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EQUILIBRIUMTOPIC 8 Review BookChapter 19 Textbook

Objectives

Reversible reactionsEquilibrium – classificationLe Chatelier’s Principle

Reversible ReactionsSome reactions occurs

simultaneously, forming and decomposing the product.

The two equations can be combined into one, by using a double arrow, which tells us that it is a reversible

reaction: 2SO2(g) + O2(g) ↔ 2SO3(g)

When the system reaches the equilibrium no net

change can be observed.

Equilibrium is a state in which there are no observable changes as time goes by.

Chemical equilibrium is achieved when:

• the rates of the forward and reverse reactions are equal

• the concentrations of the reactants and products remain constant

Chemical equilibrium

N2O4 (g)

14.1

2NO2 (g)

Physical equilibrium

Phase EquilibriumWhen a substance is changing phase

an equilibrium between phases can be established

H2O (s) H2O (l)

Physical Equilibrium

Solution Equilibrium

When a solution is saturated, if there is solid present, the solid solute and the solute in solution are at equilibrium.

The Concept of Equilibrium [A] = concentration of A [B] = concentration of B

As the reaction progresses–[A] decreases to a constant,–[B] increases from zero to a constant.

–When [A] and [B] are constant, equilibrium is achieved.

A B

Le Chatelier’s Principle

The French chemist Henri Le Chatelier (1850-1936) studied how the equilibrium position shifts as a result of changing conditions

Le Chatelier’s Principle

IF A STRESS (CHANGE) IS APPLIED TO A SYSTEM AT EQUILIBRIUM THE SYSTEM WILL REACT IN THE DIRECTION THAT COUNTERACTS THE STRESS.

In other words : the system is a REBEL!

Le Chatelier’s Principle What items did he consider to be

stress on the equilibrium?Concentration

Temperature

Pressure

Each of these will now be discussed in detail

Stress I - CONCENTRATION

Case 1 Increasing the concentration of

reactantsConsequence: more products will be

formed.Equilibrium shifts to the right

Case 2 – Increasing concentration of products

ConsequenceThe system will react to decrease the

concentration of products by forming more reactants.

Equilibrium will shift to the left or the reactants.

Case 3: Decreasing the concentration of reactants

ConsequenceThe system will react to increase the

concentration of reactants by using more products.

Equilibrium will shift to the left of the reactants

Case 4: Decreasing the concentration of products

Consequence: more products will be formed.

Equilibrium shifts to the right or the products will be favored.

This is a way to make a reaction go to completion. As products form they are taken away!

Stress II - Temperature• If temperature is increased the system will

favor the reaction that lowers the temperature (absorbing heat) then the ENDOTHERMIC RECTION will be favored.

• A decrease in temperature favors the exothermic reaction

• C + O2(g) → CO2(g) + 393.5 kJ

Increase in temperature favors endothermic reaction

Decrease in temperature favors exothermic reaction

ENDOTHERMIC APPARATUS

Air conditioners absorb heat of a room and they LOWER the temperature of the room they are in.

EXOTHERMIC APPARATUS

Heaters are exothermic, they release heat into a room and increase the temperature of the room they are in.

Stress III - Pressure• – Changes in pressure will only effect

gaseous equilibria.• Increasing the pressure will favor

side with the smaller volume• N2(g) + 3H2(g) ↔ 2NH3(g)

• this equilibrium shifts to the right with an increase in pressure because the product side occupies a smaller volume

• 4 mol of gas in reactants vs 2 in product

Effect of a Catalyst

The addition of a catalyst changes the rate of both forward and reverse reactions equally. It causes the equilibrium to be established more quickly but it does not change the equilibrium concentrations.

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

1.- Change : increase in [N2]What is the effect on the

concentration of

a. [ NH3 ]

b. [ H2 ]

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

2 Change: increase in temperatureWhat is the effect on the

concentration of

a. [ N2 ]

b. [ NH3]

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

3: Increase in pressureWhat is the effect on the

a. number of moles of N2

B. number of NH3

The Concept of Equilibrium Consider colorless frozen N2O4. At room temperature,

it decomposes to brown NO2:

N2O4(g) <- > 2NO2(g).

