Alkanes Flashcards
alkane general formula
CnH2n+2
what is a hydrocarbon
a compound made of only hydrogen and carbon atoms
saturated or unsaturated and why
alkanes and cycloalkanes are saturated hydrocarbons because they contain c-c single bonds
general formula of a cycloalkane
CnH2n
define heterolytic bond fission
when the bond breaks unevenly (one bonded atom gets both electrons)
heterolytic bond fission arrows
X—– Y -> X+ + Y-
—>
define homolytic bond fission
when the bond breaks evenly (both bonded atoms receive an electron)
homolytic bond fission arrows
X—–Y -> X* + Y*
<—–>
what is produced through homolytic bond fission and what are they
free radicals (very reactive species that have an unpaired electron)
what type of reactions are reactions with halogens
photochemical reactions and radical substitution reactions
define photochemical reaction
reaction that requires UV light
why are reactions with halogens substitution
because hydrogen is substituted
symbol equation for methane + chlorine
CH4 + Cl2 -> CH3Cl + HCl
name and describe the three steps in reactions with halogens
1- initiation (radical produced)
2- propagation (radicals used up and created)
3- termination (radicals reacted together)
methane + chlorine initiation
Cl2 –(UV) –> 2Cl*
methane + chlorine propagation
Cl* + CH4 –> *CH3 + HCl
CH3 + Cl2 –> CH3Cl + Cl
methane + chlorine termination
Cl* + Cl* –> Cl2
Cl* + *CH3 –> CH3Cl
*CH3 + *CH3 –> C2H6
Limitations of radical substitution (carbon chain)
the reactions can occur anywhere along the carbon chain
structural isomers can form
Limitations of radical substitution (too much chlorine)
- describe and show with symbol equations
Cl* + CH3Cl –> *CH2Cl + HCl
CH2Cl + Cl2 –> CH2Cl2 + Cl
with too much chlorine, the reaction continues forming dichloromethane, this continues with more sub-reactions
- forming trichloromethane, tetrachloromethane…
how to solve limits of radical substitution- too much chlorine
solve with an excess of methane
- chlorine radical more likely to collide with methane and not chloromethane
what is another name for crude oil
petroleum
what is crude oil
a mixture of long-chained and short-chained hydrocarbons
how can you separate crude oil
through fractional distillation
describe the process of fractional distillation
- crude oil is vaporised at 350degrees celsius
- vaporised crude oil then placed in a fractionating column, it rises through the trays.
- the largest hydrocarbons remain at the bottom as ‘residue’ due to their boiling point being too high, they don’t vaporise
- each fraction condenses at their boiling point, the coolest being at the top
- separate fractions are collected
- hydrocarbons with the lowest boiling point don’t condense and are drawn off as gases at the top
define cracking
breaking long-chained alkanes into smaller hydrocarbons
cracking produces which two hydrocarbons
alkane + alkene
what are the two types of cracking
thermal + catalytic
conditions required for thermal cracking
1000degrees celsius, 70atm
what is produced in thermal cracking
lots of alkenes which can become polymers
conditions required for catalytic cracking
zeolite catalyst, 450degrees celsius, slight pressure
what is produced in catalytic cracking
aromatics and motor fuels
benefits of catalytic cracking
low costs as a low temperature and pressure required. catalyst speeds up reaction without being used up
what is ‘knocking’
when alkanes explode on their own accord when the fuel mixture in engines is compressed
what type of hydrocarbons are most likely to knock
straight chained hydrocarbons
how can you make knocking less likely and combustion more efficient and what is it called
convert straight chained hydrocarbons into branched/cyclic hydrocarbons (reforming)
what is required for reforming and give an example
a catalyst (platinum stuck on aluminium oxide)
reforming of hexane
hexane -> benzene (C6H6) + 4H2
reformation of octane
2,5- dimethlyhexane
name compounds produced by reforming
cycloalkanes, branched alkanes, arenes, aromatic hydrocarbons
combustion of alkanes word equation
alkane + oxygen -> carbon dioxide + water
what state must everything be in for combustion to take place
gas (vapour)
word equation for incomplete combustion
alkane + oxygen –> carbon + carbon monoxide + water
name pollutants produced from the combustion of alkane fuels
- carbon monoxide
- oxides of nitrogen
- sulfur dioxide
- carbon particulates
- unburnt hydrocarbons
problems with carbon monoxide
- it is toxic
- binds to haemoglobin in the blood
- less oxygen is circulated around the body
- victim dies of oxygen deprivation
problems with sulfur dioxide
- burning fossil fuels that contain sulfur
- sulfur dioxide gas enters atmosphere
- dissolves in moisture, becomes sulfuric acid
- creates acid rain
problems with oxides of nitrogen
- in car engines, at high temperature and pressure oxygen and nitrogen react together
- escapes into the atmosphere
- dissolves in moisture, forming nitric acid
- creates acid rain
describe the harm created by acid rain
- destroys trees and vegetation
- corrodes buildings and statues
- kills fish in lakes
describe the role of catalytic converters
to convert dangerous pollutants into harmless/less harmful gases
what type of catalyst is used in a catalytic converter
platinum catalyst
what can a catalytic converter turn the harmful gases into
carbon dioxide, nitrogen gas, water vapour
give an example of a symbol equation for a catalytic converter
define biofuels
fuels made from living matter over a short period of time
name three types of biofuels
- bioethanol
- biogas
- biodiesel
describe how bioethanol is made
ethanol is made from the fermentation of sugar from crop such as maize
describe how biogas is made
from the breakdown of organic matter
describe how biodiesel is made
from refining renewable fats and oils ( for example vegetable oils )
why are biofuels seen as carbon neutral
they release carbon dioxide when burnt but the carbon dioxide has come originally from the plants during their growth
which fuels are made from waste going to landfill
biodiesel and biogas
name 3 problems with biofuels
- petrol car engines must be modified to handle fuels with high ethanol concentrations
- land used to grow crops for fuel cant be used for food (problem in lic who rely on agriculture for their income)
- CO2 is still produced through the farming, refining and transportation of the crops ( can’t be completely classified as carbon neutral )
