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If water is covalent, why is hydrogen H+ ?

  1. Oct 26, 2012 #1
    Water is covalent compound. But formation of water is written as:
    H+ + OH- = H2O

    I cannot understand why hydrogen is electrically positive in the above equation. Hydrogen wants one more electron to complete its first and last shell (total two valence electrons). Hence, hydrogen should be negative since it attains an extra electron.

    So, then, why is it positive?
    Please explain also.
    Thanks a lot!
  2. jcsd
  3. Oct 26, 2012 #2


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    Gold Member

    Water is covalent and it is H2O, but the nature of H-O covalent bond is polar, thus, under the influence of external factors, like dipole interaction with other water molecules, one of the H-O bonds ionizes to form H+ and OH-.

    Hydrogen you seek is positive because it's lone electron is taken by Oxygen, therefore H+ is positive and oxygen is negative in OH-.
  4. Oct 28, 2012 #3
    Also, a hydrogen ion (H+) is positive when it loses an electron. You're talking about it gaining an electron to complete its "octet", but that would be noted as H-.

    At rest, a single hydrogen has a single electron associated with it though. Atoms like to be neutral, rather than ionized in any fashion. You can either pull an electron away from that proton, or add another electron to it, in order to ionize the atom. Keep in mind that doing either takes energy, which has to come from somewhere.

    You'll end up learning later on that pure water (which is actually fairly rare) doesn't just sit there all happy. There's this thing called entropy, which causes some of the water molecules at the surface to want to find something else to play with. More importantly though, there's energy in there keeping water from crystallizing as a solid, and some of that energy will cause miniscule amounts of water to form hydroxide and hydronium.

    I'm guessing that you're learning about electrolysis right now, though. Just understand that if you throw a bunch of unassociated electrons at things (like elemental hydrogen and oxygen), then you're not talking about hydrogen and oxygen in their "normal" states. You're throwing energy at them, so they're going to do something "unusual".
  5. Oct 29, 2012 #4
    Hydrogen is less electronegative than oxygen. So, hydrogen wants to attract an electron to get a pair; oxygen wants to attract an electron to get an octet; oxygen is better at attracting and therefore succeeds in getting its octet, so hydrogen loses its electron.

    For comparison, if you react hydrogen with lithium, which is less electronegative than hydrogen, then hydrogen succeeds in getting its electron pair and lithium loses electron, forming lithium hydride.

    Since water is "covalent", free hydrogen ions are actually rare in water. Heat movement does break a few O-H bonds, normally forming H+ and OH-; but this rarely happens. Most water molecules are covalent and neutral, and pure water (distilled and also degassed) is a bad conductor of electricity. Its pH is +7, not -1,75 which it should be if water were not covalent.

    If you mix a strong acid with a strong base, the reaction
    H+ + OH-=H2O
    does happen. But it goes mainly to the right - large amount of heat is released by forming the covalent bonds, and only very few of these bonds are broken by heat.

    If you melt ionic lithium hydride (at temperature 688 Celsius) then the melt is ionic, not covalent like water, and molten salts are much better conductors of electricity than clean water. Also, if you electrolyze molten hydrides, then hydrogen goes to anode, while it goes to cathode during electrolysis of water.
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