Clear brownAt some time, the color stops changing and we have a

mixture of N2O4 and NO2.

Challenge questionIs the forward reaction endo or exo?

The Concept of Equilibrium As the substance warms it begins to

decompose: N2O4(g) 2NO2(g)

When enough NO2 is formed, it can react to form N2O4:

2NO2(g) N2O4(g). At equilibrium, as much N2O4 reacts to form

NO2 as NO2 reacts to re-form N2O4

The double arrow implies the process is dynamic. N2O4(g) 2NO2(g)

Smog…

Demonstration- Do nowCopy reaction in your notes and predict what would be the consequence of each change.

Then record your observations

Co(H2O)6 2+ + 4 Cl- <--> CoCl4 2-+ 6H2O

Pink Blue

Change 1 – Increase [Cl-] by adding HCl

Change 2- Decrease [Cl-] by adding AgNO3

Co(H2O)6 2+ + 4 Cl- <--> CoCl4 2-+ 6H2O

Pink Blue

> 0 DH Endothermic Reaction! Change 3 – Increasing temperatureConsequence

Change 4 – Decreasing temperatureConsequence

Answers to MC questions handout Le Chatelier (8741)

1 B 2 B3 D4 B5 B

6 C 7 A8 D9 B 10 B

SAMPLE ANSWERS TO LE CHATELIER’S PRINCIPLE

1 Equilibrium shifts toward the fewer number of moles of gas . Or

The reaction shifts to the side with the smaller volume.

2. Removing CO2 shifts the equilibrium towards the right lowering [ H2CO3 ]

3.-An increase in Temperature favors the endothermic reaction, which produces SO2

4.- A higher [02] causes more collisions, more effective collisions form more product, decreasing [SO2]

6.- As T increases the solubility of the gas decreases, the gas comes out the soda.

7. As the pressure decreases the solubility decrease, and the gas comes out of the soda.

8.- Increasing temperature favors the endothermic reaction.

9.- The rate of dissolving KNO3 is equal to the rate of recrystallizing KNO3

10. a) If T increases N2 increases because the endothermic reaction will be favored.

b) If pressure is increased H2 will decreased, because the equilibrium moves to the side with the smaller volume.

C) No effect because the catalyst does not affect the equilibrium position. It makes both (forward and reverse reaction ) faster.

Review Book Answers P146

18) 219) 220) 421) 222) 323) 424) 325) 426) 1

27) 328) 129) 130) 331) 432) 133) 334) 335) 4

Review Book p 149 - Entropy

36) 137) 138) 239) 440) 241) 242) 243) 244) 3

ENDOTHERMIC REACTION

NH4SCN(s) + Ba(OH)2 (s) NH3(g)+ H2O(l) + Ba2

+(aq)+NH4 +(aq)

From solid to liquid entropy increases

Why does any change occur?

Factors that determine if a physical or a chemical change would take place:

Tendency to lower the energy of the system.

Changes tend to occur if they result in a system that is more stable. Stability implies low energy.

1.-Energy factor - ENTHALPY

All exothermic reactions are favorable in terms of energy, because their products have less energy than the reactants and therefore are more stable.

Then if D H < 0 the change is favorable in terms of energy

Delta H = Heat of reaction = Enthalpy D H

2.- Tendency to greater disorder.ENTROPY

There is a tendency in nature to change to a state of greater randomness or disorder.

Entropy is a measure of the disorder or randomness in a system.

A system with high entropy is “messy” and has a lack of structure.

Entropy = Disorganization, chaos, randomness

High entropy Randomness,

messiness

Low Entropy Organized, neat

Gas = high entropy

Liquid = less entropy

Solid = little entropy

Spontaneous change

A change that occurs naturally.When a change results in an stable

and more disorganized product the reaction will be spontaneous.

The tendencies that favor a spontaneous reaction are

Low enthalpy and high entropy.

Demonstration- write your observations in your notes

Fe3+ (aq) + SCN- (aq) <--> FeSCN2+ (aq) Pale yellow dark red

In your notebook predict what would happen to the equilibrium if we Decrease [Fe3+]