Wrong PPQs

Explain the increase in boiling points of the cycloalkanes shown in the table. [2]

• More carbons (in ring) OR more (surface area of) contact [1]

• more induced dipole-dipole forces OR stronger induced dipole-dipole force forces

• More energy is needed to break the intermolecular forces [1]

Name one of the structural isomers of C6H10Br2 formed in the reaction between cyclohexane and bromine. [1]

• 1,1-dibromocyclohexane OR

• 1,2-dibromocyclohexane OR

• 1,3-dibromocyclohexane OR

• 1,4-dibromocyclohexane

What is meant by the term homologous series? [2]

• (series of compounds with the) same functional group [1]

• OR same/similar chemical properties

• OR same/similar chemical reactions

• each successive/subsequent member differing by CH₂ [1]

The reaction between cyclohexane and bromine in (f) also forms C6H10Br2

Write an equation, using molecular formulae, for the reaction of cyclohexane and bromine in the presence of ultraviolet radiation to form C6H10Br2 [1]

C6H12+ 2Br2 —> C6H10Br2 + 2HBr

State and explain the trend in boiling points of the alkanes [2]

statement:

• Increases with chain length/ number of carbon atoms / M, of molecule; [1 mark]

Explanation:

• Strength of (temporary) induced dipole-dipole forces increases as the size of the molecules increases: [1 mark]

Alkanes can undergo both complete and incomplete combustion.

State the difference between complete and incomplete combustion of a hydrocarbon fuel, such as octane. Include the products formed in your answer. [2]

Complete combustion:

• Complete combustion has an excess/plentiful supply of oxygen AND

• Complete combustion forms carbon dioxide and water/ C8H18 + 12.5 O2 —> 8 CO2 + 9 H2O [1 mark]

Incomplete combustion:

• Incomplete combustion has a limited/reduced supply of oxygen AND

• Incomplete combustion forms carbon monoxide / carbon and water/ C8H18 + 8.5 O2 ——> 8 CO + 9 H2O [1 mark]

OR

• C8H18 + 4.5 O2 ——> 8 C + 9 H2O

Suggest what might be observed to show that carbon is being formed.[1]

The observation that tells you that carbon is being formed is:

• A smoky/sooty flame; [1 mark]

Carbon monoxide is a colourless, odourless and poisonous gas.

State why carbon monoxide can cause dizziness, fainting, loss of consciousness and death in humans. [1]

Any one of the following:

• Reduces ability of red blood cells to carry oxygen; [1 mark]

• Binding irreversibly/very tightly to haemoglobin/red blood cells; [1 mark]

• Stopping oxygen attaching to haemoglobin/red blood cells; [1 mark]

• Forms carboxyhaemoglobin instead of oxyhaemoglobin; [1 mark]

State two reasons why alkanes are unreactive. [2]

• The high bond enthalpies of the C-C and C-H bonds; [1 mark]

• The very low polarity of the sigma (σ) bonds present; [1 mark]

• This means we need specific conditions in order to get alkanes to react

State what is meant by a radical [1]

A radical is:

• A species with an unpaired electron; [1 mark]

Chloromethane will be formed via several steps.

i) Write the equations for the two propagation steps. [2]

ii) Write an equation to show how ethane can be formed in this reaction. [1]

i) The two propagation steps are:

• CI+CH4 → CH3 + HCI; [1 mark]

• Cl2 + CH3CH3CI + CI; [1 mark]

ii) Ethane can be formed by:

• •CH3 + •CH3 —→ CH3CH3 OR

• •CH3 + •CH3 → C2H6; [1 mark]