Gas Phase Equilibrium - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/bojan/fa_lect/16_Chapt15_09.… · Mary J. Bojan Chem 110 1 Gas Phase Equilibrium Chemical Equilibrium

Mary J. Bojan Chem 110 1

Gas Phase Equilibrium Chemical Equilibrium Equilibrium Constant Keq Equilibrium constant expression Relationship between Kp and Kc Heterogeneous Equilibria Meaning of Keq Calculations of Kc

Solving Equilibrium Problems

USING AN EQUILIBRIUM CONSTANT Information obtained from Kc 1. Predict direction of a reaction Q ⇒ reaction quotien 2. Obtaining equilibrium concentrations of reactants and

products.


Equilibrium Equilibrium is achieved when opposing processes

proceed at equal rates.

equilibria previously discussed

Eg.


Chemical Equilibrium In chemical equilibria, forward and reverse reactions occur at

equal rates.

N2O4 (g) 2NO2(g)

colorless brown


Achieving equilibrium Initial State: reactants only Initial state: products only


Equilibrium Constant Equilibrium point of any reaction is characterized by

a single number.

Example: 2NO 2A N2O4 B

In general: jA + kB pR + qS


EXPRESSION for Keq

What is the equilibrium constant expression for the Haber process?

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


Equilibrium Constant The ratio of equilibrium concentrations will be constant:

Keq is the equilibrium constant for the reaction.

It is a NUMBER.

Keq (the number) DOES NOT depend on concentration

It’s a function of temperature only.

Notation:

Keq = Kc concentration […] is expressed in M (mol/L)

concentration is expressed using partial pressures in atm Keq = Kp


When given Keq how do I know if it is Kc or Kp?

Use context of problem.

•  If concentrations are given in M,

then Keq = Kc

•  If concentrations are given in P,

then Keq = Kp

where Δn = nprod − nreact

Relationship between Kp and Kc


Calculating Kp and Kc

Equilibrium concentrations of NH3, N2, and H2 were determined at 472°C.

[H2] = 0.1207M [N2] = 0.0402M [NH3] = 0.00272M

Find Kc and Kp.

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


Heterogeneous Equilibria involves reactants and products in more than one

phase

3Fe(s) + 4H2O(g) Fe3O4(s)+4H2(g)

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Keq =Fe3O4[ ] H2[ ]4

Fe[ ]3 H2O[ ]4What is [Fe]? [Fe3O4]?

Activities of pure solids and liquids = 1.


What is the equilibrium constant expression for these reactions?

AgCl(s)

C6H6(g) + 3H2(g)

Ag+(aq) + Cl−(aq)

C6H12(g).

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


Meaning of Keq Reactants Products

Is Keq large or small???

K ≈ 1


Problem

Which one of the following has the greatest tendency to proceed as written?

1. 4NH3(g) + 3O2(g) ⇔ 2N2(g) + 6H2O(g) Kp = 1×1022

2. N2(g) + O2(g) ⇔ 2NO(g) Kp = 5 ×10-31

3. 2HF(g) ⇔ F2(g) + H2(g) Kp = 1×10-13

4. 2NOCl(g) ⇔ 2NO(g) + Cl2(g) Kp = 4.7×10-4


Is Keq large or small?

Cu+2(aq) + 4NH3(aq) → Cu(NH3)4+2(aq)

Ni+2(aq) + 6NH3(aq) → Ni(NH3)6+2(aq)

PbI2(s) → Pb+2(aq) + 2I-(aq)


More Relationships

What is Keq if the reaction is doubled??

H2(g) + I2(g) C 2HI(g) Keq =

What is Keq for the reverse reaction?

2H2(g) + 2 I2(g) 4HI(g) B

Keq =

Keq =

H2(g) + I2(g) 2HI(g) A


At a certain temperature, Kc for the following reaction is 16.

H2(g) + I2(g)

½ H2(g) + ½ I2(g)

2HI(g)

HI(g)

At the same temperature, what is Kc for this reaction?

1.  1/16

2.  4

3.  1/4

4.  16

5.  there is not enough information to answer this question.


General Approach to Equilibrium Constant Problems

1.  Write the balanced reaction. 2.  Write the general form for Keq. 3.  Set up a data table:

(may need algebraic unknowns) -initial conditions -changes in concentrations -equilibrium concentrations

4.  Substitute equilibrium concentrations into the expression for Keq and solve.


Calculating Keq 1.00 mole of SO2(g) and 1.00 mole of O2(g) are added to a 1.00 L container and react until equilibrium is achieved. At equilibrium, the container has 0.919 moles of SO3(g).

Find Kc at 1000K.


Calculating Keq

3Fe(s) + 4H2O(g) initial

change

final

Fe3O4(s) + 4H2(g)

A one L container holds 224g of Fe and 5.00 mole of H2O(l). It is heated to 1000K and reaches equilibrium. 56g of Fe are left unreacted. What is Kc at 1000K?


