q = \frac ]

where the square brackets denote the concentrations of the various chemical species involved in the reaction in molarity, and the ' i ' denotes the instantaneous concentration at a certain period of time. This is crucial in understanding the idea of the Reaction Quotient. The Reaction Quotient is taken at a particular instant, this is at a time before equilbrium is reached.

This reaction quotient is directly related to the Law of Mass Action and the Le Chatelier Principle. For a reaction in chemical equilibrium, we define the equilibrium constant as:

[K_c = \frac]

By comparing the values of Q and K, one can determine whether the reaction will shift to the right, to the left, or if the concentrations will remain the same (equilibrium).

• If Q < K : The reaction will shift to the right
• If Q > K : The reaction will shift to the left
• If Q = K : The reaction is at equilibrium

The value of K is related to temperature according to the following expression:

[K_T = K_\infty e^}]

where ΔE is the difference in energy per mole between reactants and products, e is the base of the natural logarithm, and R is the gas constant.

The relationship with temperature makes the most sense in terms of the free energy difference, ΔF* = ΔE - TΔS*, which represents the total work that can be done by the system as it develops. At equilibrium ΔF = 0, which gives us

[\Delta F^* = RT \ln \right)}]

See also

• Chemical equilibrium
• Le Chatelier's Principle
• Equilibrium Constant
• Activity
• Activity coefficient

References

 

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