Alkanes (MDY) Flashcards
(40 cards)
What is the general formula for alkanes?
CnH2n+2
Bond angles around carbon in alkanes
109.5, tetrahedral
What type of intermolecular forces do alkanes have?
VdW
Why don’t alkanes show pD forces?
They are non polar
How does the melting/boiling point of alkanes change as they get bigger and why?
Increases as there are more electrons so stronger VdWs
How does branching affect melting/boiling points?
- Branched molecules have lower boiling points than molecules with the same amount of Carbons
- Because VdW forces are highest when there’s more surface contact
- Straight chain molecules have more surface contact
What are Van der Waals forces and how do they come about?
- intermolecular force that exists between all molecules
- Instantaneous dipole in one molecule induces a dipole in another and there is a force of attraction between the dipoles
What are permanent dipole forces?
- Inter molecular forces that occur between polar molecules
- Delta positive and negative regions are attracted to each other
What is hydrogen bonding?
- Type of pD-pD forces that occurs when N, O or F is bonded to a H
- N, O, F are very electronegative and create a very strong dipole
- H becomes delta +ve and is attracted to the lone pair in another molecule
What is crude oil
A finite resource found in rocks. It is a mixture of hydrocarbons
How is crude oil formed?
Over millions of years from the remains of plankton which were buried in mud
How do we separate crude oil?
By fractional distillation
What is the temperature gradient in a fractionating column?
Hot on the bottom, cools as you rise
Why do we separate crude oil?
To make use of the separate hydrocarbons
Which hydrocarbons will condense at the top?
Short chain
Which hydrocarbons will condense at the bottom?
Long chain
What properties change as the C chains get longer?
- Molecules become more viscous
- Harder to ignite
- Less volatile
- Higher BP
What is cracking?
Breaking long chain hydrocarbons into smaller more useful ones
What hydrocarbon is always a product of cracking?
Alkene
Conditions of thermal of cracking
- High pressure, 500 - 800kPa
- 400 - 900 degrees
- Heat as a catalyst
- Products, shorts straight chain alkanes and alkenes
- Uses of products, polymers
Conditions of Catalytic cracking
- Around 1ATM
- Around 450 degrees
- Zeolite catalyst, which has pores for a high SA
- Products, Branched, cyclic and aromatic hydrocarbons
- Uses of products includes fuels
What is combustion?
An exothermic reaction with oxygen
What are the products of the complete combustion of alkanes?
Carbon dioxide and water
What are the products of the incomplete combustion of alkanes?
Carbon monoxide and water
