Small molecules: distinguish bonds from intermolecular forces
Введено| English | Значение |
|---|---|
| intermolecular force/ˌɪntəməˈlekjʊlə fɔːs/ | межмолекулярное взаимодействие |
| small molecule/smɔːl ˈmɒlɪkjuːl/ | малая молекула |
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?
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Boiling separates the molecules from each other, overcoming only the weak attractions BETWEEN molecules. The covalent bonds inside each molecule survive - that is why the vapour is still the same substance.
Match each technical term to its precise meaning.
Difficulty 2/5 · 140 coins + a small bonus on first completion today
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?
Difficulty 2/5 · 140 coins + a small bonus on first completion today
Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
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.
Difficulty 2/5 · 140 coins + a small bonus on first completion today
The result is 35 °C. Known: boiling points -20 and 15 degrees C. Increase = 15 - (-20) = 35 degrees C. Subtracting a negative adds; the increase reflects stronger intermolecular attractions in the later member.
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: Steam is still H2O - the molecules are intact. Boiling overcomes the attractions between molecules; breaking the O-H covalent bonds would decompose water into hydrogen and oxygen, which boiling never does.
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.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Steam is still H2O - the molecules are intact. Boiling overcomes the attractions between molecules; breaking the O-H covalent bonds would decompose water into hydrogen and oxygen, which boiling never does.
An attraction between separate molecules: write the technical term.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
intermolecular force means An attraction between separate molecules.