CHEMICAL EQUILIBRIUM

Prepared by

Prof. Odyssa Natividad R. Montoya-Molo

Equilibrium

• State at which there is “balance”

– Mechanical equilibrium

• Exists when the given system is static or in a state of unchanging motion.

• Ex: book on top of a table

– Thermal equilibrium

• Exist when “balance in heat” is achieved during heat transfer (higher to lower temperature)

• Ex: a spoon in a cup of coffee

– Chemical equilibrium

Chemical equilibrium

• The condition in which the concentrations of all reactants & products in a closed system cease to change with time.

• Occurs when opposing reactions are proceeding at equal rates: the rate at which the products are formed from the reaction equals the rate at which the reactants are formed into products.

Chemical equilibrium

• Satisfies the ff conditions:

1) The amounts of the R & P are observed to be constant after some time;

[R] = K1; [P] = K2

2) The rate of the product formed from the reactant is equal to the rate of the reverse reaction

Rateforward = Ratebackward

Reversible reactions (incomplete reaction)

• The forward reaction & reverse reaction both proceed but not completely.

• The resulting equilibrium mixture is composed of finite amounts of all the components.

• Concept was first discovered by French chemist Claude Louis Berthollet.

• A complete reaction is a reaction where no reactants remain in the mixture & the reverse reaction does not occur.

Equilibrium constant expression

• Law of mass action

– Cato Maximilian Guldberg & Peter Waage

– States that the composition of a reaction mixture can vary according to the quantities of components that are present.

• Given: aA + bB cC + dD

Rateforward = Ratebackward

kf [A]a[B]b = kb [C]c[D]d

Equilibrium constant expression

• Keq = KC = or KP =

– Keq is called equilibrium constant

• Law of Chemical Equilibrium– States that the product of the molar

concentration of the product raised to powers equal to their coefficients in the balanced reversible reaction divided by the product of the molar concentration of the reactants also raised to powers equal to their coefficients is a constant (equilibrium constant).

Characteristics of Equilibrium constant

1) Different reactions have different constant.2) At a given temp, the Keq for a reaction has a

definite numerical value.Ex: H2(g) + I2(g) 2 HI(g) Keq = 45.9 @ 763°C

3) A particular reaction has different Keq at different temperature.

4) The same point of equlibrium is reached whether the reaction starts with the R () or with the products ().

5) Magnitude of Keq: – Keq >> 1 equilibrium lies to the Right; P predominates– Keq << 1 equilibrium lies to the Left; R predominates

Categories/Types of Chemical Equilibrium

• Homogeneous equilibrium

–All components are in the same phase

–2 HI(g) H2(g) + I2(g)

• Heterogeneous equilibrium

– Involves more than one phase

–H2(g) + S(s) H2S(g)

Rules in writing equilibrium expression

1) [A], [B], [C], & [D]are molar concentrations in (M) in equilibrium. PA, PB, PC, & PD are equilibrium pressures in terms of atmosphere.

2) The products are always in the numerator, & the reactants are always in the denominator.

3) The coefficient of the R & the P in the balanced chemical equations corresponds to their exponents for the expression.

4) Compounds or molecules in a solid or liquid phase are ignored. They are not included in the expression.

5) Keq does not have a unit unless it is an expression for a ratio.

6) Only substances in a gas phase are expressed in the Kp

expression.

Application of Keq1) It can predict whether the proposed

reaction is probable or attainable.– Small Keq value implies that the P is

hardly attainable– Large Keq value implies that the P is

readily attainable

2) Keq can predict the direction of the reaction– When compared with Reaction Quotient

3) It can be used to calculate equilibrium concentrations

Reaction Quotient

• The ratio of product & reactant concentrations at any given point during the reaction.

• Expression is similar to Keq, except for non-equilibrium condition.

• Same rules with Keq in writing the expression.

QC = QP =

Relationship of Qeq & Keq

• Case 1: Q < K

–The reaction will proceed forward to approach equilibrium. Q will increase until it equals K.

• Case 2: Q = K

–The system is in equilibrium.

• Case 3: Q > K

–The reaction will proceed backward to approach equilibrium. Q will decrease until it equals K.

Other Ways to Manipulate Chemical Equation & Keq values

1) The Keq of a reaction in the reverse direction is the inverse of the Keq of the reaction in the forward direction.

2) The Keq of a reaction that has been multiplied by a number is the Keq

raised to a power equal to that number.

3) The Keq for a net reaction made up of two or more steps is the product of the Keq for the individual steps.

Sample Exercises

1) N2O4(g) 2 NO2(g) Keq = 6.46 @ 100°C

– 2 NO2(g) N2O4(g) Keq = 0.155 @ 100°C

2) 2 N2O4(g) 4 NO2(g) Keq = 41.7 @ 100°C

3) 2 NOBr(g) 2 NO(g) + Br2(g)Keq = 0.42

Br2(g) + Cl2(g) 2 BrCl(g) Keq = 7.2

2 NOBr(g) + Cl2(g) 2 NO(g) + 2 BrCl(g) Keq =

3.0

Le Chatelier’s Principle• Proposed by French chemist Henri-Louis Le

Chatelier• States that when stress (addition or

removal of heat, pressure, volume, and/or components) is applied to a system in equilibrium, the system will adjust by changing the amount of components in order to relieve the external force.

• When you make a change or disturbance to a system in equilibrium, the system shifts direction so as to oppose the change or disturbance.

Effects of Changes in the Equilibrium

CONDITION EFFECT

Temperature Increase in temperature shifts in the directionwhich absorbs heatExothermic: increase in T favors backwardreactionEndothermic: increase in T favors forwardreaction

Pressure Increase in pressure shifts in the direction whichforms less amount (# of moles) of gas

Volume Increase in volume shifts in the direction whichforms more amount (# of moles) of gas

Concentration Increase in concentration of one R shifts theposition of equilibrium to where less amount ofthat R is formed

Catalyst Using catalyst does not change the position ofequilibrium