Alkene bonding — the C=C double bond
Alkenes contain a carbon-carbon double covalent bond () and are unsaturated hydrocarbons; alkanes have only single carbon-carbon bonds and are saturated. The simplest alkene is ethene, , with one and four bonds. Every alkene fits the general formula -- two hydrogen atoms fewer than the alkane , the pair the double bond replaces.
The bromine-water test for unsaturation
Shake the hydrocarbon with aqueous bromine (bromine water). An alkene decolourises it, orange to colourless; an alkane leaves it unchanged, still orange. The double bond is the reason: orange molecules add across the alkene to form a colourless dibromo-compound, using up the coloured bromine in the solution. An alkane has no for bromine to add across, so nothing reacts and the orange colour stays.
(Extended) Cracking, and addition across C=C
Cracking makes alkenes and hydrogen by breaking larger alkane molecules into smaller ones, using a high temperature and a catalyst: . In an addition reaction the opens and one atom or group joins each carbon: bromine gives 1,2-dibromoethane, hydrogen over nickel gives ethane, steam over an acid catalyst gives ethanol.
Drawn from real examiner reports.
Saturated ≠ unsaturated (not water content)
Saturated and unsaturated describe carbon-carbon bonding only -- they say nothing about water. Saturated = molecules with only single carbon-carbon bonds (the alkanes). Unsaturated = molecules with at least one double bond (the alkenes). Marks go by swapping the two terms, or by reading "saturated" in its everyday sense of water content.
Flagged Jun 2022 P41 Q2e · Nov 2023 P42 Q2c
Not showing the C=C when drawing ethene
If a question asks you to identify or draw an unsaturated hydrocarbon, the must be drawn in. Ethene is : two carbon atoms joined by a double line, with two hydrogen atoms on each. A structure drawn with all single bonds is an alkane no matter what it is labelled -- the double bond earns the mark, not the label.
Flagged Jun 2023 P32 Q8b
"Clear" is not "colourless"
The positive bromine-water result is orange to colourless. "Clear" is not accepted: in chemistry clear means transparent, and an orange bromine solution is already clear before anything reacts. Give both colours -- orange before and colourless after -- rather than the vague "turns clear", and remember the alkane result is the bromine staying orange.
Flagged Nov 2022 P33 Q2dii · Jun 2023 P42 Q2c
(Extended) Cracking ≠ fractional distillation
Cracking is a chemical reaction: larger alkanes are broken into smaller alkanes and alkenes at a high temperature with a catalyst. Fractional distillation (C11.3) is a physical separation, by boiling point, of hydrocarbons already in petroleum. Naming fractional distillation, a "catalytic converter", or a vague "heat" instead of both conditions scores zero.
Flagged Jun 2022 P41 Q2b · Nov 2023 P41 Q2c
(Extended) Balancing by inventing a molecule
When a cracking equation does not balance, fix the coefficients -- the numbers in front of whole formulae. Never invent an extra molecule to soak up the difference, and never change a subscript: that makes it a different substance. Cracking only redistributes atoms: every carbon and hydrogen in the reactant alkane must reappear across the products.
Flagged Jun 2022 P41 Q2b · Nov 2023 P41 Q2c
(Extended) Nickel catalyst ≠ acid catalyst
Both reactions add a small molecule across the , which is why the catalysts get swapped. Hydrogen addition needs a nickel catalyst and gives an alkane (ethene to ethane). Steam addition needs an acid catalyst (e.g. phosphoric acid) and gives an alcohol (ethene to ethanol). Swapping the two catalysts loses the conditions mark.
(Extended) The addition product has no C=C
An addition product is saturated -- the double bond has been used up, so no survives in the product. Both atoms of the added molecule end up in the single product: , with no bromine left over, and .
(Extended) Give BOTH cracking conditions
A "describe the manufacture of alkenes" answer needs both conditions -- a high temperature and a catalyst -- plus the products: a smaller alkane and an alkene, or an alkene and hydrogen. One condition alone is half an answer, so match your point count to the mark tariff.
Answer the command word actually used
State wants the term only; describe wants the observable steps or changes; explain wants the reason why. A scientifically true answer aimed at the wrong command word earns nothing, and the number of separate points you make should match the mark tariff.
Show the Mr working, check every subscript
For a relative formula mass, multiply each by the subscript in the formula and show the working -- -- so a method mark survives a slip. carries no unit, but any answer that does have one must show it.
Balance by counting one element at a time
Write out every formula, then count one element at a time -- carbon first, then hydrogen, then bromine or oxygen. Where a count does not match, adjust a coefficient and re-count from the start, because one change can unbalance an element you had already settled.
Cambridge 0654 spec reference: Section C11 "Organic chemistry", sub-topic C11.5 "Alkenes". Core: state that alkene bonding includes a double carbon-carbon covalent bond and that alkenes are unsaturated hydrocarbons; describe the test to distinguish saturated from unsaturated hydrocarbons by reaction with aqueous bromine. Supplement (Extended): describe the manufacture of alkenes and hydrogen by cracking larger alkane molecules using a high temperature and a catalyst; describe alkene addition reactions with bromine, with hydrogen (nickel catalyst) and with steam (acid catalyst).
| Term | Mark-scheme-precise meaning |
|---|---|
| Alkene | A hydrocarbon whose molecules contain a carbon-carbon double covalent bond (); general formula |
| Unsaturated hydrocarbon | A hydrocarbon containing at least one double bond |
| Saturated hydrocarbon | A hydrocarbon containing only single carbon-carbon bonds (the alkanes) |
| Cracking (Extended) | Breaking down larger alkane molecules into smaller alkane and alkene molecules (and/or hydrogen), using a high temperature and a catalyst |
| Addition reaction (Extended) | A reaction in which the double bond of an alkene opens up, and atoms from a second reactant add on, one to each of the two carbon atoms, forming a single product with no double bond remaining |
The bromine-water test (Core): shake the hydrocarbon sample with aqueous bromine.
| Hydrocarbon | Observation | Conclusion |
|---|---|---|
| Alkene (unsaturated) | Aqueous bromine decolourises: orange colourless | Unsaturated -- contains a double bond |
| Alkane (saturated) | Aqueous bromine stays orange | Saturated -- no double bond present |
Cracking (Extended): general form -- larger alkane smaller alkane + alkene, e.g.
or, when hydrogen itself is the product, a single alkane loses to form the alkene of the same chain length:
Addition reactions of ethene (Extended), summarised:
| Reagent | Conditions | Product |
|---|---|---|
| Bromine, | (readily, at room temperature) | (1,2-dibromoethane) |
| Hydrogen, | nickel catalyst | (ethane) |
| Steam, | acid catalyst | (ethanol) |
Define "saturated hydrocarbon".
(a) State the type of bond present between the two carbon atoms of a group in an alkene molecule. (1 mark)
(b) Explain, in terms of this bonding, why alkenes are described as "unsaturated" while alkanes are described as "saturated". (2 marks)
