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IIIIIIIVV Ch. 19 – Strengths of Acids & Bases 19.3 pp. 602 – 611.

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Presentation on theme: "IIIIIIIVV Ch. 19 – Strengths of Acids & Bases 19.3 pp. 602 – 611."— Presentation transcript:

1 IIIIIIIVV Ch. 19 – Strengths of Acids & Bases 19.3 pp. 602 – 611

2 A. Strong & Weak Acids n Strong acids are completely ionized in aqueous solution HA(aq) + H 2 O(l) H 3 O + (aq) + A - (aq)

3 Equilibrium Amounts A. Strong & Weak Acids n Weak acids ionize only slightly in aqueous solution CH 3 COOH( aq ) + H 2 O( l ) H 3 O + ( aq ) + CH 3 COO – ( aq ) ~99%~1% Initial Amounts

4 B. Acid Dissociation Constant n You can write the equilibrium-constant expression from balanced chemical equation CH 3 COOH( aq ) + H 2 O( l ) H 3 O + ( aq ) + CH 3 COO – ( aq ) The acid dissociation constant, K a, is ratio of concentrations of dissociated and undissociated form

5 B. Acid Dissociation Constant n Reflects the fraction of the acid in the ionized form If K a is small, then degree of dissociation is small Weak acids have small K a values The stronger an acid is, the larger its K a value n Often written in generic form: HA H + + A –

6 C. Strong & Weak Bases n Strong bases completely dissociate into metal ions and hydroxide ions in aqueous solution n Weak bases react with water to form the hydroxide ion and the conjugate acid of the base NH 3 (aq) + H 2 O(l) NH 4 + (aq) + OH - (aq)

7 D. Base Dissociation Constant n You can write the equilibrium-constant expression from balanced chemical equation The base dissociation constant, K b, is ratio of concentrations of dissociated and undissociated form NH 3 (aq) + H 2 O(l) NH 4 + (aq) + OH - (aq)

8 D. Base Dissociation Constant Size of K b indicates ability of weak base to compete with very strong base OH – for hydrogen ions Weak bases have small K b values The stronger a base is, the larger its K b value n Often written in generic form: B + H 2 O BH + + OH –

9 E. Calculating Dissociation Constants A 0.100M solution of acetic acid is only partially ionized. From the pH, [H + ] is determined to be 1.34 x 10 -3 M. What is K a of acetic acid? n Brackets indicate concentrations at equilibrium – you need to find equilibrium values!

10 E. Calculating Dissociation Constants Concentration[CH 3 COOH][H + ][CH 3 COO - ] Initial0.10000 Change -1.34 x 10 -3 +1.34 x 10 -3 Equilibrium 0.09871.34 x 10 -3

11 F. Percent Dissociation n Often useful to specify amount of weak acid or base that has dissociated in achieving equilibrium n Ex) 1.00M solution of HF, [H + ] = 2.7x10 -2 M

12 G. Weak Acids and Bases WS 1) Find the pH and pOH of a 0.325M acetic acid solution. K a = 1.8 x 10 -5 Concentration[CH 3 COOH][H + ][CH 3 COO - ] Initial0.32500 Change -x+x Equilibrium 0.325 - xxx HA H + + A –

13 G. Weak Acids and Bases WS 1) Find the pH and pOH of a 0.325M acetic acid solution. K a = 1.8 x 10 -5


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