Question

a)How does molar mass (or size) affect the magnitude of alcohol ∆t values?

b)How does hydrogen bonding affect ∆t values?

c)How does branching affect ∆t values among alcohol isomers?

d)How does molar mass (or size) affect the magnitude of alkane ∆t values?

e)How do ∆t values for the evaporation of 1-propanol and isopropanol compare?

f)Which type of intermolecular attractions is changing in strength as the alcohols grow larger?

delta t values that were measured: methanol -7.7, ethanol -9.1, isopropanol -9.4, n-butanol -3.1, xylene -2.8, n hexane -13.7, cyclohexene -13.3, toluene -5.4, acetone -19.6, and water -4.7.

Answer 1

1. The t of alcohol is given by t=K_fm, where m is the molality. For fixed mass of any matter, no. of moles decreases with increase in molar mass. Since, molality is directly proportional to no. of moles, molality of the solution would decrease with increase in molar mass of the alcohol. Thus, t would decrease as molar mass of alcohols increase.
2. H-bonding leads to agglomeration of many molecules, increasing the effective molar mass (since many molecules join together, giving the impression if one giant supermolecule). Thus, t would decrease if H-bonding is present.
3. Branching reduces the effective surface area of the molecule. Since there are many attractive forces like Vanderwaal's force, etc which are directly proportional to surface area, branching would decrease the effective molecular mass, and thus increase t (Reason same as point #2. Vanderwaals force would have the same effect as H-bond).
4. The effect would be same as that of alcohols.
5. Intermolecular Attractive forces are greater in 1-propanol, due to no branching. Thus it will have higher boiling point.
6. Vanderwaals attraction.