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| Question | Answer |
| What is the general formula for alkenes? | CₙH₂ₙ |
| Are alkenes saturated or unsaturated hydrocarbons? | Unsaturated |
| What is a double bond made up of? | A sigma (σ-) bond A pi (π-) bond |
| How do σ-bonds form? | Two orbitals overlap in a straight line which gives the highest possible electron density between the two nuclei. This is a single covalent bond |
| How do π-bonds form? |
When two p-orbitals overlap sideways. One part is above and one is below the molecular axis - this is because the p-orbitals which overlap are dumb-bell shaped
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| Which of the two types of bonds (σ or π) are weaker than the other? | π-bonds are weaker than σ-bonds |
| π-bonds are weaker than σ-bonds. Why? | Because the electron density is spread out above and below the nuclei. This means that the electrostatic attraction between the nuclei and the shared pair of electrons is weaker, so π-bonds are weaker than π-bonds have a relatively low bond enthalpy |
| TRUE OR FALSE: A double bond is twice as strong as a single bond | FALSE π-bonds are weaker than σ-bonds Single bond = σ-bond Double bond = π-bond + σ-bond |
| TRUE OR FALSE: Alkenes are more reactive than alkanes | TRUE |
| Why are alkenes more reactive than alkanes? | There are two pairs of electrons in the bond, the C=C double bond has a really high electron density The π-bond sticks out above and below the rest of the molecule, so it is likely to be attacked by particles that have a low electron density and can accept a pair of electrons (electrophiles) The π-bond has a low bond enthalpy |
| Complete the sentences: Carbon atoms in a C=C double bond and the atoms bonded to these carbons all lie in the same plane (they're ________). The three bonds ________ each other as much as possible and arrange themselves into a ____________ ___________ shape. | Carbon atoms in a C=C double bond and the atoms bonded to these carbons all lie in the same plane (they're PLANAR). The three bonds REPEL each other as much as possible and arrange themselves into a TRIGONAL PLANAR shape. |
| TRUE OR FALSE: In larger alkenes, only the >C=C< unit is planar | TRUE |
| In C=C double bonds, the atoms can't rotate around them like they can around single bonds. Why? | The p-orbitals have to stay in the same position to overlap and form a π-bond. Double bonds are also fairly rigid - they don't bend much. |
| Complete the sentences: Even though atoms can't rotate around the __________ bond, things can still rotate about any _________ bonds in the molecule. The restricted rotation around the C=C double bond is what causes alkenes to form __________________. | Even though atoms can't rotate around the DOUBLE bond, things can still rotate about any SINGLE bonds in the molecule. The restricted rotation around the C=C double bond is what causes alkenes to form STEREOISOMERS. |
| What is the bond angle in a double bond? |
120°
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