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Introduction
Equilibrium Constant
Although both products and reactants sync in reaction rates during equilibrium, the ratio between the concentrations of products and reactants may vary, depending on the reaction.
The equilibrium constant, K, indicates the ratio between products and reactants in a system when a reversible reaction reaches an equilibrium state.
There are many different variations of K, such as:
Kc - In terms of concentration.
Kp - In terms of pressure.
Although equilibrium is solved from experimental data only, K is unique yet consistent to each reaction as long as conditions don't change.
Reversible Reaction Example:
aA + bB ⇌ cC + dD
Capital letters represent the substance.
Lowercase letters represent the coefficients.
After you have obtained data about the concentrations/pressures of the products and reactants at equilibrium, you can use the formulas below:
The equilibrium constant formula in regards to concentration would be:
Kc = [C]ᶜ[D]ᵈ/[A]ᵃ[B]ᵇ
K: Equilibrium Constant
[X]: Concentration of substance X at equilibrium (M)
The equilibrium constant formula in regards to pressure would be:
Kp = (PC)ᶜ(PD)ᵈ/(PA)ᵃ(PB)ᵇ
K: Equilibrium Constant
Px: Partial pressure of substance X at equilibrium (atm)
Also, solids and liquids DO NOT contribute to the equilibrium constant.
Guideline for Interpreting K:
When the equilibrium shifts towards a side, that side would have a higher concentration/partial pressure.
If K > 1, then it means that the equilibrium shifts towards products.
K ≥ 10³ is usually classified as having a significant amount of products produced.
If K < 1, then it means that the equilibrium shifts towards reactants.
K ≤ 10⁻³ is usually classified as having a significant amount of reactants produced.
Manipulating K:
If you flip the products and reactants of a chemical equation, its new K value is the reciprocal of the previous one. (1/K)
If you multiply all coefficients of an equation by a factor of n, the new K value would be the old K value to the power of n. (Kⁿ)
Reaction Quotient
The reaction quotient (Q) uses the same formulas as K but applies to the system at any point, not just at equilibrium.
It helps predict the direction in which a reaction will shift to reach equilibrium, as shown:
If Q > K, the reaction is not at equilibrium and will shift left in order to reach it.
If Q = K, the reaction is at equilibrium. Therefore, no shift will occur.
If Q < K, the reaction is not at equilibrium and will shift right in order to reach it.
Tip: Visualize Q and K on a number line. The reaction will move in the direction that brings Q closer to K to restore equilibrium.
If the molarity or pressure of a substance is 0 at a given point of the reaction, the reaction will shift towards the side with that substance.
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