Exam Cards - Unit 2: Basic Concepts & Hydrocarbons

Question 1

What is the general formula for alkanes?

Answer 1

CnH2n + 2

Question 2

Define structural isomerism.

Answer 2

When molecules have the same molecular formula, but have different structural formulas.

Question 3

Name and define the three types of structural isomers.

Answer 3

Chain isomers - the carbon skeleton is arranged differently Positional isomers - The functional group is arranged in a different position Functional group isomers - The atoms of a functional group can be arranged into a different functional group

Question 4

Define stereoisomerism.

Answer 4

When molecules have the same structural formula, but arranged differently in space.

Question 5

Define E/Z isomerism/cis-trans isomerism.

Answer 5

When an alkene molecule has an E-isomer and a Z-isomer. E-isomers have the same groups across the double bond, whereas Z-isomers have the same groups both above or both below the double bond.

Question 6

How is the percentage yield of a reaction calculated?

Answer 6

Percentage yield = actual yield/theoretical yield x 100

Question 7

How is the theoretical yield calculated?

Answer 7

Number of moles (of reactant) x M (of product)

Question 8

How is atom economy calculated?

Answer 8

molecular mass of useful product/sum of molecular masses of all products x 100

Question 9

Why is the atom economy always 100% for addition reactions?

Answer 9

The reactants always form a single product.

Question 10

Why do branched-chain alkanes have lower boiling points than straight-chain alkanes?

Answer 10

The smaller molecular surface area of branched-chain alkanes means that they can’t pack closely together, reducing the van der Waals forces.

Question 11

What are the products of the combustion of an alkane?

Answer 11

Carbon dioxide and water.

Question 12

What are the products of the combustion of alkanes in limited oxygen?

Answer 12

Carbon monoxide and water.

Question 13

Outline fractional distillation.

Answer 13

• Crude oil is vaporised in a fractionating column
• The vaporised crude oil rises through the column
• As the oil rises it condenses
• Each fraction condenses at different levels

Question 14

Outline catalytic cracking.

Answer 14

• Heavier fractions are passed over a catalyst at a high temperature
  
  • THis breaks them into smaller molecules

Question 15

Define heterolytic fission.

Answer 15

• One of the bonded atoms takes both of the shared pair of electrons
• Two ions are produced
• The atom that took both shared electrons becomes an anion
• The atom the didn’t take the electrons becomes a cation
• X-Y -> X⁺ + Y^-

Question 16

Define homolytic fission.

Answer 16

• Each bonded atom takes one of the shared pair of electrons
• Each atom then has an unpaired electron (radical)
• Two species of the same type are produced
• X-Y -> X^. + Y^.

Question 17

What are the prefixes for halogenoalkanes?

Answer 17

Fluoro-, Chloro-, Bromo-, or Iodo-.

Question 18

What are the three steps in the reaction mechanism of forming halogenoalkanes?

Answer 18

1. Initiation
2. Propagation
3. Termination

Question 19

What happens in the initiation step for the chlorination of methane?

Answer 19

• A Cl-Cl bond is broken by homolytic fission, using energy from UV radiation
• Two chlorine radicals are formed

Question 20

What happens in the propagation steps for the chlorination of methane?

Answer 20

• Cl^. attacks a methane molecule - Cl^. + CH₄ -> ^.CH₃ + HCl
• The new methyl free radical can attack another Cl₂ molecule - ^.CH₃ + Cl₂ -> CH₃Cl + Cl^.
• The new Cl^. can attack another methane molecule
• These steps repat until all the Cl₂ or CH₄ molecules are wiped out

Question 21

What happens in the termination step for the chlorination of methane?

Answer 21

Two radicals combine to form a molecule. There are a number of possible combinations.

• Cl^. + Cl^. -> Cl₂
• ^.CH3 + ^.CH3 -> C₂H₃
• ^.CH3 + ^.Cl -> CH₃Cl

Question 22

What is the equation for the chlorination of methane?

Answer 22

CH₄ + Cl₂ -> CH₃Cl + HCl

Question 23

Define standard enthalpy change of reaction.

Answer 23

The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation.

Question 24

Define standard enthalpy change of formation.

Answer 24

The enthalpy change when 1 mole of a compound is formed from its elements.

Question 25

Define standard enthalpy change of combustion.

Answer 25

The enthalpy change when 1 mole of a substance is completely burned in oxygen.