Bài giảng hoá phân tích Le chatelier’s principle

If we assume that the reaction is an elementary reaction, the forward reaction rate which describes how quickly A forms B has the mathematical form The reverse reaction rate which descri

Le Chaˆtelier’s Principle

P U R P O S E

Observe Le Chaˆtelier’s principle in action as chemical systems at equilib-rium respond to different stresses

I N T R O D U C T I O N

Chemical reactions attain a reaction rate that depends upon the nature and concentration of the reactants and the reaction temperature For a given reaction performed at a constant temperature, the reaction rate depends solely

on the concentrations of the species To understand chemical equilibrium, we must realize that a chemical reaction involves two opposing processes: the reaction in the forward direction in which the reactants react to form the products, and the reaction in the reverse direction in which the products react

to form reactants For example, consider the hypothetical reaction

where a and b represent the stoichiometric coefficients and A and B rep-resent the reactants and products involved in the reaction If we assume that the reaction is an elementary reaction, the forward reaction rate (which describes how quickly A forms B) has the mathematical form

The reverse reaction rate (which describes how quickly B reforms A) has the mathematical form

Notice that the reaction rates depend on the concentrations of each species Thus, if the concentrations are changed, the rates of formation of the products and reactants also change

At equilibrium, the forward reaction rate equals the reverse reaction rate Externally, it appears that nothing is happening in chemical reactions

at equilibrium However, if we could see the atoms, ions, or molecules

E X P E R I M E N T 24

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involved in a reaction at equilibrium, they are far from static Reactants are forming products and products are forming reactants at the same rate

It should be noted that all chemical reactions, even those that ‘‘go to completion’’, attain equilibrium In those cases, the product equilibrium concentrations are very large compared to the reactant equilibrium concentrations

Because the forward and reverse reaction rates are equal, we can set

Eq 2 equal to Eq 3 and derive the equilibrium constant expression

rateforward¼ ratereverse

kf½Aa ¼ kr½Bb

kf

kr ¼½Bb

½Aa

Kc¼ ½Bb

½Aa

ðEq: 4Þ

tem-perature, their ratio, kf/kr, is also a constant This constant, Kc, is the called

con-centrations, raised to their stoichiometric powers, divided by the reactant concentrations raised to their stoichiometric powers

For a more complex reaction, such as the hypothetical reaction given in

Eq 5

the equilibrium constant expression is written as

Kc¼½Cc½Dd

reaction occurs

concentrations

concentrations

concen-trations predominate Changes (stresses) that affect a reaction rate will also affect reactant and product equilibrium concentrations Le Chaˆtelier’s principle states that a system at equilibrium changes in a manner that tends to relieve the stress placed on the system Stresses that disturb a reaction at equilibrium include changes in concentration, changes in the reaction temperature, or changes in the pressure or volume (for gaseous reactions) These stresses preferentially affect the rate of either the forward or the reverse reaction The forward and reverse reaction rates are unequal until the reaction can reestablish equilibrium For example, if the reactant concentrations are increased, the forward reaction rate exceeds the reverse reaction rate and the equilibrium shifts to the right (product side) If the product concentrations are increased, the

reverse reaction rate exceeds the forward reaction rate and the equilibrium shifts to the left (reactant side)

Effect of Concentration

Changes on Systems at

Equilibrium

Assume that the reaction shown below is at equilibrium in a closed reac-tion vessel

case, the stress applied to the equilibrium initially increases the

more NO and the equilibrium shifts to the right (favors the forward

concen-tration increases until a new equilibrium is established

What happens to the equilibrium if more NO is added to the reaction vessel? The stress applied to the equilibrium is an increase in the

equilibrium shifts to the left (the reverse reaction is favored) The NO

reestablish equilibrium

What happens to the equilibrium if some NO is removed from the equilibrium system? The stress applied to the equilibrium is a decrease in

that was removed from the system The equilibrium shifts to the right, favoring the forward reaction

In Part A of this experiment, we will study the effects of changing reactant and product concentrations in an aqueous chemical system at equilibrium One reaction that visually illustrates Le Chaˆtelier’s principle is

(SbOCl) precipitates according to Equation 8

white precipitate

By adding either distilled water or hydrochloric acid and monitoring the presence or absence of the precipitate, we can illustrate the effects of changing the reactant and product concentrations on an equilibrium system

Effect of Changing pH on a

Complex Ion Equilibrium Most d-transition metals form complex ions in aqueous solution Thesecomplexes tend to be brightly colored The dissolution of cobalt(II) nitrate

in water produces a pink colored solution from the formation of the

blue colored in solution We will use color changes (pink to blue and vice versa) to study the effects of changing the pH of the equilibrium mixture shown in Equation 9

CoðOH2Þ62þðaqÞ pink

þ 4 ClðaqÞÐ CoðClÞ42ðaqÞ

blue

Effect of Changing Reaction

Temperature on Equilibrium

Changes in concentration, pressure, or volume, for gas phase reactions, shift the position of an equilibrium system, but do not change the value of the equilibrium constant A change in the reaction temperature not only shifts the equilibrium, it also changes the numerical value of the equilib-rium constant

