Exam Cards - Unit 2: Basic Concepts & Hydrocarbons Flashcards
What is the general formula for alkanes?
CnH2n + 2
Define structural isomerism.
When molecules have the same molecular formula, but have different structural formulas.
Name and define the three types of structural isomers.
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
Define stereoisomerism.
When molecules have the same structural formula, but arranged differently in space.
Define E/Z isomerism/cis-trans isomerism.
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.
How is the percentage yield of a reaction calculated?
Percentage yield = actual yield/theoretical yield x 100
How is the theoretical yield calculated?
Number of moles (of reactant) x M (of product)
How is atom economy calculated?
molecular mass of useful product/sum of molecular masses of all products x 100
Why is the atom economy always 100% for addition reactions?
The reactants always form a single product.
Why do branched-chain alkanes have lower boiling points than straight-chain alkanes?
The smaller molecular surface area of branched-chain alkanes means that they can’t pack closely together, reducing the van der Waals forces.
What are the products of the combustion of an alkane?
Carbon dioxide and water.
What are the products of the combustion of alkanes in limited oxygen?
Carbon monoxide and water.
Outline fractional distillation.
- 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
Outline catalytic cracking.
- Heavier fractions are passed over a catalyst at a high temperature
- THis breaks them into smaller molecules
Define heterolytic fission.
- 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-
Define homolytic fission.
- 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.
What are the prefixes for halogenoalkanes?
Fluoro-, Chloro-, Bromo-, or Iodo-.
What are the three steps in the reaction mechanism of forming halogenoalkanes?
- Initiation
- Propagation
- Termination
What happens in the initiation step for the chlorination of methane?
- A Cl-Cl bond is broken by homolytic fission, using energy from UV radiation
- Two chlorine radicals are formed
What happens in the propagation steps for the chlorination of methane?
- 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
What happens in the termination step for the chlorination of methane?
Two radicals combine to form a molecule. There are a number of possible combinations.
- Cl. + Cl. -> Cl2
- .CH3 + .CH3 -> C2H3
- .CH3 + .Cl -> CH3Cl
What is the equation for the chlorination of methane?
CH4 + Cl2 -> CH3Cl + HCl
Define standard enthalpy change of reaction.
The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation.
Define standard enthalpy change of formation.
The enthalpy change when 1 mole of a compound is formed from its elements.
Define standard enthalpy change of combustion.
The enthalpy change when 1 mole of a substance is completely burned in oxygen.
