EQUILIBRIUM & BUFFERS

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

The term equilibrium constant is represented by Keq. It is a ratio of the products to the reactants. 

           [Product ]
Keq = [Reactant]

If you have more product than reactant, the Keq will be greater than 1
If you have more reactant than product, the Keq will be less than 1.

The Keq tells you which way the reaction is going.

If we did a Keq calculation for the glucose-fructose reaction

           [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 ]

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.

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

Biochemically this is important. A ---> B ----> C

In the first part A ---> B, if some of B is taken away, more of A will be converted to B to keep the equilibrium constant, constant. So if B goes to C, C pulls off B and more A goes to B, thus driving the reaction.

IONIZATION OF WATER

K_eq can also be K_i or ionization constant or K_H2O for the ionization of water or K_a for the ionization of an acid or K_b for the ionization of a base.

Given the ionization of water below, determine the K_H20

HOH -----> H⁺ + OH^-

[H₂O] = 1 M [H⁺] = 1 x 10^-7 M

[OH^-] = 1 x10 ^-7 M

K _H2O = 1 x 10¹⁴

Remember pH is the negative log of the H ion concentration. Look above pH = 7

pH + pOH = 14

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

Equilibrium constants for acids.

Keq = Ka The equilibrium constant for an acid is referred to as Ka, the 'a' for acid.

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?

If pH is the negative log of [H⁺], what do you think pKa is? Fill in the pKa's for the acids above.

 

BUFFERS Chapter 8

DEFINITION - Solutions which are able to resist pH change when a small amount of acid or base is added.

Mixture of weak acids and their salts
Mixture of weak bases and their salts

Buffers come in buffer pairs
    Acetic acid/sodium acetate
    Ammonium Hydroxide/Ammonium Chloride

pH is neg log of H⁺ concentration
pKa = neg log of Ka

A relationship exists between pH and pKa

pH = pKa + log [salt]
                         [acid]

Henderson-Hasselbalch Equation
pKa's can be gotten from tables

                                        pKa

    acetic acid                   4.74
    lactic acid                    3.86
    formic acid                  3.77
    trichloroacetic (TCA)   0.70

Which of these is strongest? Rember it is - log of K

Also can have pKa's for bases. [ammonium]⁺ = pKa = 9.26

If we had the same concentration of salt and acid the pH would be the same as pKa.

conc of H⁺ = 1 M/liter
conc of salt = 1 M/liter

pH = pKa + log [salt]
                         [acid]

pH = pKa + log 1
                         1

            log 1 is 0
pH = pKa + 0
pH = pKa

I have 1000 ml of a solution that is 0.1 N acetic acid and 0.1 M sodium acetate. pKa of acetic acid is 4.74. What is the pH?

pH = pKa + log [salt]
                         [acid]

pH = 4.74 + .1
                  1000
                     1
                 1000

pH = 4.74 + log 1
pH = 4.74 + 0
pH = 4.74

1. What is the pH of a .1M formic acid (HC ) solution that contains 0.2 moles
                                                           OH
of sodium formate per liter? The pKa of formic acid is 3.75

2. Make a buffer of pH 5.00 using sodium acetate and acetic acid. pKa = 4.76.

What ratio of molar concentrations of sodium acetate, acetic acid will be

Purpose: maintain acid-base balance in living system

Buffer system of human body.

*****bicarbonate         -         carbonic acid
            HCO₃^-                             H₂CO₃

organic phosphates
proteins
lactic acid

Blood pH 7.4 (7.35 - 7.45)
                     <7.00 > 7.8 dead

Regulatory organs for pH maintenance
        1. lungs (respiratory)
        2. kidney (metabolic)

Lets look at *****                     CO₂ + H₂O ----> H₂CO₃
                                                                            carbonic acid

Where do we get CO₂? Metabolism of sugars, fats proteins for energy. There are CO₂ receptors in blood vessels that are sensitive to CO₂ concentration. If the level of CO₂ goes up the person breathes deeper and/or faster.

After generating CO₂ the CO₂ can
        1. be dissolved in blood as a gas
        2. react with H₂O to form carbonic acid H₂CO₃
         3. react with hemoglobin Hb
*** 4. be converted to a bicarbonate HCO₃^-

CO₂ + H₂O   <----->   H₂CO₃    <---->   H⁺    +    HCO₃^-
                                                     carbonic acid      bicarbonate

Without an enzyme (carbonic anhydrase) the following reaction would be very slow
                    H₂CO₃ -----> H₂O + CO₂

Muscles generate CO₂ which goes to HCO₃^-

HCO₃^- goes to the lungs and gets a H⁺ from Hb hemoglobin

HCO₃^- + H⁺ -----> H₂CO₃ ---------> CO₂ + H₂O
                          carbonic acid                  exhale

acidosis <7.4 build up of carbon dioxide
hypoventilation - respiratory - lung alteration of CO₂

alkalosis >7.4 low levels of carbon dioxide
hyperventialtion - respiratory - lung alteration of CO₂

Condition                                          CO₂              pH                      Causes

Respiratory                                                                                        shallow breathing Hypoventilation
acidosis                                                                                             (drugs) lung damage

Respiratory                                                                                        Hyperventilation (anxiety)
alkalosis                                                                                             High Temperature

Kidney Damage
Vomiting
Diarrhea
Ingestion of large amounts of H⁺ or (OH)^-

The major buffering system is the carbonic acid H₂CO₃ / bicarbonate HCO₃^-
The pKa of carbonic acid is 6.37
The blood pH is 7.3 to 7.4, so the bicarbonate HCO₃^- to carbonic acid H₂CO₃ is 10 to 1

Use the Henderson-Hasselbach equation to show this.

Normal concentrations in the blood are 0.025 M bicarbonate to 0.0025 M carbonic acid 
The second major buffer is hydrogen phosphate ion (HPO₄)^{- 2} and dihydrogen phosphate (H₂PO₄)^-
The ratio of H(PO₄)^-2 to H₂(PO₄)^- is 1.6 to 1

Wednesday, March 21, 2007 01:28:02 PM