Intermediate Topic 465 of 1102 Kinetics & Equilibrium
Kc and Kp
Equilibrium constants in terms of concentration and partial pressure, related by Kp equals Kc (RT) to the delta n.
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In-depth explanation of Kc and Kp
Equilibrium constants for gas phase reactions can be expressed either in terms of molar concentrations, giving Kc, or in terms of partial pressures, giving Kp. The two are related by Kp = Kc(RT)^(delta n), where delta n is the change in the number of moles of gas when the balanced equation is read from left to right. Delta n equals the moles of gaseous products minus the moles of gaseous reactants. When delta n equals zero, Kp and Kc are numerically equal, because RT raised to the zero power is one. The partial pressure form is often more convenient for gas reactions, since pressures can be measured directly.
Going deeper: Kc and Kp
Key ideas: Use Kp = Kc (RT)^(delta n) with R = 0.0821 L atm per mol per K when pressure is in atm and concentration in mol per L. Compute delta n from gas coefficients only, ignoring solids and liquids. The partial pressure of a gas is P = cRT, so convert carefully, and keep temperature in kelvin.
Common mistakes and connections: The main error is using the wrong R or including solids in delta n. Students also compute delta n as reactants minus products, which gives the wrong sign of the exponent. Another mistake is applying the relation to non-gas species. When delta n is zero, such as in H2 + I2 <-> 2HI, volume changes do not shift the equilibrium. Related to the effect of pressure on equilibrium.
Visualization of Kc and Kp
Kp = Kc(RT)^(delta n) at 298 K: equal when delta n is 0, larger than Kc when gas moles increase (like N2O4 to 2NO2) and smaller when they decrease. The interactive lesson shows this as a live animation or chart.
Worked example and use case
For N2O4 <-> 2NO2 the value of delta n is 2 - 1 = 1, so Kp = Kc RT at the chosen temperature.
Frequently asked questions about Kc and Kp
What is Kc and Kp?
Equilibrium constants in terms of concentration and partial pressure, related by Kp equals Kc (RT) to the delta n.
Is Kc and Kp a beginner, intermediate or advanced topic?
Kc and Kp is a intermediate topic in Kinetics & Equilibrium. It builds on the vocabulary of beginner topics in this area.
What should I learn before Kc and Kp?
Study these earlier topics in Kinetics & Equilibrium first: Reversible Reactions, Dynamic Equilibrium, Equilibrium Constant.
What comes after Kc and Kp?
The next topics in Kinetics & Equilibrium are Reaction Quotient and Magnitude of K.
Can I practise Kc and Kp with quizzes?
Yes. The interactive lesson includes a self-check question on Kc and Kp, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Kc and Kp fits in Kinetics & Equilibrium
Kc and Kp is topic 46 of 91 in Kinetics & Equilibrium.