Key points, AQA AS level chemistry unit 1

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  • Created by: Aimee
  • Created on: 02-01-13 00:19

Fractional Distillation

Fractional Distillation:
- Fractions have different boiling points
- Boiling point of a molecule depends on chain length
- Cooler at top of column/hotter at bottom of column
- Larger molecules go to bottom of column/smaller molecules go to top of column

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Catalytsts, cracking etc.

Nitrogen Oxide (NO) is produced in the engine when a spark/high temperature causes nitrogen and oxygen to react. 
N2 + O2 = 2NO 

Problems caused by incomplete combustion in a car engine:
- More fuel needed, which costs more
- Have to fit a catalytic converter 

Long chain hydrocarbons broken into smaller chain hydrocarbons. 

Thermal Cracking:
High temperatures are needed to break the C-C bonds

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Isomerism, Homologous Series

Homologous series: Organic compounds with the same functional group, but different carbon chain length.

Characteristics of a homologous series:
- Same general formula
- Chemically similar; 
- Same functional group
- Trend in physical properties eg inc bp as Mr inc.
- Each member of the series differs from the next by CH2. 

Structural Isomerism:
Compounds with the same molecular formula, but a different structure.

Branched chain isomers have lower boiling points because...
- They have less surface area
- So less Van der Waals

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Environmental Problems

- Greenhouse gas

- Causes acid rain

- Can be neutralised by CaO

 - Causes asthma
 - Causes global dimming

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- In water in its liquid state, hydrogen bonds are free to break and reform easily as molecules are moving about.
- When water freezes, water molecules are no longer free to move around, and hydrogen bonds hold the molecules in fixed positions.
- In order to fit into this structure, the molecules are slightly less closely packed than in liquid water, meaning ice is less dense - so it floats on the top of ponds!
- This insulates ponds, allowing fish to survive through the winter.  

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Melting/boiling points

Mp of Al higher than mp of Na because...
- higher nuclear charge
- more delocalised electrons
- stronger electrostatic force of attraction between positive ions and delocalised electrons

Mp of sulfur higher than mp of phosphorus because...
- S is a bigger molecule than P
- so stronger Van der Waals

Bp increase from H2S to H2Te because...
- Atoms get larger
- so increased Van der Waals

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