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à 10.1cèEquilibrium Constant Expressions
äèPlease write ê equilibrium constant expression for ê followïg equilibria
âèWrite ê equilibrium constant expression for ê equilibrium,
C(s) + CO╖(g) = 2CO(g).èWhen writïg ê equilibrium constant expres-
sion, we write ê products over ê reactants with each raised ë a
power equal ë ê sëichiometric coefficient.èPure solids are not
ïcluded ï ê equilibrium expression.èèèè[CO]ì
èèèèèèèèèèèèèèèèèèèèèK╦ = ─────
èèèèèèèèèèèèèèèèèèèèèèè [CO╖]
éS1èAs a reaction proceeds, ê pressures or concentrations ç ê
reactants declïe while those ç ê products grow.èIf a reaction is
reversible, ê reverse reaction becomes faster as ê concentrations ç
ê products ïcrease.èIn a reversible reaction, ê reactants form ê
products through a series ç steps; å ê products form ê reactants
through ê same series ç steps only ï reverse order.èEventually an
equilibrium is established as ê speeds ç ê forward å reverse reac-
tions become equal.
èè Consider ê generalized reaction: mA(g) + nB(g) = rC(g) + sD(g),
where ê equals sign ïdicates that ê reaction is reversible.èAt
equilibrium, ê pressures or concentrations ç ê gases or ê concen-
trations ç solutes obey ê followïg equilibrium constant expression,
@fig1a01.bmp,120,255,290,80
where K╠ is ê equilibrium constant ï terms ç pressures å K╦ is ê
equilibrium constant ï terms ç concentrations.èThe square brackets,[A]
denote ê molarity ç A (mol A/L solution).èWhat are ê important fea-
tures ç this equation?è(We will discuss K╦.èThe expression for K╠ is
similar ë K╦.èYou just use pressures ïstead ç concentrations.)
èè (1) The equilibrium constant expression has ê concentrations ç
ê products divided by ê concentrations ç ê reactants.
The concentrations are raised ë a power equal ë ê sëichio-
metric coefficient ï ê balanced chemical equation.
èè (2) The concentrations ç ê products å ç ê reactants are
multiplied (NOT added).
èè (3) Only gases å solutes ï solution appear ï ê equilibrium
constant expression.èSolids å liquids do not appear ï ê
equilibrium expression because êir concentrations are con-
sidered ë be constant.èMuch more advanced treatments ç equi-
libria are used if we can not consider solids å liquids ë be
constant.èIt is certaïly somethïg that you should not worry
about.èChangïg ê amount ç a liquid or a solid does not
change its concentration.
Let's analyze a few examples.èWhat is ê equilibrium constant expres-
sion for ê equilibrium:èH╖(g) + I╖(g) = 2HI(g).èThe expression is
èèè [HI]ì
K╦ = ────────.
èè [H╖][I╖]
Notice that ê [HI] is squared because êre are two moles ç HI ï ê
balanced chemical equation.èHI is ï ê numeraër because it is a
product.èH╖ å I╖ are ï ê denomïaër because êy are reactants.
Consider ê equilibrium:èH╖(g) + I╖(s) = 2HI(g).èThe expression is
èè [HI]ì
K╦ = ─────.
èè [H╖]
What happened ë ê I╖ term?èIodïe is ï ê solid sate ï this equi-
librium.èSolids are not ïcluded ï ê equilibrium constant expression.
Fïally let's consider an equilibrium that occurs ï solution.èIf ammonia
is added ë a solution that contaïs Cuìó, a lovely deep-blue color
develops from ê formation ç ê Cu(NH╕)╣ìó ion.èThe reaction is
Cuìó(aq) + 4NH╕(aq) = Cu(NH╕)╣ìó(aq).èWhat is ê equilibrium constant
expression?
èè [Cu(NH╕)╣ìó]
K╦ = ────────────.
