4.1.2 - Alkanes Flashcards
(63 cards)
What type of hydrocarbons are alkanes and cycloalkanes?
Alkanes and Cycloalkanes are saturated hydrocarbons.
What is a sigma bond?
A sigma bond is a type of covalent bond which forms when atomic orbitals overlap head-on. Sigma
(σ) bonds can rotate freely.
What is a pi bond?
A pi bonds is a type of covalent bond which forms when adjacent p orbitals overlap sideways above and below the bonding C atoms. Pi bonds cannot be rotated.
What is the general formula of an alkane?
CnH2n+2
Why are alkenes more reactive than alkanes?
As pi bonds have low bond enthalpy, alkenes are more reactive than alkanes.
What is the shape around the carbon atom in saturated hydrocarbons?
Tetrahedral.
What is the bond angle around a carbon atom in a saturated hydrocarbon?
109.5°
Why is the shape around the carbon atom tetrahedral in saturated hydrocarbons?
Because there are four bond pairs of electrons which repel equally, arranging themselves as far apart as possible — forming a tetrahedral shape.
Why do boiling points increase down the alkane homologous series
Because larger alkanes have more electrons, leading to stronger induced dipole–dipole interactions (London forces) between molecules.
How does the shape of an alkane affect its boiling point?
Straight-chain (unbranched) alkanes have more surface area contact between molecules than branched alkanes, so they form stronger London forces and have higher boiling points.
Why do branched alkanes have lower boiling points than straight-chain alkanes?
Because they are more spherical and have less surface contact between molecules, resulting in weaker London forces.
What are the products of complete combustion of alkanes?
Carbon dioxide (CO₂) and water (H₂O).
What is a fuel?
A substance that releases heat energy when burned.
Why are alkanes used as fuels?
They burn readily in oxygen and release a large amount of heat energy because their combustion is highly exothermic.
Why are alkanes relatively unreactive?
Because C–C and C–H bonds have high bond enthalpies (they are strong), and the σ-bonds are non-polar or only very weakly polar.
What are the products of incomplete combustion of alkanes?
Carbon monoxide (CO) and/or carbon (soot).
What conditions are needed for complete combustion?
A plentiful (excess) supply of oxygen.
What happens if alkanes are burned with limited oxygen?
Incomplete combustion occurs.
Why does incomplete combustion produce less energy?
Because the fuel is not fully oxidised.
What is cracking?
The conversion of large hydrocarbon molecules into smaller, more useful ones by breaking single carbon–carbon (C–C) bonds.
How does carbon monoxide affect the body?
It binds strongly to haemoglobin in red blood cells, preventing oxygen from binding and being transported in the body.
What environmental problem can carbon (soot) contribute to?
Global dimming — it reflects sunlight back into space.
Why is carbon monoxide dangerous?
It is highly toxic and odourless. It can build up in enclosed spaces and cause death if heating systems are faulty.
What visual sign is often produced by incomplete combustion?
A sooty flame caused by carbon (soot).
