General Chemistry
In reversible chemical reactions, a point called dynamic equilibrium is reached where the rate of products forming from reactants is equal to the rate of reactants forming from products. Unlike irreversible reactions, reversible reactions continue to take place, and the concentrations of products and reactants do not experience any net change at dynamic equilibrium. At this point, entropy is at its maximum, and change in Gibbs free energy (delta G) is zero.
Two important concepts in reversible reactions are the equilibrium constant (Keq) and the reaction quotient (Qc). Keq describes the ratio of the concentration of products to the concentration of reactants at equilibrium, and the law of mass action states that this ratio is always constant if the temperature is constant. Qc uses the same equation as Keq but considers the concentrations of products and reactants at a certain snapshot in time (which, when Qc is being used, is typically not an equilibrium state). By comparing the current Qc to a reaction's Keq at the same temperature, it is possible to determine if a reaction will proceed faster in the forward or reverse direction to reach equilibrium. If Keq is greater than Qc, the reaction will move in the forward direction, while if Qc is greater than Keq, the reaction will move in the reverse direction.
Lesson Outline
<ul> <li>Irreversible vs. reversible reactions <ul> <li>In irreversible reactions, all reactants converted to products</li> <li>Dynamic equilibrium in reversible reactions <ul> <li>Entropy at maximum and Gibbs free energy at zero when dynamic equilibrium is reached</li> </ul> </li> </ul> </li> <li>Equilibrium constants (Keq) and the law of mass action <ul> <li>Keq equation for a hypothetical reversible reaction: [C]^c[D]^d/[A]^a[B]^b (where C and D are products with stoichiometric coefficients c and d, and A and B are reactants with stoichiometric coefficients a and b), using the concentrations of A, B, C, and D at equilibrium</li> <li>Exclusion of pure solids and pure liquids in equilibrium calculations</li> </ul> </li> <li>Reaction quotient (Qc) for non-equilibrium reactions <ul> <li>Calculate Qc using the same equation as Keq</li> <li>Compare Qc to Keq at the same temperature to determine reaction progress</li> <li>Direction of the reaction based on Qc and Keq</li> <li>If Keq > Qc, the reaction will move in the forward direction; if Qc > Keq, the reaction will move in the reverse direction</li> </ul> </li> </ul>
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FAQs
Dynamic equilibrium refers to a state in reversible chemical reactions where the rate of the forward reaction equals the rate of the reverse reaction, resulting in constant concentrations of reactants and products. However, the reactants and products are continuously being converted into each other. In contrast, static equilibrium is a state where there is no net change in the system, and both the forward and reverse reactions have ceased.
Gibbs free energy (ΔG) is an important thermodynamic concept that helps to predict the spontaneity of a chemical reaction and its relationship with equilibrium. At equilibrium, ΔG is equal to zero, indicating that the reaction is at its most stable state and no further net change will occur. If ΔG is negative, the reaction proceeds spontaneously in the forward direction, and if ΔG is positive, the reverse reaction is favored.
The law of mass action is a key principle governing the behavior of reversible chemical reactions at equilibrium. It states that the rate of a chemical reaction is proportional to the product of the molar concentrations of the reactants, each raised to the power of their stoichiometric coefficients in the balanced equation. At equilibrium, the rate of the forward reaction equals the rate of the reverse reaction, resulting in constant concentrations of reactants and products. The law of mass action leads to the concept of the equilibrium constant (Keq), which is calculated as the ratio of the concentrations of products to reactants, each raised to their stoichiometric coefficients.
The equilibrium constant (Keq) is a dimensionless quantity that represents the ratio of the concentrations of products to reactants, each raised to their stoichiometric coefficients, when a reversible chemical reaction has reached equilibrium. It is a characteristic constant for a particular reaction at a specific temperature and does not change unless the temperature changes. The reaction quotient (Q), on the other hand, is a similar ratio of the concentrations or activities of products and reactants at any point during the reaction. Comparing Q to Keq helps to predict the direction in which the reaction will proceed. If Q < Keq, the forward reaction is favored and will proceed until equilibrium is reached. If Q > Keq, the reverse reaction is favored. When Q = Keq, the reaction is already at equilibrium.
