Hey there! Sign in to join this conversationNew here? Join for free
    • Thread Starter
    Offline

    1
    ReputationRep:
    1) Explain why alcohols have a higher boiling point than alkanes of similar molecular mass?

    2) Describe the Volatility of alcohols compared to alkanes of similar molecular mass?

    This is AS OCR chemistry by the way.
    Thanks in advance!



    Posted from TSR Mobile
    Offline

    2
    ReputationRep:
    I don't know about OCR chemistry but a model answer for the first one might be "Alcohols contain the -OH functional group in which the O is delta positive and the H is delta negative, due to the large electronegativity difference, leading to the formation of intermolecular hydrogen bonds between the H of one molecule and the O of an adjacent molecule. These bonds are strong and thus require a lot of energy to break, whereas in alkanes there is no overall polarity, and thus alcohols have higher boiling points than alkanes."

    Volatility will be identical but you would at the end write "and thus alcohols have lower volatility than alkanes".

    Hope this helps

    Edit: By volatility I am assuming you mean how prone the substance is to subliming? Other definitions might include flammability etc. ...
    • Community Assistant
    • Study Helper
    Offline

    14
    ReputationRep:
    (Original post by Big-Daddy)
    I don't know about OCR chemistry but a model answer for the first one might be "Alcohols contain the -OH functional group in which the O is delta positive and the H is delta negative, due to the large electronegativity difference, leading to the formation of intermolecular hydrogen bonds between the H of one molecule and the O of an adjacent molecule. These bonds are strong and thus require a lot of energy to break, whereas in alkanes there is no overall polarity, and thus alcohols have higher boiling points than alkanes."

    Volatility will be identical but you would at the end write "and thus alcohols have lower volatility than alkanes".

    Hope this helps

    Edit: By volatility I am assuming you mean how prone the substance is to subliming? Other definitions might include flammability etc. ...
    Volatility in the context of 'A' levels refers to the ease with which it is vaporised.

    The more volatile a substance is the lower is its boiling point and the higher its vapor pressure at a given temperature.

    It is a measure of the intermolecular forces. Strong intermolecular forces = low volatility.
    Offline

    2
    ReputationRep:
    (Original post by charco)
    Volatility in the context of 'A' levels refers to the ease with which it is vaporised.

    The more volatile a substance is the lower is its boiling point and the higher its vapor pressure at a given temperature.

    It is a measure of the intermolecular forces. Strong intermolecular forces = low volatility.
    Thanks.

    If, as I wrote, volatility refers to ease of sublimation/vaporization then my written answer was the correct one. (Alcohols will have lower volatility than alkanes due to the stronger intermolecular forces - which come from the increased polarity in molecules.)
    Offline

    0
    ReputationRep:
    Alcohols have higher boiling points as they form hydrogen bonds between their hydroxyl groups. Hydrogen bonds are the strongest type of intermolecular force. Alkanes consist of much weaker van dear Waals' forces. More energy, hence a higher temperature is required to break the hydrogen bonding in alcohol. Thus, alcohols have a higher boiling point than alkanes of similar molecular mass.

    Volatility is the ease that something becomes a gas. It's essentially the same. Alcohols have a lower volatility, due to hydrogen bonding. Alkanes have a higher volatility, due to their much weaker van dear Waals' forces.
    Offline

    2
    ReputationRep:
    (Original post by Konflict)
    Volatility is the ease that something becomes a gas. It's essentially the same. Alcohols have a lower volatility, due to hydrogen bonding. Alkanes have a higher volatility, due to their much weaker van dear Waals' forces.
    I can't agree with this. Do alkanes really have much weaker van der Waals' forces than alcohols? No ... in fact, as the number of electrons in -OH is exactly identical to the number of electrons in -CH3, I would think they have exactly the same strength of van der Waals' forces.

    I agree with your answer for boiling point.
    Offline

    0
    ReputationRep:
    (Original post by Big-Daddy)
    I can't agree with this. Do alkanes really have much weaker van der Waals' forces than alcohols? No ... in fact, as the number of electrons in -OH is exactly identical to the number of electrons in -CH3, I would think they have exactly the same strength of van der Waals' forces.

    I agree with your answer for boiling point.
    Sorry, I meant as in comparison with the hydrogen bonds. Hydrogen bonds > van der Waals' forces. Hence, alcohols have a lower volatility as they have stronger hydrogen bond between their molecules, while in comparison the alkanes simply have van der Waals' forces.

    I hoped this clarified what I was trying to say!
    Offline

    2
    ReputationRep:
    (Original post by Konflict)
    Sorry, I meant as in comparison with the hydrogen bonds. Hydrogen bonds > van der Waals' forces. Hence, alcohols have a lower volatility as they have stronger hydrogen bond between their molecules, while in comparison the alkanes simply have van der Waals' forces.

    I hoped this clarified what I was trying to say!
    Ah ok, sorry! I misunderstood. I thought you were saying that alkanes having much weaker van der Waals' forces was the factor which I didn't see as true (they should be the same from all I know, assuming a molecule of the same shape).
    Offline

    0
    ReputationRep:
    (Original post by ItsImran)
    1) Explain why alcohols have a higher boiling point than alkanes of similar molecular mass?

    2) Describe the Volatility of alcohols compared to alkanes of similar molecular mass?

    This is AS OCR chemistry by the way.
    Thanks in advance!



    Posted from TSR Mobile
    For 1, I world write,

    Alcohols contain a hydroxy group which are able to form hydrogen bonds between the same alcohol molecules. This forms between a d+ hydrogen atom and a d- Oxygen atom. this type of inter-molecular attraction is strong, hence a lot of energy is required to separate the molecules apart, thus it has a higher boiling point that the alkene of similar molecular mass, which only form London forces and so have a much lower boiling point, thus requires less energy to separate, thus a lower boiling point than the alcohols of similar mass.
    • Thread Starter
    Offline

    1
    ReputationRep:
    Amazing!
    Combined this is the perfect model answer I was looking for.
    Thanks!!


    Posted from TSR Mobile
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    Brussels sprouts
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

    Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

    Quick reply
    Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.