<< Chapter < Page Chapter >> Page >

The equation relating K c and K P is derived as follows. For the gas-phase reaction m A + n B x C + y D:

K P = ( P C ) x ( P D ) y ( P A ) m ( P B ) n
= ( [ C] × R T ) x ( [ D] × R T ) y ( [ A] × R T ) m ( [ B] × R T ) n
= [ C ] x [ D ] y [ A ] m [ B ] n × ( R T ) x + y ( R T ) m + n
= K c ( R T ) ( x + y ) ( m + n )
= K c ( R T ) Δ n

The relationship between K c and K P is

K P = K c ( R T ) Δ n

In this equation, Δ n is the difference between the sum of the coefficients of the gaseous products and the sum of the coefficients of the gaseous reactants in the reaction (the change in moles of gas between the reactants and the products). For the gas-phase reaction m A + n B x C + y D, we have

Δ n = ( x + y ) ( m + n )

Calculation of K P

Write the equations for the conversion of K c to K P for each of the following reactions:

(a) C 2 H 6 ( g ) C 2 H 4 ( g ) + H 2 ( g )

(b) CO ( g ) + H 2 O ( g ) CO 2 ( g ) + H 2 ( g )

(c) N 2 ( g ) + 3 H 2 ( g ) 2 NH 3 ( g )

(d) K c is equal to 0.28 for the following reaction at 900 °C:

CS 2 ( g ) + 4 H 2 ( g ) CH 4 ( g ) + 2 H 2 S ( g )

What is K P at this temperature?

Solution

(a) Δ n = (2) − (1) = 1
K P = K c ( RT ) Δ n = K c ( RT ) 1 = K c ( RT )

(b) Δ n = (2) − (2) = 0
K P = K c ( RT ) Δ n = K c ( RT ) 0 = K c

(c) Δ n = (2) − (1 + 3) = −2
K P = K c ( RT ) Δ n = K c ( RT ) −2 = K c ( R T ) 2

(d) K P = K c (RT) Δ n = (0.28)[(0.0821)(1173)] −2 = 3.0 × 10 −5

Check your learning

Write the equations for the conversion of K c to K P for each of the following reactions, which occur in the gas phase:

(a) 2 SO 2 ( g ) + O 2 ( g ) 2 SO 3 ( g )

(b) N 2 O 4 ( g ) 2 NO 2 ( g )

(c) C 3 H 8 ( g ) + 5 O 2 ( g ) 3 CO 2 ( g ) + 4 H 2 O ( g )

(d) At 227 °C, the following reaction has K c = 0.0952:

CH 3 OH ( g ) CO ( g ) + 2 H 2 ( g )

What would be the value of K P at this temperature?

Answer:

(a) K P = K c ( RT ) −1 ; (b) K P = K c ( RT ); (c) K P = K c ( RT ); (d) 160 or 1.6 × 10 2

Got questions? Get instant answers now!

Heterogeneous equilibria

A heterogeneous equilibrium is a system in which reactants and products are found in two or more phases. The phases may be any combination of solid, liquid, or gas phases, and solutions. When dealing with these equilibria, remember that solids and pure liquids do not appear in equilibrium constant expressions (the activities of pure solids, pure liquids, and solvents are 1).

Some heterogeneous equilibria involve chemical changes; for example:

PbCl 2 ( s ) Pb 2+ ( a q ) + 2 Cl ( a q ) K c = [ Pb 2+ ] [ Cl ] 2
CaO ( s ) + CO 2 ( g ) CaCO 3 ( s ) K c = 1 [CO 2 ]
C ( s ) + 2 S ( g ) CS 2 ( g ) K c = [CS 2 ] [ S ] 2

Other heterogeneous equilibria involve phase changes, for example, the evaporation of liquid bromine, as shown in the following equation:

Br 2 ( l ) Br 2 ( g ) K c = [ Br 2 ]

We can write equations for reaction quotients of heterogeneous equilibria that involve gases, using partial pressures instead of concentrations. Two examples are:

CaO ( s ) + CO 2 ( g ) CaCO 3 ( s ) K P = 1 P CO 2
C ( s ) + 2 S ( g ) CS 2 ( g ) K P = P CS 2 ( P S ) 2

Key concepts and summary

For any reaction that is at equilibrium, the reaction quotient Q is equal to the equilibrium constant K for the reaction. If a reactant or product is a pure solid, a pure liquid, or the solvent in a dilute solution, the concentration of this component does not appear in the expression for the equilibrium constant. At equilibrium, the values of the concentrations of the reactants and products are constant. Their particular values may vary depending on conditions, but the value of the reaction quotient will always equal K ( K c when using concentrations or K P when using partial pressures).

A homogeneous equilibrium is an equilibrium in which all components are in the same phase. A heterogeneous equilibrium is an equilibrium in which components are in two or more phases. We can decide whether a reaction is at equilibrium by comparing the reaction quotient with the equilibrium constant for the reaction.

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)
Mmathabo
am new here can I join
Yeko
give two properties of liquid
Grace Reply
molecules are slightly packed and they follow the shape of a container.
Obedie
what is measurement
Isaiah Reply
is the comparison of an unknown quantity with a fixed quantity of the same kind
Sahada
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.
Yusuf
an element is s chemically pure substance containing a particular type of atoms.. A COMPOUND is a substance containing atoms from different elements..
Inemesit
How to get the Lewis formula of SeCl+3
Erica Reply
hi,I'm new here can I join the conversation
EZEA
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.
Abdelkarim
what is the roles of filter bed
Fathmat
what is the roles of Alu m
Fathmat
what is the roles of chlorine
Fathmat
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:
Abdelkarim
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.
Abdelkarim
what is chemistry
Maxamed
what is the meaning of exceedingly
Yushao Reply
it is an adverb which means extremely
Rohini
what is atomic chemistry?
Gladys Reply
Lewis structure for no3
Gladys
Lewis structure for no3
Gladys
Hello
Gladys
Yes
Gillian
Lewis structure for no3
Nomvelo
what is weak acid
Muhammed Reply
It is an acid which partially ionises in water.
Abdelkarim
what is incandescence
Clifton
what makes it glow
Clifton
why is it red, irange and yellow in color
Clifton
hello am new here and I want to join you
Aliyu
hello
Clifton
hi
Aliyu
too
Gillian
hello i am new here please i want to join this group
Paul
Hi, I'm also new here
Salaudeen
Hi
Keeya
hello guys !!
Sourav
what is pressure?
Slark Reply
The force applied to suction Area of the body
Ahmed
Matter composed of exceedingly small paticle called atom.
Yushao
questions related to metals
Regina Reply
occurrence and preparation of the representatives metals
Regina
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
Benita
Hydrogen, helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, neon, sodium, magnesium, aluminium, silicon, phosphorus, sulphur, chlorine, argon, potassium, calcium
Cudjoe
hey I'm also New here
Obedie
Welcome
Gillian
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
Fifa
sorry liquid
Fifa
it's a liquid substance
Fathmat
hello group
Ayomide
is the substance that dissolves in the solvent
Amos
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
Abdelkarim
Form covalenr bond*
Abdelkarim
The question marks are an emoji in the first sentence is an unread emoji. HCl Covalent compund -> molecule
Abdelkarim
Hi.
Queen
Hi
Calvin
Yh
Cudjoe
yes
Amos
what is chemistry
Chukwu Reply
is the study of composition of substances and the way they behave under different conditions
Amos
how do calculate n1 though n6 any help on understanding the concept
Clifton
is the study of properties of matter and it's component
Grace
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?

Ask