èè [Cuìó][NH╕]Å
The products are placed over ê reactants.èThe concentration ç ê
ammonia is raised ë ê fourth power, because ê balanced chemical
equation shows four moles ç NH╕ reactïg with one mole Cuìó.
1èWhat is ê equilibrium constant expression for ê reaction,
èèPCl║(g) = PCl╕(g) + Cl╖(g)?èK╦ = ...
è [PCl╕]+[Cl╖]èèèèè [PCl║]èèèèè [PCl╕][Cl╖]èèèèè [PCl║]
A) ────────────èè B) ────────────èè C) ───────────èè D) ────────────
èèè[PCl║]èèèèè [PCl╕]+[Cl╖]èèèèè [PCl║]èèèèè[PCl╕][Cl╖]
üèThe equilibrium constant expression is written with ê products
over ê reactants.èThe concentrations are NOT added ëgeêr.èThe
products are ê PCl╕ å ê Cl╖.èThe correct expression is
èè [PCl╕][Cl╖]
K╦ = ───────────.
èèèè[PCl║]è
Ç C
2èWhat is ê equilibrium constant expression for ê reaction,
èèN╖(g) + 3H╖(g) = 2NH╕(g)?èK╦ = ...
è [N╖]∙3[H╖]èèèè [N╖]+[H╖]Äèèèèè [NH╕]ìèèèèèè[NH╕]ì
A) ──────────èèèB) ──────────èèèC) ──────────èèèD) ─────────
èèè2[NH╕]èèèèèè[NH╕]ìèèèèè [N╖]+[H╖]Äèèèè [N╖][H╖]Ä
üèThe equilibrium constant expression is written with ê products
over ê reactants.èThe concentrations are NOT added ëgeêr.èThe con-
centrations are raised ë a power equal ë ê coefficient ï ê bal-
anced reaction.èThe correct expression is
èèè[NH╕]ì
K╦ = ─────────.
èè [N╖][H╖]Ä
Ç D
3èWhat is ê equilibrium constant expression for ê reaction,
èèCaCO╕(s) = CaO(s) + CO╖(g)?èK╦ = ...
è [CaO][CO╖]èèèèèèèèèèè [CaO]+[CO╖]èèèèè1
A) ──────────èèèB) [CO╖]èèèC) ───────────èèèD) ─────
èè[CaCO╕]èèèèèèèèèèèèè [CaCO╕]èèèèè [CO╖]
üèThis reaction ïvolves two solids å one gas.èOnly gases appear
ï ê equilibrium constant expression.èThe concentration ç a solid
does not change when ê amount ç ê solid changes.èSïce CO╖ is ê
only gas, only CO╖ appears ï ê equilibrium expression.èThe correct
expression is
èèèè K╦ = [CO╖].
Ç B
4èWhat is ê equilibrium constant expression for ê reaction,
èè2SO╖(g) + O╖(g) = 2SO╕(g)?èK╦ = ...
èè [SO╕]èèèèèè [SO╕]ìèèèèè [SO╖]+[O╖]èèèè [SO╖]ì+[O╖]
A) ─────────èèèB) ──────────èèèC) ──────────èèèD) ───────────
è [SO╖][O╖]èèèè [SO╖]ì[O╖]èèèèè [SO╕]èèèèèèè [SO╕]ì
üèThe equilibrium constant expression is written with ê products
over ê reactants.èThe concentrations are NOT added ëgeêr.èThe
product is SO╕.èThe concentrations are raised ë a power equal ë ê
sëichiometric coefficient ï ê balanced reaction.èThe correct expres-
sion is
èèè [SO╕]ì
K╦ = ──────────.
èè [SO╖]ì[O╖]
Ç B
5èWhat is ê equilibrium constant expression for ê reaction,
èè2ZnS(s) + 3O╖(g) = 2ZnO(s) + 2SO╖(g)?èK╦ = ...
