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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

An atom or group of atoms bearing anelectrical charge such as the sodium and chlorine atoms in a salt solution.
Adazion Reply
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Adazion
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Prince
it's a list that shows the chemical element arranged according to their properties.
Adazion Reply
what is the chemical equation for ideal gas?
Adazion
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Adazion
what's the meaning of this℃ in atomic table
Adazion
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Sinyene
What is periodic table
SIRAJO Reply
Table that shows the elements in order of their atomic number
Uzair
organization of everything known in the universe in groups and periods. The structure is based on increasing mass and reaction properties.
Kate
How to mix chemical
Ukeh Reply
why the elements of group 7 are called Noble gases
isaac Reply
they aren't. group 8 is the noble gasses. they are snobs that don't mix with others like nobles, they have full valence shells so they don't form bonds with other elements easily. nobles don't mingle with the common folk...
Jessica
the group 7elements are not the noble gases . according to modern periodic group 18 are called noble gases elements because their valence shell are completely field so that they can't gain or loss electron so they are not able to involve in any chemical reaction.
Leena
Group 7 element they are not noble gases they halogen and halogen mean salt formers
SIRAJO
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Daniel Reply
chemistry is the branch of science which deal with the composition of matter
SHEDRACK
chemistry is an organized way to think about matter
Kate
Chemistry is the branch of science that deals with the composition, properties and uses of matter
Bamgbose
The branch of science concerned with the substances of which matter is composed, the investigation of their properties and reactions, and the use of such reactions to form new substances.
Aminu
discuss the orbital stracture of the following methane,ethane,ethylene,acetylene
khadija Reply
Why phosphurs in solid state have one atom but in gas state have four atoms
Shehab Reply
Are nuclear reactions both exothermic reactions and endothermic reactions or what?
Blessed Reply
to what volume must 8.32 NaOH be diluted to its analytical concentration 0.20 M
Sheriza Reply
weight in mg 1.76 mole of I
Sheriza
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Ohanaka Reply
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aliphatic and aromatic hydrocarbons
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saturated and unsaturated
Leena
aromatic hydrocarbon aliphatic hydrocarbon
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SIRAJO
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Shalom Reply
when u move from left to right in a periodic table the negativity increases
reeza
Are you trying to say that the elctronegativity increases down the group and decreases across the period?
Ohanaka
yes and also increases across the period
reeza
for instance when you look at one group of elements in a periodic table electronegativity decreases when you go across the table electronegativity increases. hydrogen is more electronegative than sodium, potassium of that group. oxygen is more electronegative than carbon.
reeza
i hope we all know that organic compounds have carbon as their back bone
Madueke
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Adazion Reply
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Osakue
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hanna
what is the oxidation number of nitrogen, oxygen and sulphur
Osakue
5, -2 & -2
hanna
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SIRAJO
What is an atom?
Adazion Reply
is a smallest particle of a chemical element that can exist
Osakue
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Osakue
it is a substance that cannot be broken down into simpler units by any chemical reaction
Madueke
An atom is the smallest part of an element dat can take part in chemical reaction.
Idris
an atom is the smallest part of an element that can take part in a chemical reaction nd still retain it chemical properties
Precious
Is the smallest particles of an element that take part in chemical reaction without been change
John
Practice Key Terms 7

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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