Chemistry-only: hydrogen addition and alkene combustion
| English | 中文 | Pinyin |
|---|---|---|
| hydrogenation/haɪˈdrɒdʒəneɪʃn/ | 加氢反应 | jiā qīng fǎn yìng |
| addition reaction | 加成反应 | jiā chéng fǎn yìng |
What would explain this observation?
- Propene becomes propane when hydrogen adds across C=C. Its three-carbon skeleton stays intact; this reaction adds hydrogen rather than breaking the chain into shorter fragments.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Alkenes react with hydrogen in the presence of a heated nickel catalyst to form alkanes. A representative teaching condition is about 150 °C with nickel; it describes the reaction condition, not a student procedure. One H₂ adds across one C=C: its two hydrogen atoms attach to the two formerly double-bonded carbons and the carbon–carbon bond becomes single. This is hydrogenation 加氢反应, an addition reaction 加成反应.
- hydrogenation: Addition of hydrogen across a carbon–carbon multiple bond to form a more saturated product; addition reaction: A reaction adding atoms across a multiple bond without removing the existing carbon skeleton.
What happens when hydrogen adds across the one C=C of propene?
An addition reaction joins atoms to the molecule across the multiple bond without removing the existing carbon skeleton. Check the new bond counts: each carbon still makes four bonds. Separately, alkenes burn in oxygen like other hydrocarbons. With sufficient oxygen, complete combustion gives carbon dioxide and water. In air they tend to burn with smoky flames because incomplete combustion produces carbon particles; smoke does not establish complete combustion.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- An addition reaction joins atoms to the molecule across the multiple bond without removing the existing carbon skeleton. Check the new bond counts: each carbon still makes four bonds. Separately, alkenes burn in oxygen like other hydrocarbons. With sufficient oxygen, complete combustion gives carbon dioxide and water. In air they tend to burn with smoky flames because incomplete combustion produces carbon particles; smoke does not establish complete combustion.
- Draw the given alkene first, replace C=C by C–C and add one H to each affected carbon. Retain every other atom. Use the individual displayed product panels for all four named starting members. For combustion, balance carbon, hydrogen and then oxygen. Analyse approved demonstration evidence rather than conducting a pressurised hydrogen/catalyst experiment: combustible gases, heated catalysts and oxygen must remain under trained control.
Which two habits make the investigation or model in this case more defensible?
Draw the given alkene first, replace C=C by C–C and add one H to each affected carbon. Retain every other atom. Use the individual displayed product panels for all four named starting members. For combustion, balance carbon, hydrogen and then oxygen. Analyse approved demonstration evidence rather than conducting a pressurised hydrogen/catalyst experiment: combustible gases, heated catalysts and oxygen must remain under trained control.
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: C₃H₆ + H₂ → C₃H₈ conserves C=3 and H=8 on each side. In a represented particle batch, six propene molecules consume six H₂ molecules and form six propane molecules if addition is complete. Complete ethene combustion is C₂H₄ + 3O₂ → 2CO₂ + 2H₂O, with two C, four H and six O atoms on each side.
Eight represented ethene molecules undergo complete addition, each using one H₂. Find H₂ molecules consumed. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
Eight represented ethene molecules undergo complete addition, each using one H₂. Find H₂ molecules consumed.
The result is 8 molecules. Known: C₃H₆ + H₂ → C₃H₈ conserves C=3 and H=8 on each side. In a represented particle batch, six propene molecules consume six H₂ molecules and form six propane molecules if addition is complete. Complete ethene combustion is C₂H₄ + 3O₂ → 2CO₂ + 2H₂O, with two C, four H and six O atoms on each side.
Check the conclusion and its limits
- Do not add two hydrogen atoms to only one carbon or leave the double bond unchanged after addition. A catalyst is not a stoichiometric reactant. The hydrogenation example does not prescribe universal operating conditions for every industrial alkene. A smoky flame indicates incomplete combustion in the stated context, not that water can never form.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Hydrogen addition to an alkene must split the carbon skeleton. This claim is false: Do not add two hydrogen atoms to only one carbon or leave the double bond unchanged after addition. A catalyst is not a stoichiometric reactant. The hydrogenation example does not prescribe universal operating conditions for every industrial alkene. A smoky flame indicates incomplete combustion in the stated context, not that water can never form.
Chemistry-only: hydrogen addition and alkene combustion: An addition reaction joins atoms to the molecule across the multiple bond without removing the existing carbon skeleton. Check the new bond counts: each carbon still makes four bonds. Separately, alkenes burn in oxygen like other hydrocarbons. With sufficient oxygen, complete combustion gives carbon dioxide and water. In air they tend to burn with smoky flames because incomplete combustion produces carbon particles; smoke does not establish complete combustion.
Hydrogen addition to an alkene must split the carbon skeleton.
Do not add two hydrogen atoms to only one carbon or leave the double bond unchanged after addition. A catalyst is not a stoichiometric reactant. The hydrogenation example does not prescribe universal operating conditions for every industrial alkene. A smoky flame indicates incomplete combustion in the stated context, not that water can never form.
Addition of hydrogen across a carbon–carbon multiple bond to form a more saturated product: write the technical term.
hydrogenation means Addition of hydrogen across a carbon–carbon multiple bond to form a more saturated product.