Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 15.10 Hydrogen bonding and properties-Study Notes - New Syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 15.10 Hydrogen bonding and properties- Study Notes- New syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 15.10 Hydrogen bonding and properties- Study Notes -International A Level (IAL) Chemistry (YCH11) – per latest Syllabus.
Key Concepts:
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Edexcel International A Level (IAL) Chemistry (YCH11) -Concise Summary Notes- All Topics
15.10 Effect of Hydrogen Bonding on the Physical Properties of Carboxylic Acids
Carboxylic acids contain the carboxyl group:
\( \mathrm{-COOH} \)
This functional group allows carboxylic acids to form extensive hydrogen bonding, which strongly affects their boiling temperatures and solubility.
Hydrogen Bonding in Carboxylic Acids
Carboxylic acids can form hydrogen bonds because they contain:
- An \( \mathrm{O-H} \) bond (hydrogen bond donor)
- Oxygen atoms with lone pairs (hydrogen bond acceptors)
Therefore, strong intermolecular hydrogen bonds form between molecules.
Dimer Formation
Two carboxylic acid molecules commonly pair together to form:
Dimers
joined by two hydrogen bonds.
Effect on Boiling Temperatures
- Strong hydrogen bonds require large amounts of energy to break.
- Therefore, carboxylic acids have:
High boiling temperatures
compared with compounds of similar relative molecular mass.
Comparison with Other Organic Compounds
- Higher boiling temperatures than:
- Alkanes
- Aldehydes
- Ketones
because hydrogen bonding is stronger than dipole–dipole forces or London forces.
Example
- Ethanoic acid has a much higher boiling temperature than propanal despite similar Mr values.
Effect on Solubility
Carboxylic acids can also form hydrogen bonds with water molecules.
Reason
- Oxygen lone pairs accept hydrogen bonds from water.
- \( \mathrm{O-H} \) hydrogen can donate hydrogen bonds to water.
Therefore, lower carboxylic acids are:
Highly soluble in water
Trend in Solubility
- Solubility decreases as carbon chain length increases.
Reason
- Hydrocarbon chain is non-polar.
- Larger non-polar regions reduce interaction with water.
Examples
- Methanoic acid and ethanoic acid are completely miscible with water.
- Longer-chain acids are less soluble.
Summary Table
| Property | Effect of Hydrogen Bonding |
|---|---|
| Boiling temperature | Increases due to strong intermolecular hydrogen bonds |
| Solubility in water | Increases because hydrogen bonds form with water molecules |
Key Features
- Carboxylic acids form strong hydrogen bonds.
- Hydrogen bonding causes high boiling temperatures.
- Lower carboxylic acids are highly soluble in water.
- Solubility decreases with increasing chain length.
- Carboxylic acids commonly form dimers.
Example 1:
Explain why ethanoic acid has a higher boiling temperature than ethanal.
▶️ Answer/Explanation
Ethanoic acid forms intermolecular hydrogen bonds because it contains an \( \mathrm{O-H} \) group.
Ethanal cannot form hydrogen bonds between its molecules.
More energy is needed to separate ethanoic acid molecules, giving a higher boiling temperature.
Example 2:
Explain why methanoic acid is highly soluble in water.
▶️ Answer/Explanation
Methanoic acid can form hydrogen bonds with water molecules.
The carboxyl group contains oxygen atoms with lone pairs and an \( \mathrm{O-H} \) bond.
Strong interactions with water allow it to dissolve readily.