Advanced Topic 488 of 1102 Kinetics & Equilibrium
Relationship of Rate Constants and K
K equals the ratio of forward to reverse rate constants for an elementary reaction.
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In-depth explanation of Relationship of Rate Constants and K
For an elementary reversible reaction the equilibrium constant equals the ratio of the forward rate constant to the reverse rate constant, K = kf/kr. This follows because at equilibrium the forward and reverse rates are equal, so kf[A] equals kr[B], and rearranging gives the equilibrium constant expression. The relationship links kinetic and thermodynamic information and allows an equilibrium constant to be calculated from measured rate constants. It holds rigorously only for elementary steps, since the rate law of a complex reaction need not have the simple form of its equilibrium expression. It also shows that the position of equilibrium depends on how fast each direction proceeds.
Going deeper: Relationship of Rate Constants and K
Key ideas: Use K = kf/kr only for a single elementary step. For a multistep mechanism, K is the product of the ratios for each step. Example: if kf = 3.0 x 10^3 and kr = 1.5 x 10^1, K = 200. The temperature dependence follows from Ea forward minus Ea reverse = delta H, so the ratio changes with T according to the van't Hoff equation.
Common mistakes and connections: Students apply K = kf/kr to non-elementary reactions using overall rate constants, which is wrong because orders need not match the coefficients. Another mistake is thinking that a catalyst changes K: it multiplies both constants by the same factor. Also note the units: ratio of kf and kr has units that match K only if the orders correspond to stoichiometry. Related to detailed balance and microscopic reversibility.
Visualization of Relationship of Rate Constants and K
At equilibrium kf[A] = kr[B], so K = kf/kr: with kf = 100 and kr = 5 per second, K = 20 and products are favoured. The interactive lesson shows this as a live animation or chart.
Worked example and use case
If kf = 100 s^-1 and kr = 5 s^-1 for an elementary reversible step, then K = 20.
Frequently asked questions about Relationship of Rate Constants and K
What is Relationship of Rate Constants and K?
K equals the ratio of forward to reverse rate constants for an elementary reaction.
Is Relationship of Rate Constants and K a beginner, intermediate or advanced topic?
Relationship of Rate Constants and K is a advanced topic in Kinetics & Equilibrium. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Relationship of Rate Constants and K?
Study these earlier topics in Kinetics & Equilibrium first: Effect of Complexation on Solubility, Equilibrium in Gas Phase, Degree of Dissociation.
What comes after Relationship of Rate Constants and K?
The next topics in Kinetics & Equilibrium are Principle of Microscopic Reversibility and Kinetic vs Thermodynamic Product.
Can I practise Relationship of Rate Constants and K with quizzes?
Yes. The interactive lesson includes a self-check question on Relationship of Rate Constants and K, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Relationship of Rate Constants and K fits in Kinetics & Equilibrium
Relationship of Rate Constants and K is topic 69 of 91 in Kinetics & Equilibrium.