19.1. The Nature of Chemical Equilibrium http://www.ck12.org
Kp=
([C](RT))c([D](RT))d
([A](RT))a([B](RT))bKp=
[C]c[D]d
[A]a[B]b×
(RT)c(RT)d
(RT)a(RT)bThe fraction on the left is simply the generic expression for Kc. Additionally, the exponents in the fraction on the
right can be manipulated as follows:
Kp=Kc(RT)(c+d)−(a+b)
The exponent(c+d)−(a+b)is the sum of the stoichiometric coefficients for the gaseous products minus the sum
of the coefficients for the gaseous reactants. This value is sometimes abbreviated as∆n. Note that∆n can be a
negative or positive number. Using this abbreviation, the conversion factor between Kpand Kccan be written as
follows:
Kp=Kc(RT)∆n
Lesson Summary
- At equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction.
- The ratio between the concentrations of the products and the concentrations of the reactants, each raised to the
appropriate exponent, is constant at a given temperature. This is referred to as the equilibrium constant. - By convention, only gaseous and aqueous reaction components are included in the equilibrium constant
expression, since solids and liquids do not change concentration when they are consumed or formed. - The partial pressure of an ideal gas is proportional to its concentration. Kpis an alternate way to express the
equilibrium of a reaction that includes gaseous components.
Lesson Review Questions
- At chemical equilibrium, what is the relationship between the rate of the forward reaction and the rate of the
reverse reaction? - What does the law of mass action say about any reaction at equilibrium?
- Why are solids and liquids left out of the equilibrium constant expression?
- Explain why the equilibrium constant expression as derived from concentrations can be converted to the
equilibrium constant expression as derived from partial pressures. - Construct balanced chemical equations and write the equilibrium constant expression (either Kpor Kc) for
each of the following processes.
a. Sodium metal reacts vigorously with oxygen gas to produce sodium oxide.
b. Aluminum oxide decomposes to form aluminum metal and oxygen gas.
c. Nitrogen gas and oxygen gas react to form nitrogen monoxide gas. - Give an example of a chemical equilibrium that exists in our daily lives.
- Write a Keqexpression for each of the following processes.
a. Solid calcium carbonate is heated and decomposes into solid calcium oxide and carbon dioxide gas.
b. Hydrogen gas (H 2 ) and solid iodine (I 2 ) are combined to form hydrogen iodide gas (HI).