Gaseous Chemical Equilibrium

The Dynamic Nature of Equilibrium

A. What is equilibrium?• a state of balance; no net change in a dynamic process

1. Chemical equilibrium

No net change with the total amount of reactants and products remaining constant, while the reaction continues

Eek = equilibrium

2. General Characteristics• Double Headed Arrow

• Beginning of Rxn Form lots of products (before eek is established)

• Moments Later Forming both products and reactants (eek)

• **Not Necessarily equal proportions of both sides of the reaction!

B. Dynamic Equilibrium - Characteristics • two opposing processes occur at exactly the same rate

• No net change

• Dynamic Eek will not occur as soon as the reversible reaction begins, it takes time.

C. Graph

The N2O4 2NO2 Equilibrium System

D. The 2NO2N2O4 Equilibrium System

1. Description• Dynamic, reversible, no net change

2. Equilibrium Conditions• Specific for a Reaction (will be unique)

• MUST have the Balanced Written Equation

• Coefficients will matter!

• Temperature Dependent

• CONSTANT regardless of concentration

II. The Equilibrium Expression, Keq

(Equilibrium Law or Law of Mass-Action)

A. Writing Expressions for Keq

• aA + bB <-----> cC + dD

• Keq = [C]c [D]d _ for aqueous solutions use

[A]a [B]b Molarities

The Equilibrium Expression

Example: Write the Keq for:• 4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)

• N2(g) + 3H2(g) 2NH3(g)

B. Characteristics of Keq

1. Independent of :• Pure solids and pure liquids as long as some of the

substance is present

2. Dependent on:• Gaseous substances and solutions (aqueous)

C. The meaning of Keq

1. Keq = 1; reactants and products are present in equal amounts at eek

2. Keq > 1; products are present in greater proportion at eek

3. Keq < 1; reactants are present in greater proportion at eek

4. Examples:

N2(g) + O2(g) 2 NO(g) Keq=55

N2(g) + 3H2(g) 2NH3(g) Keq=5x10-6

III. Modifying Equilibrium Constant Expressions

A. Reversible reactions

1. Rule:• The Keq value for the reverse of a reaction will be

the reciprocal


Example: write the Keq for the following equation and its reverse

A(g) + B(g) C(g)


B. Summation of Reactions and modifying coefficients

Rule:

the Keq of summed reactions will be the product of the Keq’s; the factor you adjust coefficients by becomes the power

Example:

A(g) + B(g) C(g) Keq=3

A(g) + B(g) D(g) Keq=6

What is the Keq for C(g) D(g)

Example:

½ N2(g) +1/2O2(g) NO(g) Keq=6.9x10-16

NO2(g) NO(g) + ½ O2(g) Keq=6.7x10-7

N2O4(g) 2NO2(g) Keq=0.15

What is the Keq for N2(g)+2O2(g)N2O4(g)

C. Heterogeneous SystemsRule: solid and liquid substances = 1

Example: Write the Keq for:

Zn(s) + 2H+(aq) Zn2+

(aq) + H2(g)

CaCO3(s) CaO(s) + CO2(g)

IV. Determination of KA. Calculating Keq from experimental values.

Write the expression for the reversible reaction between solid ammonium chloride and gaseous products, hydrogen chloride and ammonia (NH3). At equilibrium in a 1 liter container, the following amounts are present: 12.0 mol ammonium chloride, 3.0 mol of ammonia and 5.0 mol of hydrogen chloride Determine the Keq.

B. Example When 4.29 moles of PCl3(g) and 4.29 moles of Cl2(g) are placed in a 1.00 Liter container at 250oC, the following equilibrium is established:

PCl3(g) + Cl2(g) PCl5(g)

The equilibrium concentration of phosphorus pentachloride is 2.59 mol/L. What are the equilibrium concentrations of the other two gases? Calculate Keq for the above reaction system.

C. Example: Consider the equilibrium system:

2NO(g) + Br2(g) 2NOBr(g)

At a given temperature, 1.6 mol of NO and 1.6 mol of Br2 are added to a 1.00 Liter flask and the equilibrium concentration of NOBr is found to be 0.53 M. Calculate the equilibrium concentrations of the other 2 gases and the value of Keq.

B. IRE Problems with unknown equilibrium concentrations

Description of problems• Define both the R and E lines in terms of “x”

2. Example: Carbon dioxide and hydrogen gas at concentrations of 1.00 M each are introduced into a container and the following system is established: CO2(g) + H2(g) CO(g) + H2O(g) Kc = 0.64

What are the equilibrium concentrations of all the species?

3. Example: For the system: H2(g) + I2(g) 2HI(g) at 425 oC, K = 55.5. If 2.5 atm of hydrogen and 2.5 atm of iodine are placed in a vessel and heated to 425oC and the system reaches equilibrium, what are the pressures of all species?

14.4 Qualitative Treatment of Equilibrium: Le Chatelier

A. Statement of Le Chatelier’s Principle

“When a system is stressed (changes in concentration, temp,

gas pressure, or volume of container), the system will respond by attaining new equilibrium conditions that

counteract the change”


B. For the system, N2O4(g) 2NO2(g) H = 57.2 kJ

1. Changes in amounts of species

a. Adding or removing reactant

• Add Reactant Eek shifts toward products (use it up)

• Remove Reactant Eek shifts toward reactants (make more)


b. Adding or removing product• Add Product Eek shifts toward reactants (use it up)

• Remove Product Eek shifts toward products (make more)

c. Adding or removing pure solid or solvent• Has NO effect on equilibrium (no shift)

d. Adding inert substance (inert = non-reactive)• Has NO effect on equilibrium (no shift)


2. Changes in pressure• Pressure increased Eek shifts in the direction producing

the smaller number of moles of gas

• Pressure decreased Eek shifts in the direction producing the larger number of moles of gas

• Moles of gas equal on both sides = pressure does not effect Eek.


3. Changes in volume• Decreases in volume = increase in pressure

• Increases in volume = decrease in pressure


4. Changes in temperature • Increase Temp Eek shifts in the direction of the

endothermic rxn; away from heat

• Decrease Temp Eek shifts in the direction of the exothermic rxn; toward heat


Example 14.12 List all of the ways to shift the equilibrium of the following systems to the right:

a. N2(g) + 3H2(g) 2NH3(g)


b. CO(g) + H2O(g) CO2(g) + H2(g)

End of Chapter 14!

Gaseous Chemical Equilibrium

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