Lab - Evaporation And Inter Molecular Attractions

Purpose: To study temperature

Materials: Safety: Pre-Lab Exercise: Structural Formula

Molecular Hydrogen Weight Bond

Substance

Formula

Ethanol

C2H5OH

46 g/mol

Yes

1-propanol C3H7OH

60 g/mol

Yes

1-butanol

C4H9OH

74 g/mol

Yes

N-pentane

C5H12

72 g/mol

No

Methanol

CH3OH

32 g/mol

Yes

N-hexane

C6H14

86 g/mol

No

Procedure: Data Tables: Substance Ethanol 1-propanol 1-butanol

t1 (oC) 23.62 22.38 23.50

t2 (oC) 12.44 13.37 21.24

Δt (t1-t2) (oC) 11.18 9.01 2.26

N-pentane Methanol N-hexane Substance

21.99 23.22 24.27 Predicted Δt (oC)

1-butanol

Lower than propanol

N-pentane

Higher than butanol and hexane

Methanol

Higher than ethanol

N-hexane

Lower than propanol

10.00 9.08 11.20

11.99 14.14 13.07 Explanation

The more hydrogen present and the higher the molecular weight of a substance, the higher the intermolecular forces should be. Therefore, less butanol than propanol should evaporate, thereby decreasing the temperature less, resulting in a lower Δt. Pentane is roughly the same molecular weight as butanol. Therefore, the only difference in their intermolecular forces is due to hydrogen bonding, and lack thereof. Since it lacks hydrogen bonds, pentane should therefore have weaker intermolecular forces than butanol, resulting in a higher Δt. Also, hexane also lacks hydrogen bonding, but has a higher molecular weight than pentane, so pentane should have a higher Δt that it as well. The less hydrogen present and the lower the molecular weight of a substance, the higher the intermolecular forces should be. Therefore, more methanol than ethanol should evaporate, thereby decreasing the temperature more, resulting in a higher Δt. Hexane and propane both lack hydrogen bonding, but hexane has a higher molecular weight than pentane, so it should have a lower Δt that pentane.

Processing the Data: 1. N-pentane and 1-butanol are about the same molecular weight and of very similar molecular structures, so any difference in the strength of their intermolecular forces must come from the fact that pentane lacks hydrogen bonding, while butanol has hydrogen bonding. The stronger a substance’s intermolecular forces, the less will evaporate, and so the less the temperature will change; and vice-versa. This explains the difference between the Δt’s. 2. As said above, the stronger a substance’s intermolecular forces, the less will evaporate, and so the less the temperature will change; and vice-versa. Therefore, the alcohol with the lowest Δt must have the strongest intermolecular forces, and the one with the highest Δt must have the weakest intermolecular forces, so 1-butanol has the strongest intermolecular forces of attraction and methanol has the weakest. 3. As said above, the stronger a substance’s intermolecular forces, the less will evaporate, and so the less the temperature will change; and vice-versa. Therefore, the alkane with the lower Δt must have the stronger intermolecular forces, and the one with the higher Δt must have the weaker intermolecular forces, so n-pentane has the stronger intermolecular forces of attraction and n-hexane has the weaker.