Chemical Equilibrium

 Chemical Equilibrium

Given A + B -----> C

If all of A and B reacted to from C, there would be no A or B left in mixture. What really happens is

   A  +  B    <---->       C           <---->  means reversible

 some of C will reform A and B

º                      means reaction favored to the right.
           ² 

»                   means reaction favored to the left.
            ÷ 

A         <------->                                  B

glucose                                           fructose
reactant                                          product 

Start out with 100 moles of glucose in a flask. Glucose changes to fructose slowly. After a while we have 70 moles glucose and 30 moles fructose. The reaction has reached equilibrium.

We can express this equilibrium in terms of an equilibrium constant Keq.

[  ] means concentration in moles/liter

           [Product ]
Keq = [Reactant]             

          [fructose]
Keq = [glucose] 

          3 moles/liter
Keq = 7 moles/liter 

Keq = 0.43 or 4.3 x 10-1

Suppose we had A + B  <----->    C + D

           [product]
Keq = [reactant] 

 [C] [D]
Keq =   [A] [B] 

Suppose we had H2 + I2    <------>        2 HI 

   [HI]
Keq = [H2 ] [I2 ] 

Wait - we've got 2 HI

 [HI] [HI]                         [HI]2
Keq =   [H2 ] [I2 ]         =             [H2 ] [I2 ]                                               

Write the equilibrium constant expression for 

4 NH3    +   3 O2   <-------->   2  N2 + 6H2O 

Keq = _______________________________ 

Let's try one with real concentration       N2 + O2   <----> 2 NO
[N2] = .95 moles/liter
[O2] = .95 moles/liter
[NO] = .10 moles/liter 

[product]
Keq =   [reactant] 

 [NO]2
Keq = [N2 ] [O2 ]  

    (0.1)2                0.01  
Keq = (0.95)(0.95)   =   (0.9025)   = .011 

Keq = 1.1 x 10-2 

The Keq can let you know which way the reaction is going.

If Keq is greater than 1             favors product
If Keq is less than 1                  favors reactants
How about the problem we just did? Does it favor the formation of product or reactant?
If we get 106 value for Keq reaction goes to completion.
If we get 10-6 value for Keq reaction doesn't go. 

Le Chatelier's Principle

If a chemical system in equilibrium is disturbed, the system will readjust to restore equilibrium. What changes equilibrium? 

1. Change in concentration of R or P     same Keq
            2. Change in temperature    changes Keq 

If change in reactants and products, the other concentrations will change to keep Keq constant. If temperature changes the Keq changes. 

N2 + 3 H2    <---------> 2 NH3

  Keq =     [NH3]2  
              [N2] [H2]3 

Suppose we added extra H2, to keep Keq constant [N2] would have a decrease and [NH3] increase. 

Change in temperature
Changes Keq

2  CO2 + heat   <----->            2CO + O2 

 adding more heat shifts the reaction to the right and Keq goes up 

N2 + 3 H2   <----->      2 NH3 +  heat 

which way does the reaction shift? Keq goes ___________________ 

Energy Changes

Give off  heat      exothermic
Need heat           endothermic
Calorimeter - measures increase in water temperature 

NaOH + HCl -------> NaCl + HOH     gives off heat 

 

Keq can also be Ki or ionization constant or KH2O for the ionization of water or Ka for the ionization of an acid or Kb for the ionization of a base. 

Given the ionization of water below, determine the KH20 

HOH  -----> H+   +  OH- 

[H2O] = 1 M
[H+]  = 1 x 10-7 M
[OH-] = 1 x10 -7 M

    ACIDS, BASES, SALTS 

Acids are compounds which donate a hydrogen ion. (Arrhenius)
Bases are compounds which donate a hydroxyl ion.
Bases are compounds which accept a hydrogen ion. (Bronsted-Lowry)
 

Disassociation

Breaking down of acids and bases to ions the stronger the acids, the greater the dissociation
            HCl  + HOH -------> H+ + Cl- + HOH                        Hydrochloric = HCl
            H3CCOOH + HOH -------> H+CCOO- + HOH         Acetic acid - H3CCOOH 

HCl is strong - completely H+ and Cl- in 1 M solution
H3CCOOH is weak - only .4% disassociates in 1 M solution  

Classification of Acids

Strong
HCl* - hydrochloric acid
H2SO4* - sulfuric acid
HNO3* - nitric acid 

Intermediate
H3PO4* - phosphoric acid 

Weak
H3CCOOH* - acetic acid - vinegar
H3C6H8O7 - citric - orange juice
H3BO3 - boric
*Memorize

Strength - EASE of ionization of H+
                Concentration - number of molecules per volume - moles/liter 

Ionizable Hydrogens
HCl ------> 1H+
H2SO4 ------> 2H+ Both H's are ionizable. 

This does not apply to organic acids (orgainc chemistry is carbon chemistry). 

  H      O                                    H      O         
              HC - C                                   HC - C
              H      OH                                  H      O-
                  Acetic Acid                              Acetate  Only H which can ionize off

Equilibrium constants for acids.

Keq = Ka 

HF + H2O   <------->  H+  + F - 

[HF]  = 0.0921 M 

[H=] = 7.9 x 1o-3 M 

[F- = 7.9 x 10-3 M

Calculate the Ka for this acid

The stronger the acid the higher the Ka.  We can have Keq for bases, they are referred to as Kb

Acids                               Ka

Acetic                          1.8 x 10-5
Carbonic                      4.3 x 10-7
Sulfurous                      1.3 x 10-2

Which is the strongest acid?     

Wednesday, March 21, 2007 01:28:03 PM