Chemistry | Y10 Alkenes

Question 1

Why are alkenes unsaturated?

Answer 1

Alkenes are unsaturated because they have a double carbon-carbon bond.

Question 2

Alkanes have a what type bond?

Answer 2

Alkenes Have a C=C Double Bond

Question 3

Alkenes burn with what type of flame?

Answer 3

Alkenes Burn With a Smoky Flame

Question 4

What are alkenes?

Answer 4

Alkenes are hydrocarbons which have a double bond between two of the carbon atoms in their chain.

Question 5

Why does akenes having a double carbon bond make them unsaturated?

Answer 5

They have C=C double bond means that alkenes have two fewer hydrogens compared with alkanes containing the same number of carbon atoms. This makes them unsaturated.

Question 6

How do the alkenes get reactive because of the double bond?

Answer 6

The C=C double bond can open up to make a single bond, allowing the two carbon atoms to bond with other atoms. This makes alkenes reactive.

Question 7

The first four alkenes:

Answer 7

are ethene (with two carbon atoms),
propene (three Cs), butene (four Cs) and pentene (five Cs).
far more reactive than alkanes.

Question 8

In a large amount of oxygen what do alkenes produce when burned?

Answer 8

In a large amount of oxygen, alkenes combust completely to produce only water and carbon dioxide

Question 9

________ -chain alkenes have _____ as many hydrogen atoms as ___

Answer 9

Straight-chain alkenes have twice as
many hydrogen atoms as carbon

Question 9

If there isn’t enough air for alkenes to burn (which happens usually) what happens?

Answer 9

There isn’t enough oxygen in the air for this, so when you burn them they tend to undergo incomplete combustion. Carbon dioxide and water are still produced, but you can also get carbon and carbon monoxide (CO) which is a poisonous gas.

Question 10

What can incomplete combustion result in?

Answer 10

Incomplete combustion results in a smoky yellow flame, and less energy being released compared to complete combustion of the same compound

Question 11

An example of an equation for incomplete combustion:

Answer 11

C4H8 +502 → 2CO + 2CO2 + 4H2O

Question 12

Alkenes React via ____ Reactions

Addition of Hydrogen is Known as ___________

Answer 12

Alkenes React via Addition Reactions

Addition of Hydrogen is Known as Hydrogenation

Question 13

What is a functional group?

Answer 13

A functional group is a group of atoms in a molecule that determine how that molecule typically reacts

Question 14

Why do all alkenes have a function group?

Answer 14

All alkenes have the functional group ‘C=C’, so they all react in similar ways so you can suggest the products of a reaction based on your knowledge of how alkenes react in general.

Question 15

What happens when alkenes react via addition reactions.

Answer 15

The carbon-carbon double bond will open up to leave a single bond and a new atom is added to each carbon:

Question 16

Steam can React with Alkenes to Form ________

Answer 16

Steam can React with Alkenes to Form Alcohols

Question 17

Hydrogen can react with what to become an equivalent, saturated alkane?

Answer 17

Hydrogen can react with the double-bonded carbons to open up the double bond and form the equivalent, saturated, alkane.
The alkene is reacted with hydrogen in the presence of a catalyst:

Question 18

How is alcohol formed when steam is in contact with an alkene?

Answer 18

When alkenes react with steam, water is added across the double bond and an alcohol is formed.

Question 19

How can ethanol be made using an alkene?

Answer 19

2) For example, ethanol can be made by mixing ethene with steam and then passing it over a catalyst

Question 20

The conversion of ethene to ethanol is one way of
making ________ industrially.

Answer 20

The conversion of ethene to ethanol is one way of
making ethanol industrially.

Question 21

What happens after the conversion of ethene to ethanol?

Answer 21

After the reaction has taken place, the reaction mixture is passed from the reactor into a condenser. Ethanol and water have a higher boiling point than ethene, so both condense whilst any unreacted ethene gas is recycled back into the reactor. The alcohol can then be purified from the mixture by fractional distillation.

Question 22

Can Halogens can React with Alkenes?

Answer 22

Halogens can React with Alkenes

Question 23

When alkenes react with metals like halogens what happens??

Answer 23

The molecules formed are saturated,
with the C=C carbons each becoming
bonded to a halogen atom.

Question 24

What do bromine and ethene react together to create?

Answer 24

bromine and ethene
react together to form dibromoethane