Alkenes UNIT 4

Question 1

Properties of alkenes

Answer 1

General formula CnH2n (same as cycloalkanes)
Unsaturated hydrocarbons with at least one C=C functional group. ✓
Alkenes are more reactive than alkanes undergoing addition reactions.

Question 2

Bonding in alkenes

Answer 2

Sigma bonds (σ-bonds) – end-on-end overlap of orbitals directly between atoms. ✓
Pi bonds (π-bonds) – sideways overlap of p orbitals to give a π-bond above and below.✓

Question 3

Stereoisomerism

Answer 3

The same structural formula ✓
But a different arrangement of atoms in space ✓

Question 4

Describe the two types of stereoisomerism and how to identify them

Answer 4

E/Z isomerism (geometric isomerism)
(Z) is when both groups are at the ‘same’ side ✓ (E) is when both groups are at ‘either’ side ✓

Question 5

Rules for E/Z:

Answer 5

Contain a C=C bond which restricts rotation. ✓
Two different groups attached to each carbon in the C=C. ✓

Question 6

Name of the reaction of alkenes reacting with halogens and the reaction conditions

Answer 6

Synthesising dihaloalkanes (halogenation)
Alkenes react with halogens to produce dihaloalkanes
Reaction Conditions: 20oC ✓

Question 7

Testing for alkenes/ unsaturation

Answer 7

Testing for alkenes/ unsaturation
Add bromine water to a solution and shake:
Contains double bond (unsaturated) – Orange to colourless ✓
Contains no double bond (saturated)– No visible change ✓
Bromine is added across the double bond ✓

Question 8

Name of the reaction of ALKENES reacting with ALKANES and the reaction conditions

Answer 8

Synthesising alkanes (hydrogenation)
Alkenes react with hydrogen gas to produce alkanes
Conditions: Nickel catalyst and 150oC ✓

Question 9

Name of the reaction of ALKENES reacting with ALCOHOLS and the reaction conditions

Answer 9

esising alcohols (hydration)
Alkenes react with steam to produce alcohols
Conditions: H3PO4 catalyst, 60-70atm and 300oC ✓

Question 10

Chemists have developed ways of removing hydrogen chloride from these waste gases.
Sodium hydrogencarbonate, NaHCO3(s), is frequently used in industry for this purpose.
Explain how sodium hydrogencarbonate removes hydrogen chloride.

Answer 10

Sodium hydrogencarbonate neutralises
HCl ✔

Question 11

Outline how a catalyst increases the rate of a chemical reaction

Answer 11

(allows the reaction to proceed via a route
with) lower activation energy … (1)
… so that a greater proportion of
molecules exceed the activation energy
(1)