Module 4: Section 1 Flashcards
(44 cards)
What is an isomer?
isomers have the same molecular formula but the atoms are arranged differently.
Name 2 types of isomer.
structural isomers and stereoisomers.
What are structural isomers?
structural isomers have the same molecular formula but a different structural formula (atoms connected in different ways).
Name 3 types of structural isomer.
- chain isomer
- positional isomer
- functional group isomer
What are chain isomers?
the carbon skeleton is arranged differently e.g. as a straight chain or branched in different ways.
have similar chemical properties but different physical properties because of change in shape of molecule.
e.g. C4H10 can be butane or methyl propane
What are positional isomers?
functional group attached to a different carbon atom.
have different physical properties and chemical properties might be different too.
e.g. C4H10O can be butan-1-ol or butan-2-ol
What are functional group isomers?
same atoms arranged into different functional groups.
have very different physical and chemical properties.
e.g. C4H8O2 can be butanoic acid or methyl propanoate
What is the general formula of the alkanes?
CnH2n+2
Why are alkanes classed as hydrocarbons?
the contain hydrogen and carbon only
Why are alkanes saturated?
all the carbon-carbon bonds are single bonds
What is the general formula of cycloalkanes?
CnH2n
cycloalkanes have two fewer hydrogens than alkanes
What is the shape and bond angle around each carbon in alkanes?
in each alkane molecule, each carbon atom has four pairs of bonding electrons around it. They all repel each other equally so the molecule forms a tetrahedral shape around each carbon. Bond angle = 109.5°
What does the boiling point of an alkane depend on?
its size and shape
What physical state are small alkanes, like methane, at room temp and pressure?
gas - they’ve got very low boiling points
What physical state are larger alkanes at room temp and pressure?
liquid - they have higher boiling points
Why do larger alkanes have higher boiling points than smaller alkanes?
alkanes have covalent bonds between atoms inside the molecules. Between molecules, there are London forces (induced dipole-dipole forces) which hold them all together. The longer the carbon chain, the stronger the London forces because there’s more surface contact and more electrons to interact. It take more energy to overcome these London forces and the boiling point rises.
Why does a branched-chain alkane have a lower boiling point than its straight chain isomer?
branched-chain alkanes can’t pack closely together and they have smaller molecular surface areas so London forces are reduced.
(unbranched alkane molecules can pack closely together so there is a lot of surface contact)
What is produced when alkanes are burnt (oxidised) completely in oxygen?
carbon dioxide + water
What type of reaction is it?
combustion reaction
Why do liquid alkanes need to be vaporised before combustion?
combustion reactions happen between gases, so liquid alkanes have to be vaporised first
How are smaller alkanes described as they turn into gases more easily?
smaller alkanes are more volatile (so they’ll burn more easily too)
Why do larger alkanes release more energy per mole when they’re combusted?
they have more bonds to react
Why do alkanes make good fuels?
they release so much energy when they burn
What is produced when alkanes are burnt in limited oxygen?
carbon monoxide + water
