08.26.01 10:05 pm 1 15.3 lechatelier and chemical direction 15.3 lechatelier predicting direction of...

08.26.01 10:05 PM1 15.3 LeChatelier and Chemical Direction

15.3 15.3 LeChatelier LeChatelier

Predicting Direction Predicting Direction of a Chemical of a Chemical

ReactionReaction

Two equal-weight boys at equilibrium Two equal-weight boys at equilibrium on a teeter totter. The boy at the right on a teeter totter. The boy at the right is given a five-pound weight thereby is given a five-pound weight thereby disturbing the equilibrium. The boy disturbing the equilibrium. The boy on the left scoots farther back on the on the left scoots farther back on the teeter totter to restore the equilibrium. teeter totter to restore the equilibrium.


Reaction Quotient (1)Reaction Quotient (1)For reactions not yet at equilibrium, the Law of Mass action yield information in terms of the Reaction Quotient.

Consider the following chemical process not at equilibrium.

aA + bB rR + pPA reaction quotient expression can be written:

Where the numerical value of Q will determine the direction the reaction will proceed.

Q < Keq Reaction shifts to Right

Q > Keq Reaction shifts to Left

Q [R]r[P]p

[A]a [B]b


Consider, T =532 °C, Kc = 0.19 what direction will the reaction proceed if the initial concentration is change to-

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

i 0.30 M 0.20 M 0.10 M

Q = 4.2 > Kc=0.19

Reaction Quotient (2): Reaction Quotient (2): ExampleExample

Q 0.10M 2

0.30M 0.20M 3 4.2

0.1

9

Kc

4.

2

QDirection of Reaction: Proceeds to the left


LeChatelier Principle: A LeChatelier Principle: A ReviewReview

Teeter•TotterAt Equilibrium

Stress applied

Self Adjust

Re-establish Equilibrium


Equilibrium: Stress / Relief on Equilibrium: Stress / Relief on ReactantReactantAltering Chemical ConcentrationsAltering Chemical Concentrations

Stress on Reactant, Rxn shift right Relief on Reactant, Rxn shift left


Equilibrium: Stress / Relief on Equilibrium: Stress / Relief on ProductProduct Altering Chemical ConcentrationsAltering Chemical Concentrations

Stress on Product, Rxn shift left Relief on Product, Rxn shift right


Exothermic Heats of SolutionExothermic Heats of Solution

Heating a solution in which the Hsoln is exothermic (Energy is a product) results in a shift of the reaction to the left, or more solute precipitating out of solution.

Exothermic Process. Exothermic Process. Energy is a productEnergy is a product


Endothermic Heats of SolutionEndothermic Heats of Solution

Heating a solution in which the Hsoln is endothermic (Energy is a reactant) results in a Shift of the reaction to the right, or more solute dissolving in solution.

Endothermic Process. Endothermic Process. Energy is a reactantEnergy is a reactant


Solubilities of Solids Vs TemperatureSolubilities of Solids Vs Temperature

Solubilities of several Solubilities of several ionic solid as a function ionic solid as a function of temperature.of temperature.

Some salts have negative Some salts have negative enthalpy of solution, enthalpy of solution, (exothermic process) i.e., (exothermic process) i.e., CeCe22(SO(SO44))33 and they and they

become less soluble with become less soluble with increasing temperature. increasing temperature.


2ii) Temperature & Solubility: 2ii) Temperature & Solubility: GasesGases

Temperature - (Gas)(Gas)Consider the extent in which O2 or CO2 dissolves in water. What are the conditions which will increase the solubility of gas in water.

[Solute][Solute] [Solute][Solute]

gas SolutionAs the temperature increase, both solute and solvent will be moving faster, the gas solute however will now have enough energy to leave the liquid interface.


Gas solute; Exothermic Gas solute; Exothermic HHsolnsoln

As the temperature As the temperature increase, both solute and increase, both solute and solvent will be move solvent will be move faster. The gas solute faster. The gas solute however will now have however will now have enough energy to leave the enough energy to leave the liquid interface because liquid interface because IMF can be overcomeIMF can be overcome


Disaster: (1700 dead)Disaster: (1700 dead) from Gas Solubility from Gas Solubility

In the African nation of Cameroon in 1986 a huge bubble of CO2 gas escaped from Lake Nyos and moved down a river valley at 20 m/s (about 45 mph). Because CO2 is denser than air, it hugged the ground and displaced the air in its path. More than 1700 people suffocated. The CO2 came from springs of carbonated groundwater at the bottom of the lake. Because the lake is so deep, the CO2 mixed little with the upper layers of water, and the bottom layer became supersaturated with CO2. When this delicate situation was changed, perhaps because of an earth-quake or landslide, the CO2 came out of the lake water just like it does when a can of soda is opened.