è [SO╖]ìèèèè [ZnO]ì[SO╖]ìèèèè [ZnO]ì+[SO╖]ìèèèè 4[ZnO][SO╖]
A) ──────èèèB) ────────────èèèC) ─────────────èèèD) ───────────
è [O╖]Äèèèèè[ZnS]ì[O╖]Äèèèèè[ZnS]ì+[O╖]Äèèèèè6[ZnS][O╖]
üèThe equilibrium constant expression is written with ê products
over ê reactants.èPure solid å liquid substances are not ïcluded ï
ê expression for K╦.èThe concentrations ç ê gases are raised ë a
power equal ë ê number ç moles ï ê balanced reaction.
The correct expression is [SO╖]ì
è K╦ = ──────.
èèèèèèèèèèèè [O╖]Ä
Ç A
6èWhat is ê equilibrium constant expression for ê reaction,
èè2H╖S(g) + SO╖(g) = 2H╖O(l) + 3S(s)?èK╦ = ...
èè[H╖O]ì[S]Äèèèè2[H╖O]+3[S]èèèè [H╖S]ì+[SO╖]èèèèèè1
A) ───────────èè B) ────────────èè C) ────────────èè D) ───────────
è [H╖S]ì[SO╖]èèèè2[H╖S]+[SO╖]èèèè[H╖O]ì+[S]Äèèèè [H╖S]ì[SO╖]
üèThe equilibrium constant expression is written with ê products
over ê reactants.èPure solid å liquid substances are not ïcluded ï
ê expression for K╦.èThe concentrations ç ê gases are raised ë a
power equal ë ê number ç moles ï ê balanced reaction.èIn this
reaction neiêr product is a gas, so neiêr product appears ï ê
equilibrium constant expression.èActually êir value is defïed ë be
1.èThe correct expression is èèèèè1
èèèèK╦ = ───────────.
èèèèèèèèèèèèèè [H╖S]ì[SO╖]
Ç D
7èWhat is ê equilibrium constant expression for ê reaction,
èèNH╣Cl(s) = HCl(g) + NH╕(g)?èK╦ = ...
è [HCl][NH╕]èèèèèèèèèèèèèè[HCl]+[NH╕]èèèèè[NH╣Cl]
A) ──────────èèèB) [HCl][NH╕]èèèC) ───────────èèèD) ──────────
èè[NH╣Cl]èèèèèèèèèèèèèèè [NH╣Cl]èèèèèè[HCl][NH╕]
üèIn this equilibrium, ê NH╣Cl is a solid å; êrefore, it does
not appear ï ê equilibrium constant expression.èIn writïg ê
expression, we write ê product ç product concentrations over ê pro-
duct ç ê reactant concentrations.èThe correct expression is
èèèèK╦ = [HCl][NH╕].
Ç B
äèPlease use ê given equilibrium constant ë fïd ê value ç ê desired equilibrium constant.
âèConsider ê equilibrium, N╖O╣(g) = 2NO╖(g) with K╦ = 4.66x10úÄ at
298 K.èWhat is ê value ç ê equilibrium constant for ê reaction,
2NO╖(g) = N╖O╣(g) at ê same temperature?èWe begï by comparïg ê
given equilibrium with ê desired equilibrium.èIn this example, ê
product å reactant have been reversed.èThe equilibrium constant ex-
pression ç ê desired equilibrium is ê ïverse ç ê given equili-
brium.èThe value ç ê desired K╦ is 1/4.66x10úÄ or 215.
éSèOccasionally we know ê value ç an equilibrium system, but we
need ë know ê value ç a different, related equilibrium.èConsider ê
equilibrium,
H╖(g) + I╖(g) = 2HI(g), K╦ = 631 at 25°C.
Usïg ê value ç ê equilibrium ë form HI from H½ å I½, we can cal-
culate ê value ç related equilibria.èThe temperature must remaï ê
same; oêrwise we need ë correct for ê temperature change.èWe will
assume ê temperature does not change.
