4.2 Alkanes Flashcards
What are alkanes ?
They are saturated hydrocarbons consisting only of carbon and hydrogen joined bye single bonds only.
What are alkanes the most components of ?
Main components of natural gas and crude oil. They are mainly used as fuel as they react with O2 and have low reactivity to other compounds/ elements.
What bonds are found in alkanes ?
Only single covalent bonds, known as sigma bonds, as a result as the end-on overlap of atomic orbitals from each bonding atom.
What shape are alkanes ?
Alkanes are tetrahedral shape with bond angled of 109.5. There are 4 pairs of electrons in sigma bonds to hydrogen or carbon atoms.
General formula of alkanes
CnH2n+2
What is crude oil ?
Crude oil contains hundreds of alkanes that can be separated by cracking in fractional distillation.
What do London forces act between ?
They act between molecules that are in close surface contact.
Trend in boiling point of alkanes
Boiling point increases as chains get longer because:
-Large SA so more points of surface contact.
-More London forces that are greater so more energy is required to break forces of attraction.
Do branched alkanes have lower boiling point ?
Branched isomers of alkanes have lower boiling points because:
-Less points of SA contact
-Smaller chains
-Fewer London forces
-Branches get in way from molecules being closer together.
Why don’t alkanes react with most common reagents ?
-C-C sigma bonds are strong
-C-C bonds are non-polar
-Electronegativity of C and H are similar that C-H can be considered non-polar.
What do all combustion reaction give out ?
All combustion process give out heat.
What is complete combustion of alkanes ?
When there is a plentiful supply of oxygen, alkanes burn to produce carbon dioxide and water.
What is incomplete combustion of alkanes ?
When there is a limited supply of oxygen, hydrogen atoms are oxidised to form water, but also carbon monoxide, which is toxic.
What is carbon monoxide ?
Carbon monoxide is a toxic, colourless and odourless gas. It combines irreversibly with haemoglobin to form carboxyhaemaglobin preventing binding oxygen being transported around the body. Its danger is the lack of colour as a person may be being poisoned but don’t know it.
In what conditions do alkanes react with halogens for free radical substitution ?
In the presence of sunlight, alkanes react with halogens due to UV radiation that provides energy for a reaction.
