12- Alkanes Flashcards
What are alkanes?
-main components of natural gas, crude oil
-amongst the most stable organic compounds
-lack of reactivity has allowed crude oil deposits to remain the the earth for millions of year
What are alkanes mainly used as?
fuels, exploiting their reaction with oxygen to generate heat
What is the general formula of alkanes?
CnH2n+2.
(double carbon number and add two to get hydrogen number)
What are the properties of alkanes we look into?
-Bonding
-Shape
-Variations in boiling points
-Effect of chain length on boiling points
-Effect of branching on boiling point
What is the bonding in alkanes?
-saturated hydrocarbons, containing only carbon and hydrogen atoms joined together by single covalent bonds
-each carbon atom is joined to four other atoms by a single covalent bond called a sigma bond (σ)
-σ bond is the result of the overlap of two orbitals, one from each bonding atom. Each overlap contains 1 electron, so σ bond has 2 electrons that are shared between the bonding atoms
-σ bond is positioned on a line directly between bonding atoms
-each carbon atom has 4 σ bonds (C-C or C-H)
What is the shape of alkanes?
-each carbon atom is surrounded by 4 electron pairs in 4 σ bonds which all repel each other
-repulsion results in a 3D tetrahedral arrangement around each atom
-each bond angle is approx 109.5°
-the σ bonds act as an axes around which the atoms can rotate freely, so the shapes are not rigid nut distance between bonds remains relatively the same
-evenly spaced carbon and hydrogen atoms
What is crude oil, how is it separated?
-contains hundreds of different alkanes
-oil refiners separate the crude oil into different fractions by fractional distillation in a distillation tower
-each fraction contains a range of alkanes
-this is possible due to the range of boiling points of different alkanes
What are the variations of boiling points of alkanes?
-all alkanes have different boiling points
-the greater the intermolecular forces (londons forces) the higher the boiling points
-once the forces are broke the molecules move apart from each other and the alkanes become a gas
In alkanes how does the length of the chain effect the boiling point?
-as the chain length increases, the molecules have a large surface area so more surface contact is possible between molecules
-the longer the chain = the greater the londons forces so more energy is required to overcome the forces
In alkanes how does branching have an effect on the boiling point?
-branched isomers of alkanes have lower boiling points then straight chain isomers
-the more branching there is = the fewer the surface points of contact between molecules, giving fewer lands forces
-the branches get in the way and prevent branched molecules getting as close together as straight chain molecules, decreasing the intermolecular forces further
What are the reasons for the lack of reactivity of alkanes?
-C-C and C-H σ bonds are strong
-C-C bonds are non-polar
-the electronegativity of carbon and hydrogen is so similar that the C-H bond can be considered to be non-polar
Why are alkanes used as fuels?
-give out heat
-readily available
-easy to transport
-burn in plentiful supply of oxygen without releasing toxic fuels
What is the equation that is used to balanced the complete combustion of alkanes?
CxHy + (x + y/4) O2 → xCO2 + y/2 H2O
What happens in the complete combustion of alkanes?
-react with a plentiful supply of oxygen to produce CO2 and H2O
what is the equation for the complete combustion of methane?
CH4 + 2O2 → CO2 + 2H20
