ChemEq 1 1

Chemical EquilibriumExergonic and Endergonic ReactionsThe Low of Mass Action, Keq and pKeq

Le Chatelier principle

Chemical EquilibriumExergonic and Endergonic ReactionsThe Low of Mass Action, Keq and pKeq

Le Chatelier principle

Dr. M. Sasvári: Medical Chemistry Lectures 1. Dr. M. Sasvári: Medical Chemistry Lectures 1.

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Chemical reactions:

A + BReactants

ABProducts

forward reaction

reverse reaction

Key terms and definitions

Initial state (initial concentrations)Standard conditions:1 M of both the reactants and the products

INIINI

EQEQ Equilibrium state (eq. concentrations)Dynamic equilibrium

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Exergonic reaction: Energy is released in the forward reaction

- either exothermic or endothermic

Endergonic reaction: Energy must be absorbed for the forward reaction,

otherwise reverse reaction happens

INIINIA chemical reaction: EQEQ

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ExamplesExergonic AND exothermic reactions

H2(g) + 1/2O2(g) H2O(g) -242 kJ

Water formation

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2H + 1/2O2 H2O in biological systems: Terminal oxidation

production of heat and/or energy

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Light production in an exergonic reactionBioluminescence

Light production in living organisms

The American firefly produces light (phorphorescence) by the enzyme luciferase

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Light production in an exergonic reactionChemoluminescence

Production of light in a chemical reaction

Criminology: Luminol reagent with the iron in the hemoglobin will glow with a blue light: Identification of bloodstains

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Ba(OH)2 . (H2O)8 + 2NH4Cl Ba2+ + 2Cl- + 10 H2O + 2NH3

water

Exergonic AND endothermic reactions

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2H2O 4H + O2

sunshine

Endergonic reactions (energy input is needed)

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Definition of the equilibrium constant (Keq)

Definition of the equilibrium constant (Keq)

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INIINI 1 M 1 M 1 M

EQEQ (1-x)M (1+x)M(1-x)M

A AB+ B

Keq=[Products]

[Reactants]

Keq=(1+x)

(1-x) (1-x)If Keq > 1Exergonic Reaction

If Keq < 1Endergonic Reaction

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Keq > 1Exergonic reactions

Keq > 1Exergonic reactions

Keq < 1Endergonic reactions

Keq < 1Endergonic reactions

0

1

2

time

conc (M)

reactants

products

0

1

2

time

conc (M)

reactants

products

EQ: less products than reactants

EQ: less products than reactants

EQ: more products than reactants

EQ: more products than reactants

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The Low of Mass ActionThe Low of Mass ActionCato Guldberg and Peter Waage 1867

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A AB+ BINIINI

EQEQ

forward reaction: vf = kf [A][B]

where vf is the rate, kf is the rate constant of forward reaction

forward reaction: vf = kf [A][B]

where vf is the rate, kf is the rate constant of forward reaction

reverse reaction:vr = kr[AB]

where vr is the rate, kr is the rate constant of reverse reaction

reverse reaction:vr = kr[AB]

where vr is the rate, kr is the rate constant of reverse reaction

Rate of the forward reaction = the rate of the reverse reactionRate of the forward reaction = the rate of the reverse reaction

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vf = vr

kf[A][B] = kr [AB]

Ks = kfkr

= [A][B][AB]

For a general reaction:

aA + bB cC + dD

Ks = [C]c [D]d

[A]a [B]b

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Depends on - - the temperature - the pressure (gas)

Does not depend on - - the concentrations - the reaction rate/activation energy

Ks (Equilibrium constant for reactions in solution) A characteristic value for chemical reactions

Ks (Equilibrium constant for reactions in solution) A characteristic value for chemical reactions

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Ks and pKs Ks and pKs

pKs = -log Ks

More products than reactants in

EQ.

Endergonic reactionExergonic reaction:

Ks 1

pKs 0

<

>

Ks 1

pKs 0

>

<

Less products than reactants in EQ.

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The range of Ks and pKs The range of Ks and pKs

-10 0 10-2 2pKs

1010 1 10-10100 0.01Ks

exergonic endergonicEQ

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Predicting the direction of a reactionPredicting the direction of a reaction

Reaction quotient: INIINI Q = conc. of products

conc. of reactants(Initial concentrations)

(Equilibrium concentrations)Equilibrium constant:

EQEQKs =

conc. of productsconc. of reactants

if Q < Keq : reaction will happenif Q = Keq : EQif Q > Keq : reaction will happen

forward

reverse

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K K K

Q

Q

Q

forward reversereaction

K K K

Q

Q

Q

forwardforward reversereversereaction

K K K

Q

Q

Q

forward reversereaction

K K K

Q

Q

Q

forwardforward reversereversereaction

Difference between Ks and Qshows the direction of the reaction

Difference between Ks and Qshows the direction of the reaction

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Le Chatelier principle (1884)Le Chatelier principle (1884)

Dr. M. Sasvári: Medical Chemistry Lectures Dr. M. Sasvári: Medical Chemistry Lectures

Henri LeChâtelier (1850-1936) A French chemical engineer

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Start from

EQ

The system is in EQ

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There is an effect on the system to remove it from EQ

EQ

INI

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INI

The effect will be compensated

?Ks remains the same

NewEQ

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A B EQ EQ

Effect of changing the concentrationsEffect of changing the concentrations

more reactants are added

A B

or: products are removed

A B

INIINI

The system will compensate the effect

reaction goesforward

new EQnew EQ

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Effect of changing the pressure(gaseous reactions)

Effect of changing the pressure(gaseous reactions)

N2 + 3H2 2NH3

EQ EQ

4 vol 2 volvolume decreases in the forward reaction

INIINIPressure is increased

new EQnew EQ Compenzation: Reaction goes forwardsto decrease the pressure

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Effect of changing the temperatureEffect of changing the temperature

A B

EQ EQ

endothermic reaction (heat is absorbed during forward reaction)

INIINITemperature is increased

new EQnew EQ Reaction goes forwardsto decrease the temperature

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20 oC: blue

Endothermic reaction [Co(H2O)6]2+ + 4Cl-

pink[CoCl4]2- + 6H2O

blue

OoC: turns to pink

Example