Alkanes
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What is the molecular formula of isooctane?
C8H18
What is the systematic name for isooctane?
2,2,4 trimethyl octane
Draw the displayed formula of an isooctane molecule
Describe the electronegativity of carbon and hydrogen
The electronegativities are very similar so the C-H bonds are non polar. As a result of this, alkanes are non polar molecules and have no partial positive and negative charges.
Given that each carbon atom forms four covalent bonds, what is the bonding arrangement(shape) of methane?
tetrahedral with a bond angle of 109.5 degrees
Because alkanes are non polar, what electrostatic forces do they exhibit?
temporary induced dipole-dipole forces between the molecules
As the bond length of the alkane molecule increases…?
the boiling point also increases
As the bond length of the alkane molecule increases…?
the boiling point also increases
What does the boiling point of a substance show?
It’s a good indication of the strength of forces between the particles in it.
High boiling points=?
strong intermolecular forces
What are van der waal’s forces?
they are a weak form of intermolecular force caused by electrons creating instantaneous dipole-induced dipole forces .
How do van der waals forces increase?
They increase with increasing number of electrons/ increasing chain length
When are van der waal’s forces the highest?
van der waal’s forces are highway when molecules have lots of surface contact. straight chain molecules can pack closer so have more surface contact.
The boiling point of alkanes varies with what 2 factors?
chain length branching
Explain how chain length affects the boiling point of alkanes
Temporary induced dipole-dipole forces will be very small for an alkane such as methane but will increase as the size of the molecules increase Therefore, the boiling points of the alkanes increases with the molecular size, due to the increased temporary induced dipole-dipole forces
Explain how branching affects the boiling point of alkanes.
Branched alkanes normally exhibit lower boiling points than unbranched alkanes containing the same number of carbon atoms. Temporary induced dipole-dipole forces can only operate over very short distances between one molecule and its neighbouring molecules. The unbranched alkanes have greater van der Waals’ forces of attraction because of their greater surface areas.
Draw the skeletal formulae of two branched isomers of hexane and state the type of isomerism shown by these branched isomers.
chain isomerism
What is the change from a solid to a gas called?
deposition
What is the change from a gas to a solid called?
sublimation
What is the difference between evaporation and boiling ?
Evaporation involves the change of liquid to gas, but unlike boiling, evaporation only occurs at the surface and takes place at the temperatures below boiling the point. Boiling occurs at a specific temperature and takes place when the vapour pressure reaches the external atmospheric pressure
The temperature of any substance is directly proportional to what?
the mean kinetic energy of the particles
Describe the graph
• Between 1 & 2, the particles are vibrating and gaining kinetic energy and the temperature rises • Between 2 & 3, all the energy goes into breaking intermolecular attractions - there is no increase in kinetic energy or temperature • Between 3 & 4, the particles are moving around and gaining in kinetic energy • Between 4 & 5, the substance is boiling, so intermolecular attractions are breaking and there is no increase in kinetic energy or temperature • From 5 & 6, the particles are moving around rapidly and increasing in kinetic energy
At absolute zero, how much kinetic energy to the particles have?
none
Why do liquids cool as they evaporate?
When a liquid evaporates (which occurs below the boiling point), some KE is used to overcome forces between particles to allow the particle to escape. As a consequence, the mean KE of the remaining particles is lower and so the T is lower.