In each of the following reaction please identify acid and base according to Bronsted-Lowry
a) CH3OH + NaH ⇋ RO- + H2
b) CH3COCH3 + NaOH ⇋ CH3COCH2- + H2
c) CH3COCH3 + NH2- ⇋ CH3COCH2- + NH3
d) CH3- + CH3CH2OH ⇋ CH4 + RO-
Identify the species on the left that lost a proton. That is your acid.
A) Acid: CH3OH, Base: NaH B) Acid: CH3COCH3 C) Acid: CH3COCH3, Base: NH2- D) Acid: CH3CH2OH, Base: CH3-
Lesson 1: Predicting position of acid-base equilibrium based on pKa values
Question 1
Predict which side the equilibrium is favored and calculate the equilibrium constant to verify your prediction. pKa values can be found in this table: http://evans.rc.fas.harvard.edu/pdf/evans_pKa_table.pdf
a) ROH + NaH ⇋ RO- + H2
b) CH3COCH3 + NaOH ⇋ CHCOCH2- + H2O
c) CH3COCH3 + NH2- ⇋ CH3COCH2- + NH3
d) CH3- + ROH ⇋ CH4 + RO-
Recall that equilibrium favors the side with the weaker acid
a) Equilibrium favors products. H2 has a higher pKa than ROH, which makes it a weaker acid.
Keq = Ka of ROH/Ka of H2= 10-16/10-36 = 1020
b) Equilibrium favors reactants
Keq = Ka1 of CH3COCH3/Ka of H2O = 10-25/10-15= 10-10
c) Equilibrium favors products.
Keq = Ka of CH3COCH3/Ka of NH3= 10-25/10-36= 1011
d) Equilibrium favors products.
Keq = Ka of ROH/Ka of CH4= 10-16/10-56 = 1040
Question 4
If you have two solution with equal concentrations of HCl (Ka = 103) and acetic acid (Ka =1.8 x 10-5), which solution will have lower pH?
pH = -log[H3O+] The higher [H3O+] the lower the pH
Since HCl has much bigger Ka the concentration of H3+ will be greater in HCl solutions. Since pH = -log[H3O+], pH will be lower in HCl solution.
Post-Video Questions
Pre-Video Questions
The following questions test your understanding of the following concepts: Bronsted-Lowry definition of acids bases, equilibrium constant, acid dissociation constant and pKa.
Question 3
a) Keq= 103 for the reaction A in question 3 Does equilibrium favor products or reactants?
b) Keq= 10-5 for the reaction of acetic acid with water. Does equilibrium favor products or reactants?
Remeber, if x/y is greater than 1, x is bigger than y.
A) Since Keq is greater than 1, equilibrium favors products
B) Since Keq is smaller than 1, equilibrium favors reactants
Question 2
Write expression for the equilibrium constant for the following reactions.
a) HCl + H2O ⇋ H3O+ + Cl-
b) H2SO4 + 2OH- ⇋ SO42- + 2H2O
Remeber, Keq = Products/Reactants
A) Keq = [H3O+][Cl-]/[HCl]
B) Keq = [SO42-]/[H2SO4][OH-]2