Alkanes Chapter 12

Alkane defintion

a hydrocarbon with C-C and C-H single bonds only

general formula

CnH2n + 2

saturated hydrocarbon

a compound containing only hydrogen and carbon with C-C and C-H single bonds

unbranched chains

straight hydrocarbon chains

bond angle of unbranched chains

109.5

branched chains

straight chain with a substuent bonded to the main chain

general formula of ring alkanes

CnH2n

naming straight chain alkanes

identitfy the longest unbranched chain of carbons

compounds that dont have isomers

methane,ethane, propane

pattern for the number of isomers in an alkane

4 carbon atoms = 2 isomers

5 carbon atoms = 3 isomers

physical poperties

• polarity

• boiling point

• solubility

polarity of alkanes

non polar, because there is a small difference in electronegativity

intermolecular forces in alkanes

weak van der waal’s forces only

longer chain = stronger van der waal’s forces

boiling points of alkanes

as chain length increases so does the boiling points

factors affecting boiling points of alkanes

• chain length → more van der waal’s forces

• branching→ less branching allows the atoms to pack together closely

solubility of alkanes

insoluble because the strong hydrogen bonds dont interact with alkanes in water

reactivity of alkanes

• unreactive due to strong covalent bonds

• burn in excess oxygen to form carbon dioxide

• burn in abcence of 02 to form carbon monoxide

• can react with halogens

crude oil formation

breakdown of animal and plant remains at high pressures and temperatures very slowly

crude oil

micture of branched and unbranched alkanes

other elements in crude oil

sulfur burns to form sulfur dioxide

effect: reacts with oxygen in the atmosphere to form sulfur trioxide and then with water creating sulfuric acid

fractions definition

a mixture of hydrocarbons collected over a particular range of boiling points during fractional distillation of crude oil with similar boiling points and properties

steps of fractional distillation in a fractionating tower

• crude oil gets heated in a furnace

• the vapour rises and gets condensed in a cooler fractionating tower

• short chain compounds are condensed at the top of the tower

• the residue remaining is collected at the bottom

uses of natural gases

fuel

uses of petrol

cars

uses of naptha and kerosene

jet fuel

gas oil uses

lubrication

fuel oil and waxes uses

roads and roofing

fracking

• extraction of natural gas by drilling into the shale and litting it mix with the water releasing trapped natural gas

• HCl is added to break up the shale

ethical concerns with fracking

• visual pollution

• water wastage

• concern for chemical additives

• causes small earthquakes

• releases CO2 into the atmosphere

• contributes to global warming

why is cracking used

longer chain fractions are not as usefull or high in demand so they are broken down to form shorter chains which are worth more economically

uses of alkenes

• chemical feedstock (starting materials for different products)

• converted into polymers

• petrol

• starting material for polyethene

conditions for thermal cracking

temperature:700-1200 kelvins

pressure: 700 kilopascals/ KPa

what occurs in thermal cracking

the C-C bond so an electron from the covalent bond goes to each carbon atom forming a free radical

free radical definition

an atom with an unpaired electron which are highly reactive

products of thermal cracking

• hydrogen

• alkenes

• short chain alkanes

conditions of catalytic cracking

temperature: 720 kelvins

pressure: slightly higher than 101KPa

catalyst used in catalytic cracking

• silicon dioxide

• aluminium oxide

products of catalytic cracking

• motor fuels

• branched alkanes

• cycloalkanes

• aromatic compounds

word equation for the combustion of alkanes

alkane + oxygen → carbon dioxide + water

chemical equation for combustion of alkanes

CH4 + 2O2 → CO2 + 2H2O

features of the combustion of alkanes

• release heat

• negative enthalpies of combustion

• more carbons = greater heat output

fuel definition

substances that release heat energy when they undergo combustion

incomplete combustion

a combustion reaction where there is insufficient oxygen for all carbon in the fuel ro burn to carbon dioxide

carbon monoxide forms

what is formed in the complete abcence of oxygen

soot

in what type of chain alkanes does incomplete combustion occur

long chain because they require more energy and more oxygen than shorter chain alkanes

example of incomplete combustion

alkane + oxygen → carbon monoxide + H2O

common pollutants

• carbon monoxide

• nitrogen oxides

• sulfur dioxides

• carbon particulates

• carbon dioxide

• water vapour

carbon monoxide

It is a toxic, colorless, and odorless gas that contributes to air pollution.

dangers of carbon monoxide

• Binds to hemoglobin in red blood cells, reducing oxygen transport in the body.

