Chemistry- alkanes learn eqs

Why are alkanes among the least reactive organic compounds

Their lack of a functional group

No significant diffference in electronegativity between the C and H atoms (2.5 and 2.1 respectively) making them un susceptible to attack by nucleophiles and electrophiles

2 uses of alkanes in industry

1. Fuels- short chain alkanes are very flammable and burn with a clean flame

2. They are feedstock (staring materials) for a range of other compounds eg solvents, lubricants, plastics etc

Fully describe fractional distillation process

1. Crude oil heated in furnace

2. A mixture of liquid and vapours pass into the bottom of the fractionating column where there is a temperature gradient

3. Shorter chain alkanes condense nearer the top of the column where it is cooler bc they have lower boiling points

4. Longer chain alkanes condense nearer the bottom of the column where it is hotter bc they have higher boiling points

What does the smaller the molecule mean in fractional distillation

• low boiling point

• Very volatile

• Flows easily

• Ignites easily

What does the larger the molecule mean in fractional distillation

• high boiling point

• Not very volatile

• Does not flow easily

• Does not ignite easily

What is the solubility of alkanes in water and in most organic solvents

• Alkanes are non polar due to the similarities in electronegativity between carbon and hydrogen

• and so are insoluble in water

• But soluble in most organic (non polar) solvents

Trend in bp of alkanes as chain length increases

As chain length increases the boiling point of the alkanes increases as they get more carbons in their formula, the more points of contact between molecules creating more Van der Waals forces

 This makes the Van der Waals forces stronger meaning more heat energy is required to separate the molecules when boiling 

Trend in bp as branching increases

• bp decreases

• Less points of contact between molecules

• Less van der waals forces induced

• Weaker vdw forces

• Less energy required to separate the molecules when boiling

What is cracking

The conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds

General equation for cracking

High Mr alkanes → smaller Mr alkanes + alkenes + hydrogen

2 types of cracking

Thermal cracking

Catalytic cracking

Conditions for thermal cracking and how does it occur

• High temperature 700K-1200K / 400-900ºC

• High pressure 7000 kPa

• Occurs by homolytic fission creating free radical intermediates

Products of thermal cracking and uses

Produces mostly alkenes eg ethene used for making polymers and ethanol

Sometimes produces hydrogen used in the Haber Process and in margarine manufacture

Conditions for catalytic cracking

• Low pressure- slightly below atmospheric

• High temperature 720K/450ºC

• Zeolite catalyst

Why are the temperatures much lower for catalytic cracking

The catalyst lowers the Ea of the reaction

Products of catalytic cracking, uses, and why

• Produces branched and cyclic alkanes and aromatic hydrocarbons

• Used for making motor fuels

• branched and cyclic hydrocarbons burn more cleanly and are used to give fuels a higher octane number

Which is cheaper thermal or catalytic cracking?

Catalytic cracking as it saves energy as lower temperatures and pressures are used

What other reactions may take place during thermal cracking

Dehydrogenation- the removal or loss of hydrogen from alkanes to produce alkenes

Isomerisation- occurs when un branched alkanes are converted into branched isomers or when cyclic alkanes undergo rearrangement

Cyclisation- alkanes are converted to cyclic alkanes and aromatic hydrocarbons, often with the loss of hydrogen

Structure of zeolite catalyst

• Honeycomb shape for a large surface area

• Heterogenous catalyst

• Adsorption desorption theory??

Why do alkanes make good fuels

• Readily burn in the presence of oxygen

• So highly exothermic combustion

• Fuels release heat energy when burnt

When does complete combustion take place & products

• When alkanes are burned in excess oxygen

• Products are carbon dioxide and water

When does incomplete combustion take place & products

• in limited amount of oxygen

• Produces carbon monoxide (toxic) and or carbon (producing sooty flame)

Which releases less energy per mole complete or incomplete combustion

Incomplete

Environmental consequence of carbon

Carbon (soot) can cause global dimming- reflection of the sun’s light

How is sulfur dioxide gas formed from combustion

• Sulfur containing impurities are found in petroleum fractions which produce SO2 when they are burned

• Coal is high in sulfur content and large amounts of sulfur oxides are emitted from power stations

Environmental consequence of sulfur dioxide

Dissolves in atmospheric water and produces acid rain

How can SO2 be removed from waste gases from furnaces

• Flue gas desulfurisation

• The gases pas through a scrubber containing basic calcium oxide which reacts with the acidic sulfur dioxide in a neutralisation reaction

• With CaO:

• SO2(g) + CaO(g) → CaSO3(s)

• With CaCO3:

• CaCO3(s) + SO2(g) → CaSO3(s) + CO2(g)

What is the calcium sulphite formed from flue gas desulfurisation used for

Used to make calcium sulphate for plasterboard

Environmental consequence of release of unburnt hydrocarbons

• Causes photochemical smog (caused by the reaction of unburnt hydrocarbons and nitrous oxide using sunlight)

• Unburnt hydrocarbon + NOx → (UV light) photochemical smog

How are nitrogen oxides formed and equations and environmental consequences of products

• From the reaction of N2 and O2 inside car engines

• The high temp and spark in the engine provides sufficient energy to break the strong N2 bond

• N2(g) + O2(g) → 2NO(g)

• 2NO(g) + O2(g) → 2NO2(g)

• NO is toxic and can form acidic gas NO2

• NO2 is toxic and acidic and forms acid rain

• NO2 + H2O + 0.5O2 → 2HNO3(aq)

Consequence of carbon particles (particulates)

• Exacerbate asthma and cause cancer

• Respiratory problems

What do catalytic converters do and equations

Remove CO, NOx and unturned hydrocarbons (eg octane, C8H18) from the exhaust gases, turning them into ‘harmless’ CO2, N2 and H2O