Alkanes Flashcards
What is the general formula for alkanes
CnH2n+2
What are alkanes
saturated hydrocarbons
why are alkanes non-polar molecules
the electronegativity of the carbon and the hydrogen are very similar, so there is a symmetrical distribution of the electrons. Thus, alkanes are held by only week London dispersion forces
why do boiling points of alkanes increase as the chain length increases
larger molecules have more electrons, thus greater electron density and greater induced dipoles. This causes greater London dispersion forces of attraction, thus more energy is needed to separate the bonds
why does boiling point decrease as the number of branches increases
because the long chain molecules of an unbranched alkane can pack together much more readily with a greater area of contact, than with branched alkanes
why are alkanes insoluble in water
because alkanes are non-polar, meaning it doesn’t have any dipole-dipole interactions, thus cannot dissolve in water
why do alkanes dissolve in non-polar organic liquids
because the energy required to break the week London dispersion forces between the alkane and the liquid molecules is given back when forming the London dispersion forces between the alkane and the solvent
meaning polarity
caused by an asymmetry in the +ve and -ve charge centres within a molecule, causing an induced dipole
meaning electronegativity
the relative abilities of atoms to attract electrons in a covalent bond
meaning London Dispersion forces
the weakest form of intermolecular force caused by small, naturally occurring, oscillating dipoles in one molecule, inducing another dipole in the electron density of other molecules, causing the molecules to attract each other
meaning hydrogen bonds
the strongest of the intermolecular forces created when lone pairs of electrons on a highly electronegative atom becomes attracted to a slightly positive hydrogen atom covalently bonded to a highly electronegative F, O or N atom
why does lime water turn cloudy in the presence of carbon dioxide
because white precipitate of calcium carbonate is formed
why is carbon monoxide toxic
because it is absorbed by the haemoglobin in the blood to prevent transport of oxygen
what are the products of complete combustion
carbon dioxide and water
what are the products of incomplete combustion
carbon monoxide, soot and water
meaning fractional distillation
the process of separating crude oil into fractions of hydrocarbons with similar numbers of hydrocarbons and similar boiling points
Describe the process of fractional distillation
- Evaporation: Crude oil is evaporated and the vapour is put into a fractionating column at the bottom and rises upwards
- The temperature at the bottom is the highest, so long-chain hydrocarbons condense at the bottom and are collected as liquids. Short-chain hydrocarbons have lower boiling points, so pass up the columns and condense at lower temperatures
- The fractions and collected and processed to create fuels
meaning cracking
the process of converting long-chained alkanes into shorter alkanes and alkenes, as shorter hydrocarbons are more valuable
meaning thermal cracking
uses harsher conditions such as 1000°C and 70 atm to break long-chained alkanes into a high percentage of alkenes
meaning catalytic cracking
uses lower temperature (450°C) and atmospheric pressure to break down long-chained alkanes into benzine-containing molecules and shorter alkanes, with the use of the catalyst zeolite
which flame does complete combustion produce
blue
which flame does incomplete combustion produce
orange smokey flame, due to the small particles of pure carbon glowing red hot
how is sulfur dioxide removed from car engines
by reaction sulfur dioxide with calcium carbonate or calcium oxide. This is because sulfur dioxide is an acid, and calcium carbonate is a base, resulting in a neutralisation reaction
explain the steps of free radical substitution
- Initiation: UV light produces reactive radicals
- Propagation: Radicals react in chain reactions
- Termination: Radicals join to form stable molecules
explain the Initiation step in the free radical substitution reaction with chlorine and methane
UV light breaks the Cl-Cl bond in chlorine via homolytic fission to give 2 chlorine radicals. The unpaired electron makes the Cl radicals very reactive
explain the Propagation step in the free radical substitution reaction with chlorine and methane
The chlorine radicals react with a methane molecule in a substitution reaction. The methyl radical then reacts with another chlorine molecule. This propagation cycle continues until the reagents are used up
explain the Termination step in the free radical substitution reaction with chlorine and methane
Two radicals react together to form a stable covalent bond. If there is excess methane, chloromethane is the major product. If there is excess chlorine, chloromethane would have further been substituted to form secondary and tertiary structures
What are 2 issues with free radical substitution
- With an excess of halogens, additional substitution reactions can occur, resulting in a mixture of halogenoalkanes that must be separated
- The propagation radicle can substitute at any position along a carbon chain, leading to the production of positional isomers
