<< Chapter < Page Chapter >> Page >

Homogeneous equilibria

A homogeneous equilibrium is one in which all of the reactants and products are present in a single solution (by definition, a homogeneous mixture). In this chapter, we will concentrate on the two most common types of homogeneous equilibria: those occurring in liquid-phase solutions and those involving exclusively gaseous species. Reactions between solutes in liquid solutions belong to one type of homogeneous equilibria. The chemical species involved can be molecules, ions, or a mixture of both. Several examples are provided here.

C 2 H 2 ( a q ) + 2 Br 2 ( a q ) C 2 H 2 Br 4 ( a q ) K c = [ C 2 H 2 Br 4 ] [ C 2 H 2 ] [ Br 2 ] 2
I 2 ( a q ) + I ( a q ) I 3 ( a q ) K c = [ I 3 ] [ I 2 ] [ I ]
Hg 2 2+ ( a q ) + NO 3 ( a q ) + 3 H 3 O + ( a q ) 2 Hg 2+ ( a q ) + HNO 2 ( a q ) + 4 H 2 O ( l )
K c = [ Hg 2+ ] 2 [ HNO 2 ] [ Hg 2 2+ ] [ NO 3 ] [ H 3 O + ] 3
HF ( a q ) + H 2 O ( l ) H 3 O + ( a q ) + F ( a q ) K c = [ H 3 O + ] [ F ] [ HF ]
NH 3 ( a q ) + H 2 O ( l ) NH 4 + ( a q ) + OH ( a q ) K c = [ NH 4 + ] [ OH ] [ NH 3 ]

In each of these examples, the equilibrium system is an aqueous solution, as denoted by the aq annotations on the solute formulas. Since H 2 O( l ) is the solvent for these solutions, its concentration does not appear as a term in the K c expression, as discussed earlier, even though it may also appear as a reactant or product in the chemical equation.

Reactions in which all reactants and products are gases represent a second class of homogeneous equilibria. We use molar concentrations in the following examples, but we will see shortly that partial pressures of the gases may be used as well.

C 2 H 6 ( g ) C 2 H 4 ( g ) + H 2 ( g ) K c = [ C 2 H 4 ] [ H 2 ] [ C 2 H 6 ]
3 O 2 ( g ) 2 O 3 ( g ) K c = [ O 3 ] 2 [ O 2 ] 3
N 2 ( g ) + 3 H 2 ( g ) 2 NH 3 ( g ) K c = [ NH 3 ] 2 [ N 2 ] [ H 2 ] 3
C 3 H 8 ( g ) + 5 O 2 ( g ) 3 CO 2 ( g ) + 4 H 2 O ( g ) K c = [ CO 2 ] 3 [ H 2 O ] 4 [ C 3 H 8 ] [ O 2 ] 5

Note that the concentration of H 2 O( g ) has been included in the last example because water is not the solvent in this gas-phase reaction and its concentration (and activity) changes.

Whenever gases are involved in a reaction, the partial pressure of each gas can be used instead of its concentration in the equation for the reaction quotient because the partial pressure of a gas is directly proportional to its concentration at constant temperature. This relationship can be derived from the ideal gas equation, where M is the molar concentration of gas, n V .

P V = n R T
P = ( n V ) R T
= M R T

Thus, at constant temperature, the pressure of a gas is directly proportional to its concentration.

Using the partial pressures of the gases, we can write the reaction quotient for the system C 2 H 6 ( g ) C 2 H 4 ( g ) + H 2 ( g ) by following the same guidelines for deriving concentration-based expressions:

Q P = P C 2 H 4 P H 2 P C 2 H 6

In this equation we use Q P to indicate a reaction quotient written with partial pressures: P C 2 H 6 is the partial pressure of C 2 H 6 ; P H 2 , the partial pressure of H 2 ; and P C 2 H 6 , the partial pressure of C 2 H 4 . At equilibrium:

K P = Q P = P C 2 H 4 P H 2 P C 2 H 6

The subscript P in the symbol K P    designates an equilibrium constant derived using partial pressures instead of concentrations. The equilibrium constant, K P , is still a constant, but its numeric value may differ from the equilibrium constant found for the same reaction by using concentrations.