Predicting the direction of a reaction

Note: the concentrations used are NOT equilibrium concentrations.

Reaction quotient ⇒ Q

aA + bB cC + dD

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Q =C[ ]c D[ ]d

A[ ]a B[ ]b


Predicting the Direction of a Reaction

•  When Q < Kc reaction moves to right (produces product)

•  When Q > Kc reaction moves to left (produces reactant)

When Q = Kc system IS at equilibrium


2HI(g)

If we put 0.1 mole of HI in a 1L container, what will happen? 1  reaction shifts to right → 2  reaction shifts to left ← 3  no change occurs

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Kc =H2[ ] I2[ ]HI[ ]2

=1.25 ×10−3

If the initial concentrations of all three gases in the vessel are 0.1 mol/L, what will happen?

H2(g) + I2(g)


Using Keq to obtain equilibrium concentrations of reactants and products.

Initially [IBr] = [I2] = [Br2] = 0.0500 M

What are the final concentrations of reactants and products? Kc = 2.50 × 10-3

2 IBr(g) Br2(g) + I2(g)


Haber process Industrial process used to make ammonia

N2(g) + 3H2(g) 2NH3(g) + heat


Factors that affect equilibrium What happens to a system at equilibrium when it is disturbed by changing

  concentration

  volume

  pressure

  temperature

Answer: Le Chatelier’s Principle

If a system at equilibrium is disturbed, the system will shift its equilibrium position to minimize the effect of the disturbance.


Le Chatelier’s Principle To illustrate this principle use the following chemical

system in a closed 1L container:

ΔHrxn = -92.4kJ/mol T = 700°C Initial conditions:

[N2] = 3.00 M [H2] = 4.50 M [NH3] = 0 At equilibrium:

[N2] = 2.00M [H2] = 1.50 M [NH3] = 1.41 M What is Kc?

N2(g) + 3H2(g) 2NH3(g) + heat


Use LeChatelier’s principle to answer the following questions

1.  What will happen to [NH3] if 3.0 moles of H2 are added to the system at equilibrium? Increase the amount of a reactant or product

2.  What will happen if the volume decreases from 1.0L to 0.5L? Increase P by decreasing volume

4.  What will happen if 5.0 moles of inert gas is added to the system? Increase pressure by adding an inert gas.

6.  What will happen if the temperature is increased to 1000K?


1. What will happen to [NH3] if 3.0 moles of H2 are added to the system at equilibrium?

System is no longer at equilibrium.

Which direction will it go to get to equilibrium?

N2(g) + 3H2(g) 2NH3(g) + heat

Using LeChatelier’s principle:

Reactant or product added to a mixture at equilibrium will cause reaction to shift in the direction that consumes part of the added material.


2. Which direction will the reaction proceed if the volume is decreased to 0.5L?

LeChatelier’s principle:

Since pressure is increased by disturbance, reaction will shift in direction that will reduce pressure

N2(g) + 3H2(g)

4 moles

2NH3(g)

2 moles


3. What happens when 5.0 moles of an inert gas are added?

Inert gas: gas that does not participate in the reaction. How does inert gas effect concentrations of reactants and products?

LeChatelier’s principle:

N2(g) + 3H2(g)

4 moles

2NH3(g)

2 moles


Examples

FeSCN+2(aq) Yellow colorless red

N2O2(g) colorless

Fe+3(aq) + SCN-(aq) Change concentration

Change pressure 2NO2(g) brown


  changing concentration (or V so that [ ] changes) puts a stress on the system.

  Stresses do not change Keq!

  Q changes; system shifts to re-establish equilibrium

Q→K

Summary

WHAT IF TEMPERATURE CHANGES? •  Keq changes •  change depends on whether the reaction is

exothermic or endothermic. ↑ ↑

-ΔH +ΔH


4. What happens when the temperature is increased to 1000K?

Treat heat like a product (exothermic) or reactant (endothermic).

ΔHrxn = −92.4 kJ/mol

N2(g) + 3H2(g) 2NH3(g) + heat

Le Chatelier’s Principle: heat is added to a system, the reaction will shift in the direction that absorbs heat Reaction is exothermic (heat is a product) so adding heat will cause reaction to shift to left.


4. What happens when the temperature is decreased?

heat + Co[H2O]62+ (aq) + 4Cl−(aq)

CoCl42− (aq)+ 6H2O()

pink blue

Treat heat like a product (exothermic) or reactant (endothermic).

Example: endothermic reaction


Summarize N2(g) + 3H2(g) 2NH3(g) + heat

Problem: rate of reaction increases as T increases, BUT equilibrium constant decreases at higher T.

To produce ammonia:

Do we want high or low temperature?

Do we want high or low pressure?

Liquefy ammonia as process proceeds. WHY?

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Gas Phase Equilibrium - Chemistry Coursescourses.chem.psu.edu/chem110/faculty/bojan/fa_lect/16_Chapt15_09.… · Mary J. Bojan Chem 110 1 Gas Phase Equilibrium Chemical Equilibrium