Raw Notes

Chemical Equilibrium (Dynamic)

Focusing on stoichiometry and the nature of chemical reactions.

1. Introduction to Reaction Types

Equilibrium is a critical state for many chemical and biochemical reactions.

  • Irreversible Reactions: Proceed in one direction until a limiting reactant is consumed.
    H2 + ½ O2 → H2O
  • Reversible Reactions: Products can convert back into reactants.
    A + B ⇌ C + D

2. Defining Dynamic Equilibrium

Equilibrium is reached when the rate of the forward reaction equals the rate of the reverse reaction. At this point, the concentrations of reactants and products stop changing.

Gas Phase Example:
N2O4(g) ⇌ 2NO2(g)
At equilibrium, if PNO2 ≈ 2.0 mm Hg and PN2O4 ≈ 280 mm Hg:
Kp = (PNO2)2 / PN2O4 = (2.0)2 / 280 ≈ 0.014.

3. The Equilibrium Constant (K)

The mathematical relationship between the amounts of reactants and products.

For: aA + bB ⇌ cC + dD

K = [C]c[D]d / [A]a[B]b
  • K is dimensionless (no units).
  • Pure solids (s) and liquids (l) are omitted from the expression.
Worked Example: 2NO + Cl2 ⇌ 2NOCl
[NO] = 0.050 M; [Cl2] = 0.015 M; [NOCl] = 0.500 M
K = (0.500)2 / [(0.050)2(0.015)] = 6.7 x 104
Result: K >> 1, products are strongly favored.

4. Complex Ion Equilibrium (Experimental Data)

Determination of K for Iron(III) Thiocyanate formation.

Fe3+ + SCN− ⇌ FeSCN2+
Species Initial [M] Change [M] Equilibrium [M]
Fe3+ 1.00 x 10-3 - 4.5 x 10-5 9.55 x 10-4
SCN− 2.5 x 10-3 - 4.5 x 10-5 2.455 x 10-3
FeSCN2+ 0 + 4.5 x 10-5 4.5 x 10-5
K = [FeSCN2+] / ([Fe3+][SCN−])
K = (4.5 x 10−5) / [(9.55 x 10−4)(2.455 x 10−3)] ≈ 19.2

5. Le Chatelier's Principle

If a system at equilibrium is disturbed (Concentration, Pressure, or Temperature), it shifts to counteract the disturbance.

  • Add Reactant: Shift Right (→)
  • Remove Product: Shift Right (→)
  • Increase Pressure: Shift toward side with fewer gas moles.
  • Change Temp: Treats heat as a reactant (endothermic) or product (exothermic).

6. Acid-Base Dissociation (Ka)

Acidity defined by the equilibrium of acid dissociation in water.

H-A + H2O ⇌ A− + H3O+

Ka = [A−][H3O+] / [H-A]
  • pKa = −log(Ka)
  • Stronger acids = Larger Ka = Lower pKa
  • Neutral water concentration (55.5 M) is omitted from the Ka expression.
End of Chapter 6 Notes

7. Biological Application: Blood pH Buffering

The human body maintains blood pH within a very narrow range (7.35–7.45) using the bicarbonate buffering system. This is a classic application of dynamic equilibrium.

CO2(g) + H2O(l) ⇌ H2CO3(aq) ⇌ HCO3−(aq) + H+(aq)
  • Respiratory Acidosis: If breathing is suppressed, CO2 levels increase. According to Le Chatelier’s Principle, the equilibrium shifts to the right, increasing [H+] and lowering blood pH.
  • Hyperventilation: Rapid breathing exhales CO2 too quickly. The system shifts to the left to replace the lost CO2, consuming H+ ions and causing blood pH to rise (alkalosis).

8. Competitive Binding: Hemoglobin and Poisoning

Hemoglobin (Hb) is a protein that reaches equilibrium with Oxygen (O2). However, other molecules can compete for the same binding sites, often with much higher equilibrium constants (K).

Normal Oxygen Transport:
Hb + O2 ⇌ HbO2 (Oxyhemoglobin)

Carbon Monoxide (CO) Poisoning

Carbon monoxide binds to hemoglobin with an affinity 200–300 times greater than oxygen.

HbO2 + CO ⇌ HbCO + O2
  • Because the equilibrium constant for HbCO is so large, even small amounts of CO shift the equilibrium far to the right.
  • Treatment: Patients are often placed in hyperbaric oxygen chambers. By drastically increasing the concentration (pressure) of O2, Le Chatelier’s Principle forces the equilibrium back to the left to displace the CO.

Cyanide (CN−) Poisoning

Unlike CO, which competes for the oxygen site on hemoglobin, cyanide binds to the iron in Cytochrome c oxidase within the mitochondria.

  • This halts the electron transport chain, stopping cellular respiration.
  • Equilibrium Treatment: One treatment involves administering nitrites to convert some hemoglobin into methemoglobin. Methemoglobin has a high equilibrium affinity for cyanide, acting as a "sink" to pull the cyanide away from the mitochondria and back into the blood where it can be detoxified.