• Can cause headaches, dizziness, unconsciousness

• reacting with other pollutants to form harmful ground-level ozone (smog).

equation for the formation of nitrogen oxides

N2(g) + O2(g) → 2NO(g)

how are nitrogen oxide formed

when the temperature of petrol engines reaches high temperatures forming the oxides

dangers of nitrogen oxide (2)

• react with water vapour and oxygen to form nitric acid

• contribute to acid rain and petrochemical smog

dangers of sulfur dioxide

reacts with water to produce sulfuric acid which contributes to acid rain

dangers of carbon particulates

exacerbate asthma and cause cancer

effect of carbon dioxide on the atmosphere

• necessary to keep earth habitable

• large amounts increase earths average temperature

• leads to climate change

uses of fossil fuels

burned to generate electricity

flue gas definition

a mixture of gases given out by power stations

formation of sulfur dioxides

natural gas contains sulfur impurities which form sulfur dioxide. this reacts with the water in the atmosphere to form acid rain

chemical and word equation of sulfur dioxide forming sulfuric acid

sulfur dioxide + oxygen + water → sulfuric acid

flue gas desulfurisation

process pf removinng SO2 from flue gas

2 methods of removing flue gas desulfurisation

1. uses calcium oxide

2. uses calcium carbonate

method 1 of desulfurisation using calcium oxide

• a slurry of calcium oxide and water are sprayed into the flue gas to form calcium sulfite (gypsum)

method 2 of desulfurisation using calcium carbonate

flue gas is passed through a suspensions of calcium carbonate

uses of catalytic converters

reduce the output of carbon monoxide, nitrogen oxides and unburnt hydrocarbons in the exhaust

shape of catalytic converters and the benefit of this shape

honeycomb shape that provides a large surface area. the honeybome is made of ceramic material that is coated with platinum and rhodium

reactions that occur in catalytic converters

carbon monoxide + nitrogen monoxide →nitrogen + carbon dioxide

greenhouse effect explanation

visible rays pass through the atmosphere and the energy gets absorbed and reradiated.

aenergy with longer wavelengths and infrared cannot escape the atmosphere

uses of the greenhouse effect

maintain earth’s average temperature keeping it warm enough to sustain life

what do green house gases do

absorb infrared heating up the earth

why has the concentration of CO2 increased in recent years

due to the undustrial revolution where fossil fuels have been used and burned for energy in industrial plants

effect of increased greenhouse gases in the atmosphere

a general increase in the earth’s average temperature contributing to global warming

explain water as a greenhouse gas

• very abundant but doesnt affect the earth as the levels of H2O in the atmosphere are constant

• increase in earths average temperature increases water vapour and greater cloud formation

carbon neutral activities defintion

activities that produce no carbon dioxide emissions overall

reaction type of formation of halogenoalkanes

substitution reactions

substitution word equation

alkane+ halogen → halogenoalkane + hydrogen halid

conditions for substitution

1 photon of ultra violet light

observations of substitution reactions

red/brown liquid goes colourless

3 key steps of free radical substitution

1. initiation

2. propagation stage 1 and 2

3. termination

free radical

a chemical species with an unpaired electron that makes it highly reactive

how does intitation occur

Cl2 absorbs energy from uv that breaks the covalent bond

equation to show initiation

Cl-Cl→ 2CL.

propagation stage 1

the free radical reacts with a hydrogen atom to form a HCl and methyl free radical

propagation stage 2

methyl radical is formed and reacts with Cl2 to form a halogenoalkane such as chloromethane

termination 3 types

• forming a halogen

• forming an alkane

• forming a halogenoalkane

termination by forming a halogen

2 free radical halogen atoms required

termination by forming a alkane

2 free radical alkanes required

termination by forming a halogenoalkane

a mixture of both a chlorine radical and alkane radical

limitations of chain reactions

• not as usefull

• micture of products

• can occur in the abundance of light

effect of CFC on the environment

destroys the ozone layer as CFC contribute to ozone layer decomposition increasing exposure to UV

role of ozone

absorbs infrared radiation and protect from exposure that can cause cancer

break down of ozone

radicals act as a catalyst to decompostion