Ap Chem. Ch. 17 Flashcards
What are buffers?
Solutions of a weak conjugate acid-base pair.
What does the common-ion effect state?
A weak electrolyte will ionize less when a strong electrolyte with a common ion is added to the solution.
Ex: +
CH3COOH + H2OH3O + CH3COO-
-if NaCH3COO is added, the acetic acid will ionize less!
Buffers are resistant to pH changes, even when strong acid or base is added, why?
Because it contains both an acid to neutralize OH- ions and a base to neutralize H+ ions
What are three ways of making buffers?
- by mixing a weak acid or weak base with a salt
- by partial neutralization of a weak acid with a strong base.
- by partial neutralization of a conjugate base from a salt
3 factors that affect the solubility of ionic compounds:
- presence of common ions
- the pH of the solution
- the presence of complex agents
When a small amount of OH- is added to the buffer (HXH+ + X-), the OH- reacts with HX to produce X- and water. What happens to the [HX]/[X-] ratio and pH?
The [HX]/[X-] ratio remains more or less constant, so the pH is not significantly changed.
When a small amount of [H+] is added to a buffer ( HX H+ + X- ), X- is consumed to produce HX. What happens to the ratio of [HX]/[X-] and pH?
Their ratio is more or less constant, so the pH does not change significantly.
Henderson-Hasselbalch equation
PH= pKa + log[base/acid]
At [base]=[acid],
pH?pKa
pH=pKa
At [base]<[acid]
pH?pKa
pH<pKa
[base]>[acid]
pH?pKa
pH>pKa
When do buffers resist a change in pH in either direction ?
If the weak acid and its conjugate base are equimolar.
In this technique a known concentration of base(or acid) is slowly added to a solution of acid(or base).
Acid-base titrations
Titration of a strong acid with a strong base:
1. At the start the pH is determined by the acid.(no base has been added)
What equation is used?
pH=-log[acid]
Titration of a strong acid with a strong base.
2.after the start of the titration to near the equivalence point, the pH goes up slowly(when adding base). What equation is used?
[H+]=(mol H+) - (mol OH-)/total volume
Titration of a strong acid with a strong base:
3.at the equivalence point, moles acid=moles base, and the solution only contains water and the salt from the cation of the strong base and the anion of the strong acid.
PH=7
Titration of a strong acid with a strong base
4. As more base is added, the increase in pH again levels of (excess base). What is the equation?
[OH-]=(mol OH-) - (mol H+)/total volume
Titration of a Weak Acid with a strong base. Three things to remember!
- the conjugate Base of the acid affects the pH when it is formed.
- At the equivalence point the pH>7
- Phenolphthalein is commonly used as an indicator in these titrations.
Titration of a Weak Acid with a strong base.
1.at the start, the pH is determined by the concentration of the weak acid and Ka.what equation is used?
Ka= x^2/ HA-x (just a normal Ka problem)
Titration of a Weak Acid with a strong base.
2.Between the initial pH and the equivalence point, a partial neutralization creates a buffer solution.
What equation is used?(half equivalence point. [HA]=[A-] and pH=pKa)
Find the[HA] and the [A-] and use Henderson-Hasselbalch to calculate
At what point during the titration should the indicator change color?
At the equivalence point
Titration of a Weak Acid with a strong base.
3. At the equivalence point, the H+ and OH- neutralize each other, but the weak base, [A-] WILL CAUSE THE SOLUTION TO BE SLIGHTLY BASIC. What equation should you use?
Use the Kb of the weak base,[A-], to calculate the pH.
Titration of a Weak Acid with a strong base.
4. After the equivalence point, the weak base contribution is NEGLIGIBLE, compared to the [OH-].
What equation is used?
[OH-] = (mol OH-) - (mol H+)/ total volume
When one titrates a polypeptide acid with a base, there is an __________ for each dissociation.
Equivalence point
