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Chem 266: Acid-Base Review Notes

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University of Waterloo
CHEM 266
Steve Forsey

Using pK aalues to predict the Position of Equilibrium  In a reaction equation, the equilibrium will favour the formation of the weaker acid (higher pK )a o The higher the pK a the less acidic (ex. HCl has a negative pKavalue; ∴ it is very acidic)  The stronger the acid, the smaller the pK a  The stronger the acid, the weaker the conjugate base Acid-Base Strength  Acidity increases with increasing conjugate base stability o The less reactive the conjugate base, the stronger the acid is + + o Equilibrium is shifted towards the formation of H (H O3) - + HA + H 2 ⟷ A + H O 3  The larger the anion, the more stable the conjugate base Rules of comparing acidity:  When comparing acidity of different acids, find their conjugate bases to see what atom the charge is on  When going across the periodic table (ex. B, C, N, O, F), look at electronegativity o The more electronegative the charged atom, the more stable the conjugate base and the stronger the acid  When going down the periodic table (ex. F, Cl, Br, I), size matters more than electronegativity o Atomic size increases as you go down the periodic table (Br is larger than Cl) o The larger the atom, the more stable the conjugate base and the stronger the acid *Size has a greater stabilizing effect than electronegativity! Trends in pK aalues  When going across the periodic table, pK values decrease  When going down the periodic table, pK values decrease a Acid-Base Strength: Inductive Effects  Induction – the stability of partial or full charges by electron-donating or electron-withdrawing atoms or groups  The proximity of an electronegative atom to an acidic atom can help stabilize the conjugate base  Inductive effect: electronegative atoms (ex. Cl) pull electron density through the sigma bond from adjacent carbons making the carbon more positive (“electron-withdrawing effect”); if the carbon is close to the anion, this carbon can then pull electron density from the region that has the negative charge to make it more stable o The closer the electronegative atom is to the anion, the more stable the conjugate base, the lower the pK , and the more acidic the acid a o When proximity between the EN atom and the anion is the same (when comparing two molecules), the # of EN atoms present as well as the EN of the atom matters  The more EN atoms present, the more stable the acid  The more electronegative the atom is (F vs. N), the
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