Physical Properties of Alkanes: BP Determination and Trends Flashcards
What kinds of liquids with have higher boiling points?
those with strong intermolecular forces
the point at which the vapor pressure of a chemical is equal to the atmospheric pressure
boiling point
Why do molecules with more intermolecular forces have higher boiling points?
more kinetic energy is required to break the bonds holding the molecules together
non-polar, single bonded compounds with the general formula Cn H2n+2
alkanes
What type of intermolecular forces to alkanes, branched alkanes, and cycloalkanes experience?
Van der Waals
What is the boiling point trend for straight chain alkanes?
an increase in molecular weight = an increase in BP
Why do branched alkanes have a lower boiling point than both straight chained and cyclo alkanes?
Less surface area for intermolecular attraction
General formula for cycloalkanes?
CnH2n
Why do cycloalkanes have higher boiling points than their straight chain counterparts?
More surface area for intermolecular associations
Trend for boiling points of alkanes?
branched alkanes < straight-chain alkanes < cycloalkanes
Why is it important in the boiling point experiment that the thermometer be above the liquid and not in it?
We are measuring the temperature of the vapors not the liquid
Steps to determine boiling point of liquid?
- add sample to small test tube along with boiling chip and clamp onto stirring hot plate
- place thermometer about 2 mm above liquid
- heat slowly and record data points as function of time
- observe the point at which the temperature appears to “level off.” This is the boiling point
Why did we have to use a 360*C thermometer in our experiment?
Boiling points of possible compounds exceeded 150*C
Why did we practice with 5 known liquids?
To get a grip on the procedure and show that the boiling points may not be spot on but close to the expected value.
Why do alcohols and water have higher boiling points than alkanes?
They exhibit hydrogen bonding (stronger intermolecular forces).