In the African nation of Cameroon in 1986 a huge bubble of CO2 gas escaped from Lake Nyos and moved down a river valley at 20 m/s (about 45 mph). Because CO2 is denser than air, it hugged the ground and displaced the air in its path. More than 1700 people suffocated. The CO2 came from springs of carbonated groundwater at the bottom of the lake. Because the lake is so deep, the CO2 mixed little with the upper layers of water, and the bottom layer became supersaturated with CO2. When this delicate situation was changed, perhaps because of an earth-quake or landslide, the CO2 came out of the lake water just like it does when a can of soda is opened.

Lake Nyos in Cameroon, the site of a natural disaster. In 1986 a huge bubble of CO2 escaped from the lake and asphyxiated more than 1700 people.


The effect of a Change in TemperatureThe effect of a Change in Temperature in terms of Reaction Quotient and LeChatelierin terms of Reaction Quotient and LeChatelier

In and In and endothermicendothermic process, energy is a reactant and an increase in temperature process, energy is a reactant and an increase in temperature results in a shift of the reaction to the right. When the temperature is decreased the results in a shift of the reaction to the right. When the temperature is decreased the reaction shifts to the left.reaction shifts to the left.

In and In and exothermicexothermic process, energy is a reactant and an increase in temperature process, energy is a reactant and an increase in temperature results in a shift of the reaction to the left. When the temperature is decreased the results in a shift of the reaction to the left. When the temperature is decreased the reaction shifts to the right.reaction shifts to the right.

The question is raisedThe question is raised, why does the reaction adjust itself when according to the , why does the reaction adjust itself when according to the Mass Action Expression, the concentrations of chemicals are not altered with Mass Action Expression, the concentrations of chemicals are not altered with temperature change? What causes the Mass Action Expression temperature change? What causes the Mass Action Expression k keqeq

Consider an Endothermic reaction: E + R Consider an Endothermic reaction: E + R P: K P: Keqeq = [P] / [R] . If the = [P] / [R] . If the

temperature is raised, the reaction shifts to the right, (Q < Ktemperature is raised, the reaction shifts to the right, (Q < Keqeq ))

A shift to the right means that [P] = A shift to the right means that [P] = will raised) and [R] = will raised) and [R] = (will lowers). This (will lowers). This will only occur if the [P] / [R] (or Q) is now less than Kwill only occur if the [P] / [R] (or Q) is now less than Keqeq @ new temperature. In @ new temperature. In

other words when there is a change in temperature, equilibrium is disturb, the other words when there is a change in temperature, equilibrium is disturb, the current [P] / [R] ratio =is still equal to the old Keq which is now Q (the reaction current [P] / [R] ratio =is still equal to the old Keq which is now Q (the reaction quotient) and the reaction shifts to re-establish equilibrium; to attain the new Kquotient) and the reaction shifts to re-establish equilibrium; to attain the new Keqeq..


Temperature Effect and Reaction QuotientTemperature Effect and Reaction Quotient

Direction

Reaction

When the temperature is raised for an endothermic reaction, the Keq constant changes. Because the current concentrations yields a reaction quotient less than Keq (new) the reaction must shift to the right.

Direction

Reaction

When the temperature is raised for an exothermic

reaction, the Keq constant changes. Because the current

concentrations yields a reaction quotient greater

than Keq (new) the reaction

must shift to the left.)

