What is the functional group of the alkenes?

A carbon–carbon double bond, C=C.

Why are alkenes called unsaturated?

They contain a C=C double bond, so they hold fewer hydrogen atoms than the alkane with the same number of carbon atoms.

Give the names and formulae of ethene, propene, butene and pentene.

C₂H₄, C₃H₆, C₄H₈, C₅H₁₀.

Why is there no alkene called methene?

A double bond needs at least two carbon atoms.

What is an addition reaction?

A reaction in which a small molecule adds across a C=C bond, forming a single product.

Write the equation for ethene reacting with hydrogen.

C₂H₄ + H₂ → C₂H₆ (nickel catalyst, about 150 °C).

Write the equation for ethene reacting with bromine.

C₂H₄ + Br₂ → C₂H₄Br₂

What is made when ethene reacts with steam?

Ethanol, C₂H₅OH, with a phosphoric acid catalyst at high temperature and pressure.

What is the test for an alkene?

Shake with bromine water. An alkene changes it from orange-brown to colourless.

Why should you write 'colourless' rather than 'clear'?

Clear only means see-through; the result of the test is the loss of the orange-brown colour.

Why are alkenes more reactive than alkanes?

The C=C double bond can open up so that other atoms can add on.

Name one use of an addition reaction of an alkene.

Adding hydrogen to vegetable oils to make margarine (or adding steam to make ethanol).

AlkenesIB MYP Chemistry: Flashcards

Card 1 of 140 of 14 known

Question

What is the general formula of the alkenes?

Tap or press Space to reveal

Tap card or press Space to flip

See all 14 cards

What is the general formula of the alkenes?: CₙH₂ₙ
What is the functional group of the alkenes?: A carbon–carbon double bond, C=C.
Why are alkenes called unsaturated?: They contain a C=C double bond, so they hold fewer hydrogen atoms than the alkane with the same number of carbon atoms.
Give the names and formulae of ethene, propene, butene and pentene.: C₂H₄, C₃H₆, C₄H₈, C₅H₁₀.
Why is there no alkene called methene?: A double bond needs at least two carbon atoms.
What is an addition reaction?: A reaction in which a small molecule adds across a C=C bond, forming a single product.
Write the equation for ethene reacting with hydrogen.: C₂H₄ + H₂ → C₂H₆ (nickel catalyst, about 150 °C).
Write the equation for ethene reacting with bromine.: C₂H₄ + Br₂ → C₂H₄Br₂
What is made when ethene reacts with steam?: Ethanol, C₂H₅OH, with a phosphoric acid catalyst at high temperature and pressure.
What is the test for an alkene?: Shake with bromine water. An alkene changes it from orange-brown to colourless.
What happens when bromine water is added to an alkane?: It stays orange-brown.
Why should you write 'colourless' rather than 'clear'?: Clear only means see-through; the result of the test is the loss of the orange-brown colour.
Why are alkenes more reactive than alkanes?: The C=C double bond can open up so that other atoms can add on.
Name one use of an addition reaction of an alkene.: Adding hydrogen to vegetable oils to make margarine (or adding steam to make ethanol).

Exam questions on Alkenes

A student compares two colourless gases, ethane, C₂H₆, and ethene, C₂H₄. Both gases burn, but they behave differently when she bubbles each through bromine water.
Explain why ethene is described as an unsaturated hydrocarbon.2 marks
A technician has two unlabelled colourless liquids. One is pentane, an alkane, and the other is pentene, an alkene. She adds 2 cm³ of orange-brown bromine water to 2 cm³ of each liquid in separate test tubes and shakes each tube.
Describe what the technician sees in each test tube and how she can tell which liquid is pentene.2 marks
A student has four colourless liquids labelled P, Q, R and S. Each is either an alkane or an alkene. She adds 2 cm³ of bromine water to 2 cm³ of each liquid in a test tube, shakes it and records the colour of the mixture after 1 minute. P and R stay orange-brown. Q and S become colourless.
Identify the independent variable, the dependent variable and one control variable in the student's investigation.3 marks

See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).