Consider the following exothermic reaction at equilibrium

Because the reaction is exothermic, heat is a product of the reaction Increasing the reaction temperature has the same effect as increasing the concentration of C or D The equilibrium responds by shifting to the left (favors the reverse reaction) The additional heat is absorbed by C and D and they react to produce A and B The concentrations of A and B increase while the concentrations of C and D decrease until equilibrium is rees-tablished Lowering the reaction temperature shifts the equilibrium to the right (favors the forward reaction) A and B react to produce C and D and

to replace the heat that is removed when the reaction temperature is lowered The concentrations of C and D increase while the concentrations

of A and B decrease until equilibrium is reestablished

Endothermic equilibrium reactions absorb heat as represented by Equation 11

Because heat is a reactant in endothermic reactions, increasing the reaction temperature has the same effect as increasing the concentration of A or B The equilibrium responds by shifting to the right (favors the forward reaction) The additional heat is absorbed by A and B and they react to produce C and D The concentrations of C and D increase while the con-centrations of A and B decrease until equilibrium is reestablished Low-ering the reaction temperature shifts the equilibrium to the left (favors the reverse reaction) C and D react to produce A and B and to replace the heat that is removed when the reaction temperature is lowered The concen-trations of A and B increase while the concenconcen-trations of C and D decrease until equilibrium is reestablished

We can summarize the effects of changing the reaction temperature of

a system at equilibrium as follows:

For exothermic reactions increasing the reaction temperature favors the reverse reaction decreasing the reaction temperature favors the forward reaction For endothermic reactions

increasing the reaction temperature favors the forward reaction decreasing the reaction temperature favors the reverse reaction

In Part C of this experiment, we will reexamine the reaction presented

in Eq 9 for the effects of changing reaction temperature From the color changes, we can determine if this is an exothermic or endothermic reaction

P R O C E D U R E

C A U T I O N

Students must wear departmentally approved eye protection while performing this experiment Wash your hands before touching your eyes and after completing the experiment.

Part A ^ Effect of

Concentration Changes on

Systems at Equilibrium

distilled water to a 50-mL beaker Stir the mixture with a stirring rod Should you record your observations in the Lab Report?

C Chemical Alert

Concentrated HCl is extremely corrosive Do not allow it to contact your skin If it does contact your skin, wash the affected area with copious quantities of water and inform your laboratory instructor.

Do not inhale concentrated HCl vapors Perform this experiment in a hood or well-ventilated area.

stirring, until you observe a chemical change Should you record your observations in the Lab Report?

Justify your answer based on your observations from the previous step

stirring, until you observe a chemical change Should you record your observations in the Lab Report?

the previous step

Part B ^ Effect of Changing

pH on a Complex Ion

Equilibrium

tube 1 until you observe a chemical change Should you record your observations in the Lab Report?

change after the addition of 12 M HCl? Justify your answer based on your observations from the previous step

you observe a chemical change Should you record your observations

in the Lab Report?

14 How did the relative concentrations of Co(OH2)62þ and CoCl42

on your observations from the previous step

Part C ^ Effect of Changing

Reaction Temperature on an

Equilibrium System

solution and 2 mL of 12 M HCl solution to test tube 2 Why is HCl added to test tube 2? Should you record the color of the solution in the Lab Report?

Step 21

aqueous cobalt(II) chloride solution to each of the test tubes Add 12 M HCl drop-wise to each test tube until the solutions turn purple The

C

Chemical Alert

Note if too much HCl is added, the solution from Step 18 will turn blue If that happens, pour the solutions into the Waste Container and repeat the process.

mL of water to the beaker Place one of the test tubes from Step 18 into the ice bath for 10 minutes Remove the test tube from the ice bath Should you record the color of the solution in the Lab Report?

Step 18 into a 400-mL beaker Heat the beaker and test tube in a microwave for 15 seconds Remove the test tube from the boiling bath Should you record the color of the solution in the Lab Report?

If a microwave oven is not available, add 200 mL of water to a

400-mL beaker Place the beaker on a hot plate and bring the water to a gentle boil Place the remaining test tube from Step 18 into the boiling water bath for 5 minutes Remove the test tube from the boiling bath Should you record the color of the solution in the Lab Report?

tubes from Step 16 Based upon your observations, is this reaction endothermic or exothermic? Justify your answer with an explanation

waste container

Name Section Date

Instructor

24 E X P E R I M E N T 2 4

Lab Report

Part A – Effect of Concentration Changes on Systems

at Equilibrium

and SbOCl increase or decrease? Justify your answer based on your observations from the previous step

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How does the addition of H2O affect the equilibrium? How did the relative concentrations of SbCl3,

the previous step

Part B – Effect of Changing pH on a Complex Ion Equilibrium

How did the addition of 12 M HCl affect the equilibrium?

Justify your answer based on your observations from the previous step

Part C – Effect of Changing Reaction Temperature on an Equilibrium System

Why is HCl added to test tube 2?

Observations of CoCl2 þ 12 M HCl solution in boiling water bath

Based on your observations, is this reaction endothermic or exothermic? Justify your answer with an explanation

Name Section Date

Instructor

24 E X P E R I M E N T 2 4

Pre-Laboratory Questions

equilibrium reaction in Question 1

321

5 Consider the following system at equilibrium.

brown

Ð N2O4ðgÞ colorless

explanation

Name Section Date

Instructor

24 E X P E R I M E N T 2 4

Post-Laboratory Questions

your answer with an explanation

Justify your answer with an explanation

323

4 Some inexpensive humidity detection systems consist of a piece of paper saturated with Na2CoCl4 that changes color in dry or humid air What color is the piece of paper in dry air? What color is the piece of paper in humid air? Justify your answer with an explanation