Predict position of the equilibrium for the following reaction: CH3NH3+ CH3PH3+ = CH3NH3++ CH3PH2
a) Equilibrium favors right
b) Equilirbium favors left
Which atom has larger atomic radius?
Equilibrium favors right hand side. Phosphorous has a larger atomic radius and can support the lone pair better than nitrogen. Therefore, CH3PH2 is more stable species
Question 7
Upon reaction of CH3SH with OH- which ion is more likely to form, -CH2SH or CH3S-?
Which anion is more stable?
The second anion is more stable, because sulfur is larger than carbon. Therefore, the second anion is more likely to form.
Question 3
Arrange the following atoms in the order of increasing electronegativity:
a) S, Na, P, Mg, Si, Al;
b) C, O, Fe, Se, N,
The closer the atom is to fluorine, the more electronegative it is. Fluorine is the most electronegative atom
a) Na, Mg, Al, Si, P, S
b) Fe, Se, C, N, O
Question 6
Which one is a stronger acid, methylamine (CH3NH2) or methanol (CH3OH) ? Provide a brief explanation for your choice
Draw the conjugate bases. Which atom is more electronegative?
Methanol CH3OH is a stronger acid than methylamine, CH3NH2. Let's look at the respective conjugate basis, CH3O- and CH3NH-. Oxygen is more electronegative than nitrogen. Therefore, oxygen anion is more stable than nitrogen anion. More stable anion means weaker conjugate base. Weaker conjugate base corresponds to the stronger acid. FYI, pKa of methanol is 16. pKa of methyl amine is 36.
Lesson 2: Predicting position of acid-base equilibrium based on structure (electronegativity, radius)
Question 2
Arrange the following atoms in the order of increasing radius:
a) Ar, Na, P, Mg, Si, Al;
b) Ge, C, Si, Pb, Sn
Radius increases down the column. Radius decreases across the period.
a) Ar, P, Si, Al, Mg, Na
b) C, Si, Ge, Sn, Pb
Post-Video Questions
Question 1
In the examples below indicate which species is acting as an acid and which species is acting as a base according to Bronsted-Lowry definition. Identify acid/conjugate base pair. Determine if conjugate base is strong or weak.
a) HCl + H2O ⇋ HO+ + Cl-
b) NH3 + CH3- ⇋ CH4 + NH2-
c) CH3OH + H- ⇋ CH3O- + H2
Weak acid acid (pKa is 15 or greater) produces strong conjugate base.
a) Acid: HCl; Conjugate base: Cl-
Since HCl is a strong acid (pKa = -3), Cl- is a weak conjugate base
b) Acid: NH3; Conjugate Base: NH2-
Since NH3 is a weak acid (pKa = 36), NH2-is a strong conjugate base
c) Acid: CH3OH; Conjugate Base: CH3O-
Since CH3OH is a weak acid (pK = 16), CH3O- is a strong conjugate base
Question 4
Predict hybridization and bond angles of carbon in the following molecules:
a) CH3OH
b) HCOH
c) C2H2
d) HCN
Draw the Lewis Dot Structure of the molecule. If you are having trouble with this, review the Lewis Dot Structures first
a) Hybridization = sp3; Bond angle = 109.5 Single bond = sp3 hybridization, tetrahedral geometry, 109.5 bond angle
b) Hybridization = sp2; Bond angle = 120
Double bond = sp2 hybridization, trigonal planar geometry, 120 bond angle
c) Hybridization = sp; Bond angle = 180
Triple bond = sp hybridization, linear geometry, 180 bond angle.
Pre-Video Questions
The following questions test your understanding of the following concepts: