Acid-base%20equilibria%202%20Powerpoint[2]

Acid-base%20equilibria%202%20Powerpoint[2] - Introductory...

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Unformatted text preview: Introductory University Chemistry 2 Acid – Base Equilibria 2 Buffer Systems The common-ion effect Common-ion effect occurs when a reactant containing a given ion is added to an equilibrium mixture that already contains that ion . For example: For example: HA + A- This is the common ion The common-ion effect A solution of acetic acid: CH 3 COOH (aq) + H 2 O ( l ) a CH 3 COO- (aq) + H 3 O + (aq) Common ion If sodium acetate is added: CH 3 COONa (s) + H 2 O ( l ) → Na + (aq) + CH 3 COO- (aq) Remember: Ionic compounds are 100% dissociated The common-ion effect Acetic acid Sodium acetate CH 3 COO- H 3 O + CH 3 COO- Na + CH 3 COOH (aq) + H 2 O ( l ) a CH 3 COO- (aq) + H 3 O + (aq) Acetic acid + sodium acetate CH 3 COO- H 3 O + Na + Solution with common ion – acetate ion What is the effect of adding a common ion to an equilibrium? • Acetate ion is removed. • pH is increased. Buffer solutions A buffer is a solution that resists a change in its pH when either an acid or a base is added to it. • A solution that contains an acidic component (reserve) – to react with OH- and a basic component (reserve) – to react with H 3 O + . • The two components must not neutralize each other : A strong acid and a strong base cannot act as a buffer. Buffer solutions What constitutes a buffer ? Most commonly, the components of a buffer are the conjugate acid-base pair (preferably with similar concentrations) of a weak acid / base Which of the following pair of compounds would form a buffer solution? • H 2 SO 4 / NaHSO 4 • NaHCO 3 / Na 2 CO 3 • H 3 PO 4 / KH 2 PO 4 • NH 4 Cl / NH 3 NO. Because H 2 SO 4 is a strong acid Yes Yes Yes How does an acid-base buffer work? For example: CH 3 COOH / CH 3 COO- buffer system The equilibrium of the buffer is: CH 3 COOH (aq) + H 2 O ( l ) a CH 3 COO- (aq) + H 3 O + (aq) hen a strong acid is added When a strong acid is added CH 3 COO- (aq) + H 3 O + (aq) a CH 3 COOH (aq) + H 2 O ( l ) When a strong base is added CH 3 COOH (aq) + OH- (aq) a CH 3 COO- (aq) + H 2 O + ( l ) How does an acid-base buffer work? What is the change in pH when 1.00 mL of a solution of HCl (1.0M) is added to 1.0 L of water? HCl (aq) + H 2 O ( l ) → H 3 O + (aq) + Cl- (aq) The number of mole of H 3 O + = 1.0 mol / L x 1.00 L / 1000 = 1.0 x 10-3 mol [H O + ] = 1.0 x 10-3 mol / 1.0 L = 1.0 x 10-3 M 3 pH = - log [H 3 O + ] = - log 1.0 x 10-3 = 3.00 ∆ pH = 7.00 – 3.00 = 4.00 Water is a poor buffer What is the change in pH when 1.00 mL of NaOH (1.00 M) is added to 1.0 L of water? (Homework) Buffers pH calculations Given an acetic acid/sodium acetate buffer with the following initial concentrations: [CH 3 COOH] = 0.700 M [CH 3 COONa] = 0.700 M K a = 1.8 x 10-5 Calculate the pH of this buffer solution....
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Acid-base%20equilibria%202%20Powerpoint[2] - Introductory...

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