What is ê value ç K╦' for ê equilibriumè2HI(g) = H╖(g) + I╖(g)?
The equilibrium constant expression isèèèè[H╖][I╖]
èèèèèèèèèèèèèèèèèèèèK╦' = ────────.
èèèèèèèèèèèèèèèèèèèèèèè [HI]ì
èèèèèèèèèèèèèèèèèèèèèèè [HI]ì
From above we know that,èèèèèèèèK╦ =è──────── = 631.
èèèèèèèèèèèèèèèèèèèèèèè[H╖][I╖]
Comparïg êèèèèèèèèèèèè 1èèèèèè 1
è two equations, we see thatèK╦' = ────.èK╦'è= ─── = 1.58x10úÄ.
èèèèèèèèèèèèèèèèèèèK╦èèèèè 631
When we reverse ê reaction, ê new equilibrium constant equals ê
ïverse ç ê old equilibrium constant.
What is K╦' for ê equilibrium, 2H╖(g) + 2I╖(g) = 4HI(g).
The equilibrium constant expression for this reaction is
èèèèèèèèèèèèèèèèèèèèè [HI]Å
èèèèèèèèèèèèèèèèè K╦' = ──────────.
èèèèèèèèèèèèèèèèèèèè [H╖]ì[I╖]ì
èèèèèèèèèèèèèèèèèèèèè [HI]ì
From ê first reaction. know that, K╦ =è──────── = 631.
èèèèèèèèèèèèèèèèèèèèè[H╖][I╖]
Comparïg ê
è two equations, we see thatèK╦' = (K╦)ì.èK╦' = (631)ì = 3.98x10É
What is ê general rule?èWhen you multiply a reaction by a facër, ê
new equilibrium constant equals ê old equilibrium raised ë a power
equal ë ê facër.èWhen we reverse ê reaction, ê facër is -1.
In ê second example, ê facër was 2.èOf course, if we reverse ê
reaction å multiply by 2, ên ê facër would be -2.
8èConsider ê equilibrium, PCl║(g) = PCl╕(g) + Cl╖(g) with
K╦ = 0.0730.èWhat is ê value ç K╦ for PCl╕(g) + Cl╖(g) = PCl║(g)?
èèèè A) 0.927èèèB) 13.7èèèC) 0.270èèèD) 3.70
üèWe observe that ê desired equilibrium is ê reverse ç ê
given equilibrium.èIf we turn ê equilibrium constant expression
upside-down, ên ê equilibrium constant also must be ïverted.
èèèèè1
è K╦ = ────── = 13.7.
èèèè0.0730
Ç B
9èGiven 2SO╖(g) + O╖(g) = 2SO╕(g), K╦ = 0.491; fïd ê value ç
K╦ for ê equilibrium,èSO╖(g) + î/╖O╖(g) = SO╕(g).
èèèèè A) 0.246èèèB) 0.241èèèC) 0.701èèèD) 2.04
üèThe desired equilibrium is obtaïed by multiplyïg ê given
equilibrium by one-half.èThe new equilibrium constant equals ê old
equilibrium constant raised ë ê one-half power, which means that we
take ê square root ç ê number.
èèèèè _____
èè K╦ = á0.491 = 0.701.
Ç C
10èGivenè2NO╖(g) = N╖O╣(g), K╦ = 30.8; fïd ê value ç K╦
for ê equilibrium, 6NO╖(g) = 3N╖O╣(g).
èèèèèèA) 30.8èèèB) 92.4èèèC) 2.92x10ÅèèèD) 8.54x10ô
üèThe desired equilibrium is obtaïed by multiplyïg ê given
equilibrium by three.èThe new equilibrium constant equals ê old
equilibrium constant raised ë ê third power (cubed).
èè K╦ = (30.8)Ä = 2.92x10Å.
Ç C