valdosta state university experiment 3 le chatelier’s principle valdosta state university

Valdosta State University

Experiment 3Le Chatelier’s Principle


http://www.chemguide.co.uk/physical/equilibria/introduction.html#top


Purpose

Demonstrate Le Chatelier’s Principle in equilibrium reactions.



Background

Consider the reaction:

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

Once some amount of ammonia is produced a competing reaction starts:

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



Background

Both these reactions occur at the same time in the same vessel:

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



Background

Dynamic Equilibrium - A process in which no net change occurs in the concentration of the substances in two competing processes.



Background

• An equilibrium can be upset when the reaction conditions are changed.

• How it responds to this change in described by Le Chatelier’s Principle.



Background

Le Chatelier’s Principle – If a stress is applied to a chemical system at equilibrium, the equilibrium will shift to minimize the effect of the stress.



Background – Le Chatelier’s Principle

Types of Stress• Change of concentration of reactants. • Change of concentration of products.• Change of temperature of reaction.




Change of concentration of reactants.

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

• If H2 is added, the reaction shifts to the right, consuming the excess hydrogen and making more ammonia. In other words, the equilibrium shifts away from the added species.





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

• If additional nitrogen gas is added, the equilibrium will shift to the right. Again, the equilibrium shifts away from the added species.

• If ammonia is removed from the reaction the equilibrium will shift to the right. In this case, the equilibrium shifts toward the species that was removed.





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

• If one of the reactants was removed or the product concentration was increased the reaction would shift to the left. Again, if a reactant is removed, the equilibrium would shift toward the reactant that was removed (left), or if the concentration of the product was increased, the equilibrium will shift away from the added product.



Prelab

3.

Ca2+ (aq) + CO32- (aq) CaCO3 (s)

if Ca(NO3)2 (aq) is added

if CaCO3 (s) is added


Laboratory Reactions

Copper(II) – Ammonia equilibrium

CuSO4(aq) + NH3(aq) Cu(NH3)42+(aq) + SO4

2-(aq) MIX WELL!

Cu2+(aq) + 4 NH3(aq) Cu(NH3)42+(aq)

• The equilibrium can be shifted by adding an acid to remove the ammonia. In this case, think of this as removing the ammonia.

NH3(aq) + HCl(aq) NH4Cl(aq)




Silver Equilibrium

2 AgNO3 (aq) + Na2CO3(aq) Ag2CO3(s) + 2 NaNO3(aq)

2 Ag+(aq) + CO32-(aq) Ag2CO3(s)

• The equilibrium can be shifted by adding an acid to remove the carbonate.

2 HNO3(aq) + CO32-(aq) H2O(l) + CO2(g) + 2 NO3

-(aq)

ADD to the SAME test tube




Silver Equilibrium

Ag+(aq) + Cl-(aq) AgCl(s)• The equilibrium can be shifted by adding ammonia to

remove the silver ion.

Ag+(aq) + 2 NH3(aq) Ag(NH3)2+





Silver Equilibrium

Ag+(aq) + l-(aq) Agl(s)• The equilibrium can be shifted by adding a sulfide to

remove the silver ion.

2 Ag+(aq) + S2-(aq) Ag2S





Buffer Solution

A solution containing a weak acid (or weak base) and its conjugate (a salt).

• The important feature of this solution is that it resists large changes in pH.

• The solution contains an acid and a base to neutralize incoming acid and base.




Buffer Solution

HC2H3O2(aq) + H2O(aq) H3O+(aq) + C2H3O2-(aq)

pH

NaC2H3O2(aq) Na+ (aq) + C2H3O2-(aq)

• Addition of NaOH neutralizes the acetic acid, shifting the equilibrium.

NaOH(aq) + HC2H3O2(aq) H2O(l) + C2H3O2-(aq) + Na+(aq)




Buffer Solution

HC2H3O2(aq) + H2O(aq) H3O+(aq) + C2H3O2-(aq)

• Addition of HCl neutralizes the conjugate base (acetate ion), shifting the equilibrium.

C2H3O2-(aq) + HCl(aq) HC2H3O2(aq) + H2O(aq) + Cl-(aq)

• The changes in equilibrium are followed by monitoring the pH (H3O+ concentration).




• Prove that acetic acid and acetate make a buffer solution.– Test changes of pH using pure water



Co(H2O)62+ / CoCl4

2-

Co(H2O)62+ + 4 Cl-(aq) CoCl4-(aq) + 6 H2O(l)

• The equilibrium can be shifted by adding water to the reaction.

• The forward reaction is endothermic, therefore the addition of heat shifts the reaction to the right. Think of heat as being a reactant / product. If adding heat shifted the equilibrium to the right, then the heat is on the left side of the equilibrium, since the equilibrium shifts away from the added species




CrO42- / Cr2O7

2-

2 H+(aq) + 2 CrO42-(aq) Cr2O7

2-(aq) + H2O(l)

• The equilibrium shifts on the addition of ammonia. Remember that ammonia reacts with the H+.

• NH3(aq) + H+(aq) NH4+(aq)



Safety• The 6M and concentrated acids are corrosive. If you

spill any on your skin, wash the affected area for five minutes. If the acids get in your eyes, wash the eyes for ten minutes. Seek immediate medical attention!

• Concentrated ammonia (NH3) is corrosive. If you spill any on your skin, wash the affected area for five minutes. If the ammonia get in your eyes, wash the eyes for ten minutes. Seek immediate medical attention! The fumes of concentrated ammonia are very irritating. Use adequate ventilation (i.e. mini hoods).

• Potassium chromate is a strong oxidizing agent. If you spill some on your skin, wash the affected area for five minutes.

• Silver salts stain the skin brown upon exposure to light, especially UV light (i.e. like tanning beds).



Waste Disposal

• All waste materials should be placed in waste containers marked “Recovered Metals and Metal Ions”.

• Do not dispose of any reagents in the sink or the trash can.


valdosta state university experiment 3 le chatelier’s principle valdosta state university

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