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GCSE Chemistry - Hydrocarbons - Alkanes & Homol ...
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Flashcards in this deck (23)

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  • What does organic chemistry study?

    • Compounds that contain carbon
    organic definition

  • Why is carbon useful for making large compounds?

    • Because each carbon atom can form four strong bonds
    carbon bonding

  • Which atoms are the most common partners bonded to carbon in large organic molecules?

    • Other carbon atoms
    • Hydrogen atoms
    bonding atoms

  • What is a hydrocarbon?

    • Any compound formed from carbon and hydrogen only
    hydrocarbon definition

  • What specific class of hydrocarbons is the video focusing on?

    • Alkanes
    alkanes topic

  • What defines a 'hydrocarbon'?

    A compound that contains only carbon and hydrogen atoms.

    hydrocarbons definition

  • Why is butane a hydrocarbon but butanol is not?

    Butane contains only C and H (C4H10); butanol also has an oxygen atom, so it is not a hydrocarbon.

    examples hydrocarbons

  • What are 'alkanes'?

    The simplest group of hydrocarbons that are saturated (only single bonds).

    alkanes definition

  • What is the general formula for alkanes?

    The general formula is \(\mathrm{C_nH_{2n+2}}\).

    alkanes formula

  • What is the first alkane and its formula?

    • Methane
    • CH4
    alkanes members

  • Name the first four alkanes with their formulas.

    • Methane — CH4
    • Ethane — C2H6
    • Propane — C3H8
    • Butane — C4H10
    alkanes members

  • How does each consecutive alkane in the series differ?

    Each successive alkane adds one CH2 unit to extend the carbon chain.

    homologous growth

  • What is a homologous series in organic chemistry?

    A homologous series is a group of organic compounds that have similar properties and react in a similar way.

    homologous organic

  • Which are the first four alkanes shown?

    • Methane
    • Ethane
    • Propane
    • Butane
    alkanes names

  • What is the general formula for alkanes?

    The general formula for alkanes is \(\mathrm{C_nH_{2n+2}}\).

    alkanes formula

  • What does the alkane general formula \(\mathrm{C_nH_{2n+2}}\) mean in words?

    If the compound contains 'n' carbon atoms, then it contains 2 times 'n' plus 2 hydrogen atoms.

    alkanes interpretation

  • Using the general formula, what is the hydrogen count when n = 3?

    For n = 3 the calculation is \((2\times 3)+2 = 8\), so there are 8 hydrogen atoms.

    example calculation

  • What is the molecular formula of propane?

    Propane has the molecular formula C3H8.

    propane formula

  • What formula gives the number of hydrogen atoms in an alkane with n carbon atoms?

    The number of hydrogens is given by \(2n+2\).

    alkanes formula

  • Using the general alkane hydrogen formula, how many hydrogen atoms does octane (n=8) have?

    Calculate \((2\times 8)+2 = 18\), so octane has 18 hydrogens.

    octane calculation

  • What is the molecular formula of octane?

    Octane's molecular formula is \(C_8H_{18}\).

    octane molecular-formula

  • What does it mean for alkanes to be 'saturated compounds'?

    Every carbon atom has four single covalent bonds and there are no double bonds.

    saturated alkanes

  • What happens to a propane molecule if one C–C single bond is changed to a double bond?

    Each carbon involved loses a hydrogen and the molecule is no longer an alkane but an alkene.

    propane alkene
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Alkanes — concise study notes

Big picture

  • Organic chemistry studies compounds containing carbon. Carbon commonly bonds to other carbons and to hydrogen because each carbon forms four strong bonds.

Hydrocarbons

  • Hydrocarbon: a compound containing only carbon and hydrogen. Example: butane (C4H10) is a hydrocarbon; butanol (contains O) is not.

Alkanes (definition and examples)

  • Alkanes are the simplest hydrocarbons and are saturated (only single covalent C–C bonds).
  • You must know the first four alkanes and their molecular formulas:
  • Methane — \(\mathrm{CH_4}\)
  • Ethane — \(\mathrm{C_2H_6}\)
  • Propane — \(\mathrm{C_3H_8}\)
  • Butane — \(\mathrm{C_4H_{10}}\)
  • Each successive alkane differs by a –CH_2– unit (chain growth by one carbon).

Homologous series and general formula

  • Alkanes form a homologous series: members have similar chemical properties and follow a pattern.
  • General molecular formula for alkanes: \(\mathrm{C_nH_{2n+2}}\).
  • Example (propane): if \(n=3\) then hydrogen atoms \(=2\times3+2\), so \(\mathrm{C_3H_8}\).
  • Example (octane): if \(n=8\) then hydrogen atoms \(=2\times8+2\), so \(\mathrm{C_8H_{18}}\).

Saturation vs unsaturation

  • Saturated means every carbon has four single covalent bonds; there are no double bonds in alkanes.
  • Introducing a C=C double bond produces an alkene and reduces the number of hydrogens (e.g., propane \(\mathrm{C_3H_8}\) → propene \(\mathrm{C_3H_6}\)).

Key points to remember

  • Carbon is tetravalent: forms up to four bonds, enabling long chains and rings.
  • Hydrocarbons = only C and H; adding other atoms (O, N, etc.) means "not a hydrocarbon."
  • Alkanes: saturated, general formula \(\mathrm{C_nH_{2n+2}}\), first four as above.
  • Homologous series = predictable formulas and similar reactivity; add CH2 to go to the next member.
  • Combustion and detailed properties of alkanes are typically covered separately.

Quick revision checklist

  • Can you list the first four alkanes and formulas? Yes: CH4, C2H6, C3H8, C4H10.
  • Can you apply \(\mathrm{C_nH_{2n+2}}\) to compute formulas for any n? Try \(n=5\) or \(n=10\).
  • Do you know the difference between saturated (alkane) and unsaturated (alkene) hydrocarbons?