Small molecules: distinguish bonds from intermolecular forces
| English | Français |
|---|---|
| intermolecular force/ˌɪntəməˈlekjʊlə fɔːs/ | force intermoléculaire |
| small molecule/smɔːl ˈmɒlɪkjuːl/ | petite molécule |
What would explain this observation?
- Water can boil without splitting into hydrogen and oxygen. The change separates molecules while the covalent bonds within each molecule remain.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Small molecular substances usually have relatively low melting and boiling points and are often gases or liquids. Strong covalent bonds hold atoms inside each molecule, but weaker intermolecular forces 分子间作用力 act between molecules. Melting or boiling overcomes these weaker attractions, rather than breaking every covalent bond. Typical neutral molecular substances do not conduct because they have no mobile charged particles.
- intermolecular force: An attraction between separate molecules; small molecule 小分子: A discrete covalently bonded group containing relatively few atoms.
What is mainly overcome when a typical molecular liquid boils?
For comparable molecules, intermolecular forces generally increase with molecular size, so larger molecules tend to have higher melting and boiling points. Molecular shape and other interactions also matter; a size trend is not an exact universal temperature rule. A compound such as hydrogen chloride can form ions on dissolving, so the pure molecular substance and its reacting aqueous solution must be distinguished.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- For comparable molecules, intermolecular forces generally increase with molecular size, so larger molecules tend to have higher melting and boiling points. Molecular shape and other interactions also matter; a size trend is not an exact universal temperature rule. A compound such as hydrogen chloride can form ions on dissolving, so the pure molecular substance and its reacting aqueous solution must be distinguished.
- Use paired molecular drawings that label strong internal bonds and weaker attractions between separate molecules. Identify what is overcome during a specified physical change and what remains intact. Compare supplied boiling-point data for a related series at the same pressure. Do not infer a substance’s identity from boiling point alone without composition and other evidence.
Which two habits make the investigation or model in this case more defensible?
Use paired molecular drawings that label strong internal bonds and weaker attractions between separate molecules. Identify what is overcome during a specified physical change and what remains intact. Compare supplied boiling-point data for a related series at the same pressure. Do not infer a substance’s identity from boiling point alone without composition and other evidence.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: supplied rounded boiling points for a fictional related series are −10 °C and 30 °C. The increase is 30−(−10)=40 °C. This is consistent with stronger attractions in the larger member if other relevant features are comparable, but it does not show that an internal covalent bond gained forty units of strength.
A supplied related series has boiling points −20 °C and 15 °C. Calculate the increase. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
A supplied related series has boiling points −20 °C and 15 °C. Calculate the increase.
The result is 35 °C. Known: supplied rounded boiling points for a fictional related series are −10 °C and 30 °C. The increase is 30−(−10)=40 °C. This is consistent with stronger attractions in the larger member if other relevant features are comparable, but it does not show that an internal covalent bond gained forty units of strength.
Check the conclusion and its limits
- Low boiling point does not mean weak covalent bonds. Molecules are usually electrically neutral overall, not devoid of protons and electrons. Dissolving a molecular solute does not always create ions, while a chemical reaction with water can change which particles are present.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Boiling water necessarily breaks the O–H covalent bonds in its molecules. This claim is false: Low boiling point does not mean weak covalent bonds. Molecules are usually electrically neutral overall, not devoid of protons and electrons. Dissolving a molecular solute does not always create ions, while a chemical reaction with water can change which particles are present.
Small molecules: distinguish bonds from intermolecular forces: For comparable molecules, intermolecular forces generally increase with molecular size, so larger molecules tend to have higher melting and boiling points. Molecular shape and other interactions also matter; a size trend is not an exact universal temperature rule. A compound such as hydrogen chloride can form ions on dissolving, so the pure molecular substance and its reacting aqueous solution must be distinguished.
Boiling water necessarily breaks the O–H covalent bonds in its molecules.
Low boiling point does not mean weak covalent bonds. Molecules are usually electrically neutral overall, not devoid of protons and electrons. Dissolving a molecular solute does not always create ions, while a chemical reaction with water can change which particles are present.
An attraction between separate molecules: write the technical term.
intermolecular force means An attraction between separate molecules.