Alkanes: connect the formula to the displayed bonds
| English | Français |
|---|---|
| displayed formula/dɪˈspleɪd ˈfɔːmjʊlə/ | formule développée |
| alkane/ˈælkeɪn/ | alcane |
What would explain this observation?
- Ethane is written C₂H₆, but its displayed formula 展开结构式 also shows where the six hydrogen atoms attach. Both representations describe the same molecule at different levels of detail.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- A homologous series is a family of compounds with the same functional group and general formula and similar chemical reactions. The general formula of the alkane 烷烃 homologous series is CnH2n+2. For the first four members, methane is CH₄, ethane C₂H₆, propane C₃H₈ and butane C₄H₁₀. Successive members differ by CH₂. A displayed formula shows each atom and bond; carbon makes four bonds and hydrogen one. In these alkanes all carbon–carbon bonds are single. Their molecules are saturated.
- alkane: A saturated hydrocarbon in the homologous series with general formula CnH2n+2; displayed formula: A representation showing every atom and each bond in a molecule.
Which is the correct molecular formula for propane?
For a straight chain, each end carbon has three hydrogen atoms, while an internal carbon has two. Methane has no carbon neighbour and so has four hydrogens. Count the bond lines at each carbon to check a displayed structure rather than adding hydrogen randomly. The general formula identifies the expected molecular composition of this series; molecular composition alone need not establish a unique arrangement of atoms.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- For a straight chain, each end carbon has three hydrogen atoms, while an internal carbon has two. Methane has no carbon neighbour and so has four hydrogens. Count the bond lines at each carbon to check a displayed structure rather than adding hydrogen randomly. The general formula identifies the expected molecular composition of this series; molecular composition alone need not establish a unique arrangement of atoms.
- Construct the first four molecules using a model kit or the original displayed diagrams. Count carbons, total hydrogens and bonds at each atom. Translate between displayed and molecular formulae, then test a supplied formula against H=2n+2. A question may supply a longer-chain formula without requiring its specific name. Keep the required names limited to the four in the acquired section.
Which two habits make the investigation or model in this case more defensible?
Construct the first four molecules using a model kit or the original displayed diagrams. Count carbons, total hydrogens and bonds at each atom. Translate between displayed and molecular formulae, then test a supplied formula against H=2n+2. A question may supply a longer-chain formula without requiring its specific name. Keep the required names limited to the four in the acquired section.
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: for n=3, expected hydrogen number is H=2n+2=2×3+2=8, giving propane C₃H₈. Its end carbons contribute 3+3 hydrogen atoms and its middle carbon contributes 2, also giving 8. For a supplied C₆ formula, the alkane series predicts 14 hydrogens; its specific name is not needed for this calculation.
An alkane molecule has seven carbon atoms. Find its hydrogen atom count. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
An alkane molecule has seven carbon atoms. Find its hydrogen atom count.
The result is 16 atoms. Known: for n=3, expected hydrogen number is H=2n+2=2×3+2=8, giving propane C₃H₈. Its end carbons contribute 3+3 hydrogen atoms and its middle carbon contributes 2, also giving 8. For a supplied C₆ formula, the alkane series predicts 14 hydrogens; its specific name is not needed for this calculation.
Check the conclusion and its limits
- Do not confuse the small subscript with the coefficient in an equation. C₂H₄ does not satisfy the alkane formula. A carbon with five ordinary covalent bonds is not a valid displayed alkane here. These flat drawings represent bonding, not actual straight planar molecules, bond angles or relative atomic sizes.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Ethane needs a carbon–carbon double bond to satisfy carbon valency. This claim is false: Do not confuse the small subscript with the coefficient in an equation. C₂H₄ does not satisfy the alkane formula. A carbon with five ordinary covalent bonds is not a valid displayed alkane here. These flat drawings represent bonding, not actual straight planar molecules, bond angles or relative atomic sizes.
Alkanes: connect the formula to the displayed bonds: For a straight chain, each end carbon has three hydrogen atoms, while an internal carbon has two. Methane has no carbon neighbour and so has four hydrogens. Count the bond lines at each carbon to check a displayed structure rather than adding hydrogen randomly. The general formula identifies the expected molecular composition of this series; molecular composition alone need not establish a unique arrangement of atoms.
Ethane needs a carbon–carbon double bond to satisfy carbon valency.
Do not confuse the small subscript with the coefficient in an equation. C₂H₄ does not satisfy the alkane formula. A carbon with five ordinary covalent bonds is not a valid displayed alkane here. These flat drawings represent bonding, not actual straight planar molecules, bond angles or relative atomic sizes.
A saturated hydrocarbon in the homologous series with general formula CnH2n+2: write the technical term.
alkane means A saturated hydrocarbon in the homologous series with general formula CnH2n+2.