Given the data in the table below and your knowledge of the "chemical standard state" (X) and the "biochemical standard state" (X°'), answer parts a) to e). a) For the dissociation of acetic acid at 298.15 K, calculate A,G and the corresponding pka. b) Even though by definition, AG (H+ (aq)) = 0 and AG (H+ (aq)) = 0, these are different physical quantities. What precisely does each represent? c) Calculate at 298.15 K, A,G for CH3COOH(aq) and CH3COO(aq). Gibbs free energy of formation, in units of kJ mol-¹, at T = 298.15 K AGⓇ A Gor -396.5 -369.3 0 CH3COOH(aq) CH3COO- (aq) H+ (aq) 0

Chemical standard state is defined by standard conditions in chemical systems. These standard conditions are a temperature of 250C ( i.e. 298.15K) , initial concentrations of reactions and products being 1M ,for gases the partial pressure being 101.3kPa (i.e. 1 atm) and the hydrogen ion concentration of reactions involving hydrogen ions being 1M (i.e. [H+]= 1M or pH=0). Biochemical standard state is defined by standard conditions in biochemical systems. These standard conditions are similar to the chemical standard conditions mentioned above, except the the hydrogen ion concentration of reactions involving hydrogen ions will be 10-7 M ( i.e. a pH of 7) . This is because biochemical systems are generally buffered to maintain a pH of 7 (i.e. neutral pH).∆rG⊝ =sum of ∆fG⊝(products) - sum of ∆fG⊝(reactants) =∆fG⊝(CH3COO-) + ∆fG⊝(H+) -∆fG⊝(CH3COOH) =-369.3kJ mol-1 + 0 - -396.6 kJ mol-1 =27.2 kJ mol-1 =27200 J mol-1 ∆fG⊝ can be expressed in yet another equation, which is given below. ∆fG⊝ = -RT ln[product][reactant] here, 'R' is the Universal gas constant and has a value of 8.315 J mol-1 K-1 . 'T' is temperature in kelvin. ∆fG=-8.315 J mol-1K-1×298.15K× ln[CH3COO-][H+][CH3COOH]We know,∆fG⊝ =27200 J mol-1[H+] under chemical standard state is 1M, as pH is 0.Lets substitute these values to the above equation.27200 J mol-1=-8.315Jmol-1K-1×298.15K× ln[CH3COO-]×1[CH3COOH][CH3COO-][CH3COOH] =1.7182×10-5 The Henderson-Hasselbalch equation relates the pH, pKa of acid and the concentrations of protonated and deprotonated states of a species . The Henderson-Hasselbalch equation is given below. pH = pKa + log[deprotonated][protonated] For deprotonation (i.e. dissociation) of acetic acid, the above equation becomes, pH = pKa + log[CH3COO-][CH3COOH] We know the pH is 0 and [CH3COO-][CH3COOH] =1.7182×10-5 . 0 = pKa + log1.7182×10-5pKa = 4.765 ∆fG⊝H+ (aq) is the standard Gibbs free energy change for the formation of 1 mole of H+ in aqueous medium from its elements under chemical standard conditions of 298.15K temperature, 1M initial concentration and at a pH of 0 . ∆fG0'H+ (aq) is the standard Gibbs free energy change for the formation of 1 mole of H+ in aqueous medium from its elements under biochemical standard conditions of 298.15K temperature, 1M initial concentration and at a pH of 7 . As you can see, the only difference between both is that the pH under chemical standard condition is 0 , while it is 7 under biochemical standard condition.

Related questions