CHEM 1A Lecture Notes - Lecture 23: Sodium Bicarbonate, Conjugate Acid, Weak Base

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29 Nov 2016

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UCSB

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Chemistry and Biochemistry

Course

CHEM 1A

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Joseph C A

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CHEM 1A Full Course Notes

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Recall: ka and kb given for two equations. Whichever reaction has greater k value, that means that will be greater in the reaction. Ka is greater = [h+] > [oh-] Kb is greater = [h+] < [oh-: higher [h+] = lower ph, lower [h+] = higher ph. Contain cations of strong bases combine with anions of strong acids. Reaction: mix: baking soda, nahco3, and vinegar, ch3cooh (aq) Write the major specie in solution before acid-base reaction occur. Major species in solution before acid-base reactions occur: na+, hco3-, ch3cooh, h2o. Strongest acid in solution: acids: hco3-, ch3cooh, h2o, strongest acid: ch3cooh. Strongest base in solution: bases: hco3-, h2o, strongest base: hco3- Strongest acid reacts with strongest base: ch3cooh + hco3- ch3coo- + h2co33, k = ka (reactant acid)/ ka (product acid) Recognize acids and bases: nh3, clo-, hno2, ch3nh2, nh4+ Buffer: when an acid or base is added.

Related Questions

Salts formed by reaction of a strong acid with a weak base areacidic, and salts formed by reaction of a weak acid with a strongbase are basic. This is due to the equilibrium the conjugate haswith its acid or base. For example, sodium bicaronate NaHCO3contains the conjugate of the weak acid H2CO3. The conjugate willreform the acid through the equilibrium

HCO3?(aq)+H2O(aq)?H2CO3(aq)+HO?(aq)

and, thus, acts as a weak base. A solution of NaHCO3yields abasic solution.

If a salt is derived from a weak acid and a weak base, the pH ofthe solution depends on the relative acid strength (Ka) ofthe cation and base strength (Kb) of the anion. IfKa>Kb, the solution of the salt is acidic. IfKa<Kb, it is basic. If Ka?Kb, it is neutral.

Given that

Ka for HCN is 4.9

For conjugate acid-base pairs, how are K_a and K_b related?

Consider the reaction of acetic acid in water.

where

a. Which two bases are competing for the proton?

b. Which is the stronger base?

c. In light of your answer to part b, why do we classify the acetate ion (CH₃CO₂^-) as a weak base? Use an appropriate reaction to justify your answer.

In general, as base strength increases, conjugate acid strength decreases. Explain why the conjugate acid of the weak base NH₃ is a weak acid.

To summarize, the conjugate base of a weak acid is a weak base and the conjugate acid of a weak base is a weak acid (weak gives you weak). Assuming K_a for a monoprotic strong acid is 1x10⁶, calculate K_b for the conjugate base of this strong acid. Why do conjugate bases of strong acids have no basic properties in water? List the conjugate bases of the six common strong acids. To tie it all together, some instructors have students think of Li⁺, K⁺, Rb⁺, Cs⁺, Ca²⁺, Sr²⁺, and Ba²⁺ as the conjugate acids of the strong bases LiOH, KOH, RbOH, CsOH, Ca(OH)₂, Sr(OH)₂, and Ba(OH)₂. Although not technically correct, the conjugate acid strength of these cations is similar to the conjugate base strength of the strong acids. That is, these cations have no acid properties in water; similarly, the conjugate bases of strong acids have no basic properties (strong gives you worthless). Fill in the blanks with the correct response. The conjugate base of a weak acid is a ______ base. The conjugate acid of a weak base is a ______ acid. The conjugate base of a strong acid is a ______ base. The conjugate acid of a strong base is a ______ acid. (Hint: Weak gives you weak and strong gives you worthless.)

limeswan13

For conjugate acid-base pairs, how are Ka and Kb related?

Consider the reaction of acetic acid in water:

CH₃CO₂H (aq) + H₂O (l) ⇌ CH₃CO₂⁻ (aq) + H₃O⁺ (aq) where Ka = 1.8 × 10⁻⁵

Which two bases are competing for the proton?
Which is the stronger base?
In light of your answer to part b. why do we classify the acetate ion (CH₃CO₂⁻) as a weak base? Use an appropriate reaction to justify your answer.
In general, as base strength increases, conjugate acid strength decreases. Explain why the conjugate acid of the weak base NH₃ is a weak acid.

To summarize, the conjugate base of a weak acid is a weak base and the conjugate acid of a weak base is a weak acid (weak gives you weak). Assuming Ka for a monoprotic strong acid is 1× 10⁶, calculate Kb for the conjugate base of this strong acid.

Why do conjugate bases of strong acids have no basic properties in the water?

List the conjugate bases of the six common strong acids.

To tie it all together, some instructors have students think of Li⁺, K⁺, Rb⁺, Cs⁺, Ca²⁺, Sr²⁺, and Ba²⁺ as the conjugate acids of the strong bases LiOH, KOH, RbOH, CsOH, Ca(OH)₂, Sr(OH)₂, and Ba(OH)₂. Although not technically correct, the conjugate acid strength of these cations is similar to the conjugate base strength of the strong acids. That is, these cations have no acidic properties in water; similarly, the conjugate bases of strong acids have no basic properties (strong gives you worthless). Fill in the blanks with the correct response. The conjugate base of a weak acid is a_____base. The conjugate acid of a weak base is a_____acid. The conjugate base of a strong acid is a_____base. The conjugate acid of a strong base is a_____ acid. ( Hint: Weak gives you weak and strong gives you worthless.)

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CHEM 1A Lecture Notes - Lecture 23: Sodium Bicarbonate, Conjugate Acid, Weak Base

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