Conversion between a value for K c    , an equilibrium constant expressed in terms of concentrations, and a value for K P , an equilibrium constant expressed in terms of pressures, is straightforward (a K or Q without a subscript could be either concentration or pressure).

Questions & Answers

wat are hydrocarbon s
Opio Reply
I think they are molecules that comprise only of hydrogen and carbon atoms ( they are organic if I'm not mistaken)
am new here can I join
give two properties of liquid
Grace Reply
molecules are slightly packed and they follow the shape of a container.
what is measurement
Isaiah Reply
is the comparison of an unknown quantity with a fixed quantity of the same kind
How does an element differ from a compound? How are they similar?
Adeola Reply
an element is an indivisible particles that can take part in a reaction and consist of smaller or tiny particles i.e proton, neutrons and electron while a compound is when two or more element chemically combine together. They are similar when they are homogeneous compound. they take the same rxn.
an element is s chemically pure substance containing a particular type of atoms.. A COMPOUND is a substance containing atoms from different elements..
How to get the Lewis formula of SeCl+3
Erica Reply
hi,I'm new here can I join the conversation
what is the structural formula for starch
EZEA Reply
Starch is a mixture (of chemicals) of amylose and amylopectin. Both are macromolecules and polymers. You can search on wikipedia.
what is the roles of filter bed
what is the roles of Alu m
what is the roles of chlorine
Roles can be classified or correlate it to different areas: For example: Chlorine can be used in reactions (in industry) to manufacture HCl, which then can be used for other things. Or in swimming pools to kill bacteria. Or as a component in compounds with pharmaceutical roles (drugs). For Al:
Its dentisty value is suitable to be used in alloys (mixture of metals) in aircraft bodies. Also, Aluminium foils, Tin cans,.. Some of them are also in Al overhead cables in streets and long roads.
what is chemistry
what is the meaning of exceedingly
Yushao Reply
it is an adverb which means extremely
what is atomic chemistry?
Gladys Reply
Lewis structure for no3
Lewis structure for no3
what is weak acid
Muhammed Reply
It is an acid which partially ionises in water.
what is incandescence
what makes it glow
why is it red, irange and yellow in color
hello am new here and I want to join you
hello i am new here please i want to join this group
Hi, I'm also new here
hello guys !!
what is pressure?
Slark Reply
The force applied to suction Area of the body
Matter composed of exceedingly small paticle called atom.
questions related to metals
Regina Reply
occurrence and preparation of the representatives metals
list the 20, periodic table and their symbols
Fathmat Reply
hydrogen:h helium;he lithium:l beryllium:be Boron:b Carbon;C Nitrogen:n Oxygen:O FLUORINE:f Neon:n Sodium:s Magnesium:mg Aluminum:a Silicon:s Phosphorus:p Sulphur:s Chlorine:c Argon;a Potassium:p Calcium:c
Hydrogen, helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, neon, sodium, magnesium, aluminium, silicon, phosphorus, sulphur, chlorine, argon, potassium, calcium
hey I'm also New here
there are 118 known elements ...you numbnuts
what is a solute
Ekezie Reply
Any substance that is disolved in a liqid solvent to create a solution
sorry liquid
it's a liquid substance
hello group
is the substance that dissolves in the solvent
so is HCl ionic compound
Honest Reply
No, covalent compound ➡️ molecule. As both H and Cl are non-metals and and form covalent bind by sharing valence e-. But can fully ionice in water forming H+ (a proton, a reason for acidity) and Cl- (anion =Chloride) Hydrogen Chloride is a gas at room; Hydrochloric acid = HCl (aq), dissolved in w
Form covalenr bond*
The question marks are an emoji in the first sentence is an unread emoji. HCl Covalent compund -> molecule
what is chemistry
Chukwu Reply
is the study of composition of substances and the way they behave under different conditions
how do calculate n1 though n6 any help on understanding the concept
is the study of properties of matter and it's component
Practice Key Terms 7

Get the best Chemistry course in your pocket!

Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Chemistry' conversation and receive update notifications?