3.3.2 Alkanes Flashcards
What is an alkane?
A saturated hydrocarbon.
What does saturated mean?
Molecule containing no double bonds.
What does petroleum consist mainly of, how is this separated?
Alkane hydrocarbons, fractional distillation.
Define fraction.
Contains compounds that have similar b.ps.
Different fractions have different what? What does this depend on?
B.ps, depends on the size or Mr of the hydrocarbons.
What is the temperature gradient of a fractioning column?
Cold at the top (25), hot at the bottom (360).
What does cracking involve?
Breaking C-C bonds in alkanes.
Why do we do crack?
Longer chain fractions aren’t as useful and have a lower value economically.
What does cracking produce?
Shorter, more useful chains. Alkenes.
What are the conditions for thermal cracking? What does it produce?
400-900 C, 7000 KPa. Produces high proportion of alkenes to make plastics.
What are the conditions for catalytic cracking? What does it produce?
450 C, slightly above atmospheric pressure, zeolite catalyst. Produces high proportion of branched alkanes, cycloalkanes and aromatic hydrocarbons.
What is the general equation for complete combustion of alkanes?
CnH2n+2 + XO2 -> XCO2 + XH2O
What is the general equation for incomplete combustion?
CnH2n+2 + XO2 -> XCO + XH2O
or
CnH2n+2 + XO2 -> XC + XH2O
What are the problems with complete combustion of alkanes?
Produces CO2 + H2O, both greenhouse gases.
What are the problems with incomplete combustion of alkanes?
Produces CO (toxic) and C (carbon particulates exacerbate asthma and causes cancer).
What do exhaust fumes contain that’s harmful?
Contain unburnt alkanes, greenhouse gases and contribute to photochemical smog.
What is harmful about burning petrol?
Burns at 1000 C, enough activation energy for N2 + O2 in the air-petrol vapour mix to react. Produces NOx gases.
What are the equations for the reaction between N2 and O2? What do the gases produce?
N2 + O2 -> 2NO (Colourless)
2NO + O2 -> 2NO2 (Brown gas)
Builds up to give a brown haze and also reacts in water to give HNO3 (acid rain).
What is harmful about using impure fossil fuels. What are the equations?
S impurities, major contributor to acid rain.
S + O2 -> SO2
SO2 + H2O -> H2SO3
2H2SO3 + O2 -> 2H2SO4
What do catalytic converters do? How?
Remove carbon monoxide, nitrogen oxides and hydrocarbons from our car exhausts. Contain a honeycomb of ceramic material onto which metals such as platinum, palladium and rhodium are spread in a thin layer.
Give the equations for reactions with catalytic converters. What do we still produce?
2CO + 2NO -> 2CO2 + N2
C8H18 + 25NO -> 8CO2 + 12N2 + 9 H2O
We still produce greenhouse gases.
What happens to a mixture of methane and chlorine under UV light at room temp? What is the mechanism for this reaction called?
Reacts, very high temperature breaks Cl-Cl bond. Free radical substitution.
What is a free radical?
A species that contains an odd number of electrons, with one e- not paired up with anyother.
How are radicals formed?
When a covalent bond breaks with an equal splitting of the bond pair of electrons (homolytic fission).
What is the first step in free radical substitution?
The initiation step. The UV provides enough energy needed to start the reaction by splitting some molecules into atoms (radicals).
Cl2 —-UV light—-> 2Cl•
Cl-Cl is weaker than the C-H bond in methane.
What is the second step in free radical substitution?
Propagation step. In each step, a radical is used and a new radical is formed. Each step is exothermic, so the chain reaction might produce an explosion.
•Cl + CH4 -> •CH3 + HCl
•CH3 + Cl2 -> CH3Cl + Cl•
Overall reaction of the two propagation steps are
CH4 + Cl2 -> CH3Cl + HCl (radicals cancel)
What is the equation for catalytic converter reaction?
2CO + 2NO -> 2CO2 + N2
Why do we use a thin layer of metal in a catalytic converter?
Less material needed, more SA.
Bromine water reaction?
Orange/ yellow to colourless.
Saturated meaning?
Only single bonds, no double bonds.
Why are sulphur impurities bad?
Produce SO2 (not sulphur oxides!!), contributes to acid rain.
Explain how oxides of N are formed?
React with oxygen when burnt, at high temps.
Catalyst for catalytic cracking?
Zeolite.
Why is the m.p of dodecane higher than m.p of straight chain alkane when cracked?
More bonds, longer chain for dodecane, stronger VDW forces between molecules.
What compound reacts with SO2 before it enters the atmosphere?
Calcium oxide/carbonate because it neutralises SO2.
Why ethanol higher b.p than ethaniol?
Ethanol has H bonding, stronger VDW.
Functional group isomer of enes?
Cyclic alkane.
Ethene?
Antifreeze.
What are the things you forget about the fractional distillation process?
Crude oil heated to vapourise it, fractions have different b.p, b.p depends on size.
Characteristics of homologous series?
Same general formula, gradual change in physical properties, similar chemical properties, differs by CH2.
Catalyst in converter?
Platinum / palladium / rhodium
Why is cracking commercially good?
More high value products.
