CH 117 Spring 2015Worksheet 10
1. What are the two theories used to define acids and bases? According to these, what is an acid and what is a base?
Arrhenius theory – An acid is a compound that produces H+ or H3O+ when dissolved in water. A base is a compound that produces OH- when dissolved in water.
Bronsted-Lowry theory – An acid is a proton donor, while a base is a proton acceptor.
Bronsted-Lowry is more useful for us right now.
2. Given an acid, how do you determine its conjugate base? Given a base, how do you determine its conjugate acid?
Given an acid, take away a proton (H+) to find the conjugate base. Given a base, add a proton (H+) to find the conjugate acid. This is a direct application of the Bronsted-Lowry acid/base theory.
3. Identify the conjugate acid-base pairs in each of the following reactions. Acids in yellow, bases in green.
a). HSO3- + H2O ® SO32- + H3O+ Pairs: HSO3- and SO32-, H2O and H3O+
b). HSO3- + H2O ® H2SO3 + OH- HSO3- and H2SO3, H2O and OH-
c). HSO4- + CO32- ® SO42- + HCO3- HSO4- and SO42-, CO32- and HCO3-
d). HCl + H2PO4- ® Cl- + H3PO4 HCl and Cl-, H2PO4- and H3PO4
4. What is the conjugate base for each of the following acids?
a). HClO4 ClO4-
b). H2S HS-
c). PH4+ PH3
d). HCO3- CO32-
5. What is the conjugate acid of each of the following bases?
a). CN- HCN
b). SO42- HSO4-
c). H2O H3O+
d). HCO3- H2CO3
e). CO HCO+
6. How would you define a strong acid or base? Weak acid or base? List the seven strong acids and the strong bases.
Strong acids/bases are ones that completely dissociate when dissolved in water, while weak acids/bases are ones that only partially dissociate when dissolved in water.
Strong acids: HCl, HI, HBr, H2SO4, HClO4, HNO3, HClO3
Strong bases: Alkali or alkaline earth metals paired with a hydroxyl group (OH)
7. What can you say about the relative strengths of acids and their conjugate bases? Or of bases and their conjugate acids?
Generally inversely related. A strong acid will have a weaker conjugate base, and a weak acid will have a stronger conjugate base. Same for bases.
8. What is the relationship between the equilibrium constant (Ka or Kb) and the relative strength of a weak acid or base?
Generally directly related. A large K indicates a stronger acid/base, while a low K indicates a weaker acid/base.
9. Given that the Ka of CH3COOH is 1.8 x 10-5 and the Ka of H2O is 1.8 x 10-16, which is the strongest acid? Which is the strongest conjugate base?
Strongest acid is the one with the biggest K value ® CH3COOH is stronger.
Weakest acid will have the strongest conjugate base, so the conjugate base of H2O will be the strongest ® OH-