CH 3 COOH l H 2 O l H 3 O aq CH 3 COO aq Common weak acids are H 3 PO 4 HF H 2

Ch 3 cooh l h 2 o l h 3 o aq ch 3 coo aq common weak

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CH 3 COOH (l) + H 2 O (l) H 3 O + (aq) + CH 3 COO - (aq) Common weak acids are H 3 PO 4 , HF, H 2 CO 3 , and HCN
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Base Strength When a strong base dissolves in water, 100% of the base dissociated into ions. NaOH (s) + H 2 O (l) Na + (aq) + - OH (aq) - Common strong bases are NaOH and KOH - A strong base readily accepts a proton, forming a weak conjugate acid . When a weak base dissolves in water, only a small fraction of the base dissociates into ions. NH 3 (g) + H 2 O (l) NH 4 + (aq) + - OH (aq)
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Acid/Base Strength HCl in H 2 O CH 3 COOH in H 2 O
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Examples-Acid/Base Strength Which is the stronger acid? H 2 SO 4 or H 3 PO 4 HF or HCl H 2 CO 3 or NH 4 + Which has the stronger conjugate base? HCN or HF H 2 O or CH 3 COOH H 2 O or HF
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Acid Ionization Constant The acid ionization constant , K a , describes the equilibrium that forms when an acid reacts with water. The larger the K a value, the stronger the acid. Equilibrium favors formation of the weaker acid (the acid with the smaller value of K a CH 3 CO 2 H (aq) + H 2 O (l) H 3 O + (aq) + CH 3 CO 2 - (aq) K a = 1.8 x 10 -5 HCN (aq) + H 2 O (l) H 3 O + (aq) + CN - (aq) K a = 6.2 x 10 -10
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K a Values
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Examples-K a Which acid in each pair is stronger? - HCN or HSO 4 - - CH 3 COOH or NH 4 + Rank the acids in order of increasing acid strength: - H 3 PO 4 , H 2 PO 4 - , HPO 4 2-
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Dissociation of Water Water can behave as both a Brønsted-Lowry acid and a Brønsted- Lowry base . Thus two water molecules can react together in an acid-base reaction: One molecule of H 2 O donates a proton , forming its conjugate base - OH . One molecule of H 2 O accepts a proton, forming its conjugate acid H 3 O + . H 2 O + H 2 O - OH + H 3 O + loss of H + gain of H +
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Dissociation of Water H 2 O + H 2 O - OH + H 3 O + The equilibrium constant expression for this reaction can be written as: Multiplying both sides by [H 2 O] 2 yields K w , the ion-product constant for water. - K w is a constant, 1.0 x 10 -14 , for all aqueous solutions at 25 o C. K = [H 3 O + ][ - OH] [H 2 O] 2 K w = [H 3 O + ][ - OH] K w = [H 3 O + ][ - OH] = 1.0 x 10 -14
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Dissociation of Water, K w To calculate [ - OH] when [H 3 O + ] is known: K w = [H 3 O + ] [ - OH] [ - OH] = [ - OH] = To calculate [H 3 O + ] when [ - OH] is known: K w = [H 3 O + ] [ - OH] [H 3 O + ] = [H 3 O + ] = [H 3 O + ] K w [H 3 O + ] 1.0 x 10 -14 [ - OH] K w [ - OH] 1.0 x 10 -14
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Meaning of [H 3 O + ] and [ - OH] If the [H 3 O + ] in a cup of coffee is 1.0 x 10 -5 M, then the [ - OH] can be calculated as follows: Since [H 3 O + ] > [ - OH], the solution is acidic.
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