Common Ion Effect, Buffers, and Titration

March 4/6, 2015

Warm Up

• What is the pH of a solution of 0.3 M HC2H3O2?

Common Ion Effect

• Whenever a weak electrolyte and a strong electrolyte containing a common ion are together in solution, the weak electrolyte ionizes less than it would if it were alone in solution.– Le Chatelier’s principle– Ex: acetic acid and sodium acetate

pH of acetic acid and sodium acetate

• What is the pH of a solution made by adding 0.3 mol acetic acid and 0.3 mol sodium acetate to enough water to make 1 L solution?

Practice

• Whiteboard: 1 or 2 or 3

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Buffers• Buffers are solutions of a

WEAK conjugate acid-base pair.– HCO3

- and CO32-

• They are particularly resistant to pH changes, even when strong acid or base is added.– Why?

Buffers resist changes because…

• They contain both• acid to neutralize added OH- ions (if a strong

base is added)• base to neutralize H+ ions (if a strong acid is

added)

Thus, we can prepare buffers by…

• Mixing weak acid or weak base with a salt of that acid or base– Example: NH4

+ — NH3 buffer

– NH3 added to NH4Cl

• Adjusting relative concentrations and components creates buffers at virtually any pH

Three Examples of Buffer Solution

• (skip forward to page 17)• List three conjugate acid-base pairs

Stop and Check

• What do buffers need? • What makes a solution buffered?• (bottom of page 17)

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How Buffers Work (adding base)

If a small amount of hydroxide is added to an equimolar solution of HF in NaF, for example, the HF reacts with the OH− to make F− and water.

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How Buffers Work (adding acid)

Similarly, if acid is added, the F− reacts with it to form HF and water.

Buffer Capacity

• The amount of acid or base the buffer can neutralize before the pH begins to significantly change

• How much H+ or OH- a solution can absorb before pH begins to significantly change

Calculating pH of a Buffer

1. Common ion effect2. Henderson-Hasselbalch equation (pg 18 top)

works when you can use starting concentrations of acid and base components directly (5% rule)

– ONLY FOR WEAK ACIDS AND WEAK BASESpH = pKa + log([base]/[acid])

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1. Determine how the neutralization reaction affects the amounts of the weak acid and its conjugate base in solution. (stoichiometry)

2. Use the ICE table to determine the new pH of the solution. (equilibrium)

When Small Amounts of Strong Acids or Bases Are Added to a Buffer…

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When Small Amounts of Strong Acids or Bases Are Added to a Buffer…

…it is safe to assume that all of the strong acid or base is consumed in the reaction.

Page 17, Number 4

Titrations

• Used to find the concentration of unknown solutions

• Add acid to base or vice versa• Amount added can be used to calculate the

concentration of the unknown solution

The color change occurs at a different pH for different

indicators.The pH at which the indicator

changes color is dependent on the Ka of the indicator as

a weak acid.

Titration Curve

• Graph of pH as a function of the volume of the titrant added

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Titration of Strong Acid with a Strong Base

From the start of the titration to near the equivalence point, the pH goes up slowly.

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Titration of Strong Acid with a Strong Base

Just before (and after) the equivalence point, the pH increases rapidly.

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Titration of Strong Acid with a Strong Base

At the equivalence point, moles acid = moles base, and the solution contains only water and the salt from the cation of the base and the anion of the acid.

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Titration of Strong Acid with a Strong Base

As more base is added, the increase in pH again levels off.

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Titration of a Weak Acid with a Strong Base

• Unlike in the previous case, the conjugate base of the acid affects the pH when it is formed.

• At the equivalence point the pH is >7.

• Phenolphthalein is commonly used as an indicator in these titrations.

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Titration of a Weak Acid with a Strong Base

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Titration of a Weak Acid with a Strong Base

At each point below the equivalence point, the pH of the solution during titration is determined from the amounts of the acid and its conjugate base present at that particular time.

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Titration of a Weak Acid with a Strong Base

With weaker acids, the initial pH is higher and pH changes near the equivalence point are more subtle.

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Titrations of Polyprotic Acids

When one titrates a polyprotic acid with a base there is an equivalence point for each dissociation.

Titration Calculations

• Page 23