In electrochemistry, Q represents the reaction quotient. According to the provided reference, it is the ratio of the thermodynamic activities of the products to the reactants at any given point in a reaction.
Here's a breakdown:
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Definition: Q, the reaction quotient, indicates the relative amount of products and reactants present in a reaction at a particular time. It helps determine which direction a reversible reaction will shift to reach equilibrium.
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Formula: Generally, for a reaction aA + bB ⇌ cC + dD, the reaction quotient (Q) is expressed as:
Q = (aCc aDd) / (aAa aBb)
Where 'a' represents the thermodynamic activity of each species (A, B, C, and D), and a, b, c, and d are the stoichiometric coefficients. In dilute solutions, activities are often approximated by concentrations.
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Comparison with K (Equilibrium Constant): The reference highlights that K (the equilibrium constant) is a specific value of the reaction quotient (Q) when the reaction has reached equilibrium.
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Significance: By comparing Q and K, we can predict the direction a reaction will shift to reach equilibrium:
- If Q < K: The ratio of products to reactants is less than that at equilibrium. The reaction will proceed in the forward direction (more reactants will be converted to products) to reach equilibrium.
- If Q > K: The ratio of products to reactants is greater than that at equilibrium. The reaction will proceed in the reverse direction (more products will be converted to reactants) to reach equilibrium.
- If Q = K: The reaction is at equilibrium, and there will be no net change in the concentrations of reactants and products.
