Unit 2 - Nature's Chemistry (Systematic Carbon Chemistry)

Question 1

What is the definition of a homologous series?

Answer 1

A family of hydrocarbons with similar chemical properties who share the same general formula.

Question 2

What is the general formula for each series?

Answer 2

• Alkanes (CnH2n+1)
• Alkenes (CnH2n)
• Cycloalkanes (CnH2n)
• Alcohols (CnH2n+1OH)
• Carboxylic acids (CnH2n+1COOH)

Question 3

Which chemical test can be carried out to distinguish between saturated and unsaturated compounds?

Answer 3

The bromine test - unsaturated compounds will decolourise bromine solution quickly, whereas it will remain a brown colour with saturated compounds. This is due to the bromine-bromine bond breaking and adding to the double bonds of unsaturated compounds. Note that cycloalkanes will not decolourise bromine solution as they are saturated (despite having the same molecular formula as alkenes).

Question 4

What is an addition reaction?

Answer 4

A reaction in which two or more molecules combine to produce a larger molecule and nothing else. The double bond of an alkene partially breaks when the reactant molecule attacks and adds on across it.

Question 5

What are isomers?

Answer 5

Compounds with the same molecular formulae but different structural formulae. They have the same number of each type of atom. They can be part of the same or different homologous series, e.g. alkenes and cycloalkanes can be isomers.
Due to their different structures their physical properties (melting point, boiling point etc) are usually different.

Question 6

Why does boiling point increase with molecular size?

Answer 6

Increasing intermolecular forces - when comparing molecules of the same size, the type of intermolecular forces present need to be considered by looking for O-H or N-H bonds (hydrogen bonding) or polar bonds (permanent dipole-permanent dipole interactions). The stronger the type of intermolecular forces present, the higher the boiling point as they will require more energy to break apart.

Question 7

What is volatility?

Answer 7

A measure of the ease of evaporation. This is also affected by the intermolecular forces present.

Question 8

What must a molecule be in order to be volatile?

Answer 8

Reasonably small in structure and have weaker intermolecular forces.