Chemistry
Stuctures
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Chemical Reactions
Terry Acree
Chemical Reaction - molecules made of atoms in fixed proportions react (change) to form molecules in fixed proportions. " Small whole numbers!" The speed of a chemical reaction is indicated by a rate constant, k. The larger the constant faster the reaction. The reaction in wine making that converts sugar to alcohol and CO2 has a kz measured in days.
grape sugar (glucose)
ethyl alcohol + carbon dioxide (gas)
C6H12O6
2 C2H6O
+ 2 CO2
Dissociation Reaction & Equilibrium:
AH  A- + H-
where k1/k2 = Ka
Strongest
- C4H6O6
C4H5O6-
+ H+ pKa = 2.98, 4.32.
tartaric acid
Weaker
- C4H6O5
C4H5O5-
+ H+ pKa = 3.04, 5.05.
malic
- C3H6O3
C3H5O3-
+ H+ pKa = 3.08.
lactic acid
- C2H4O2
C2H3O2-
+ H+ pKa = 4.75.
acetic acid
- C6H8O7
C6H7O7-
+ H+ pKa = 3.14, 4.77, 6.39.
citric acid
Henderson-Hasselbach Equation - pH = -log[H+]
where k1 > k2 >>> kz i.e. ionization is much faster than fermentation.
Total Acid Measurement - (titration w/ strong base)
C4H6O6 + NaOH
C4H4O6Na2 + 2 H2O
tartaric acid + sodium hydroxidesodium tartrate
Dissociation of Weak Acids
- C4H6O6
C4H5O6-
+ H+....... pKa1 = 2.98.
tartaric acid
bitartrate ion + hydrogen ion
- C4H5O6-
C4H4O6-2
+ H+........ pKa2 = 4.32.
bitartrate ion
tartrate ion + hydrogen ion
The relative concentration of each species, ion, salt or acid, depends on the pH. A plot of concentration vs. pH shows that at pH 3 the undisocciated tartaric acid equals the concentration of the bitartrate ion and that the tartrate ion is just a few percent of the solution. Malic acid has more of the undissociated form than the bimalate and only trace amounts of the malate at pH 3.
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How does this affect the taste of wine?
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FS430: Understanding Wine and Beer