KKeqeq

@ New

Temperature

Endothermic Rxn: Increase in Temperature

Exothermic Rxn: Increase in Temperature

K eq(old Temp)

= Q

K eq(old Temp)

= Q


Temperature Effect onTemperature Effect on [Co(H [Co(H22O)] O)] [CoCl [CoCl44]]2-2-

Direction

Reaction

Consider the reaction: H > 0H > 0

[Co(H[Co(H22O)]O)] + 4Cl- + 4Cl- [CoCl[CoCl44]]22-- ++ 6H6H22OO

pinkpink blueblue

In this experiment when the solution was placed in cold water, the In this experiment when the solution was placed in cold water, the solution turned vivid pink. The pink color indicates a shift to the left solution turned vivid pink. The pink color indicates a shift to the left for the reaction shown above. This will only occur if the new Kfor the reaction shown above. This will only occur if the new Keq eq at the at the

instant the temperature is altered, is now lower than the old Kinstant the temperature is altered, is now lower than the old Keqeq, (which , (which

is now called Q). Therefore the Product concentration is decreased, is now called Q). Therefore the Product concentration is decreased, the reactant concentration increase as the reaction adjust itself so that the reactant concentration increase as the reaction adjust itself so that the Mass action equals Kthe Mass action equals Keqeq: K: Keqeq Q Q

K eq(old Temp)

= QK eq (new Temp)


3 i) Pressure on Solubility:3 i) Pressure on Solubility: Solids / Liquid Solids / Liquid

Pressure - (Solid and Liquid)(Solid and Liquid)The solubility of solids and liquids are hardly affected by pressure.

Solids and liquids are already very close to each other. An increase in pressure will not affect solubility.


3 ii) Pressure on Solubility: 3 ii) Pressure on Solubility: GasGas

Pressure - (Gas)(Gas)Solubility of gas is greatly affected by pressure

Gas solutes are very sensitive to pressure. Because gas particles are separated by void space, an increase in pressure will increase the solubility of the gas.

Divers must be careful when diving to great depths because the potential of dissolved N2 gas in the blood will lead to the bends.


Pressure Affect:Pressure Affect:Teeter Totter AnalogyTeeter Totter Analogy

In utilizing LeChatelier Principle to determine the direction of solubility for a gaseous solute with variation in pressure, the first thing that must be establish is which side is more sensitive to pressure. In our case the gas is more sensitive than the solution.

[Solute][Solute] [Solute][Solute]gas Solution

Pressure Sensitive


The effect of a Change in PressureThe effect of a Change in Pressure in terms of Reaction Quotient and LeChatelierin terms of Reaction Quotient and LeChatelier

For a chemical reaction, LeChatelier Principle can be verified in terms of the Reaction For a chemical reaction, LeChatelier Principle can be verified in terms of the Reaction Quotient. Consider the reaction: NQuotient. Consider the reaction: N22 + 3H + 3H22 2NH 2NH33

Let [HLet [H22] = 0.1207 M, [N] = 0.1207 M, [N22] = .0402 M and [NH] = .0402 M and [NH33] = 0.00272 M for a 1-L vessel.] = 0.00272 M for a 1-L vessel.

In which direction will the reaction shift if the reaction vessel is decreased by half such In which direction will the reaction shift if the reaction vessel is decreased by half such that V = Vthat V = Voo/2. The concentrations will now double for all specie at the instant the volume /2. The concentrations will now double for all specie at the instant the volume

is decreased. Molarity = mol / L [His decreased. Molarity = mol / L [H22] = 0.2414 M, [N] = 0.2414 M, [N22] = .0804 M and [NH] = .0804 M and [NH33] = 0.00544 ] = 0.00544

M . Plugging in to the mass action expression and solving for Q, M . Plugging in to the mass action expression and solving for Q,

In the reaction quotient equation Q < Keq which means that in order to regain equilibrium, In the reaction quotient equation Q < Keq which means that in order to regain equilibrium, the product must increase and the reactant decrease, a shift to the right in the overall the product must increase and the reactant decrease, a shift to the right in the overall reaction.reaction.

Q [NH3]

2

[H2 ]3 [N 2 ]

[5.44 • 10-3 ]2

[.2414]3 [.0804]0.26 • 10 2

Keq [NH

3]2

[H2

]3 [N2

] [2.72•10-3]2

[.1207]3 [.0402]0.103

Direction

Reaction KKeqeq = 0.103Q= .026


Summary of Pressure, Temperature Summary of Pressure, Temperature Affect on SolubilityAffect on Solubility

H (s, l or g) Temp Direction Solubility

(+) Endothermic Prod increase

(+) React decrease

(-) Exothermic React decrease

(-) Exothermic Prod increase

Pressure Direction Solubility

Gas solute Prod increase

Gas solute React decrease

08.26.01 10:05 pm 1 15.3 lechatelier and chemical direction 15.3 lechatelier predicting direction of...

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