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Thread: Why is hydroxide an anion?

  1. #1 Why is hydroxide an anion? 
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    I was going back over my chemistry basics when I thought I'd try to "visualise" the formation of water at the atomic level.

    Hydroxide is an anion formed by the covalent bonding of Oxygen and Hydrogen.

    I make the assumption that they come together to lower their potential chemical energy. In other words, they are more stable as one.

    What doesn't make sense to me is that Oxygen by itself (in this case), is not in terms of electronegativity, strong enough (I assume this also means that it is not unstable enough/too high in energy) to completely rip an electron from Hydrogen, to make an oxygen anion, instead it shares the electron in a covalent bond.

    In the form of a hydrogen and oxygen bonded together, it should be a neutral molecule. It may be polar, but it should have considerably less electronegativity (and so be more stable and have less energy) than when it was separated oxygen and hydrogen. Yet, the final formation of water, involves H losing an electron to OH to make OH- and H+.

    Questions:

    1. I want to know, with reference to chemical potential energy and/or electron kinetic energy, why hydroxide forms, instead of two hydrogen atoms covalently bonded to oxygen.

    Remember:

    Total Energy = Kinetic Energy + Potential Energy.

    In order to become more stable (less energy), either the potential energy or kinetic energy of an atom must decrease.


    Last edited by Guidelines; November 5th, 2012 at 04:53 PM.
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  3. #2  
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    Think about it: water is a weak acid so H20 dissociating into H+ and OH- in itself unfavorable process, but still occurs since O is pretty electronegative. It can hang on to one negative charge fairly decently, but H2O is still more stable. On the other hand, O 2- would make oxygen bear two negative charges and this is extremely unfavorable. Whenever we think about dissociation it's important to remember that charges are high in energy and generally unfavorable.


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  4. #3  
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    Thank you for your reply OneAtATime,

    However, I am looking for a different type of explaination.

    I should also clarify, I am talking about the formation of water from hydrogen (2 moles) and Oxygen (1 mole).

    I want to know, with reference to chemical potential energy and/or electron kinetic energy, why hydroxide forms, instead of two hydrogen atoms covalently bonded to oxygen.

    When we talk about atoms preferring a stable configuration, we are actually referring to a configuration with decreased chemical potential energy (this means the distance between particles (on average) in the atom is less than it would be if it had higher chemical potential energy) and/or electron kinetic energy.

    Any thoughts on this are welcome.
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  5. #4  
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    I've been trying to get my head around the basics of chemistry recently, so i'm a complete novice

    oxygen is presumably a neutral atom... 8 protons and eight neutrons is it? or 5 neutrons... either way it has as many eletrons as protons so it is neutral.

    Anyway, Hydrogen has one electron orbiting one proton making it easy for this atoms electron force field to be penetrated and making it easy for it to bond with other atoms. presumable the negatively charge eletrons orbiting oxygen are attracted to the positive charge of a hydrogen atom and can penetrate easily due to the limited number of electrons in the hydrogen.

    Presumably during the bonding of H and O... which are both neutral atoms(?) some of the charge (a positron) is lost resulting in an anion which is a negatively charged ion.

    If H loses an electron it doesn't become H+... it becomes a proton. This is because according to chemistry an atom must have an electron.

    As you can see there is a lot of presumtion on my part. I'm not sure i've helped to answer your questions but hopefully this is some aid to you.
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  6. #5  
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    Thank you for your reply *question for you*.

    The number of neutrons that oxygen has is dependent on it isotope. Generally speaking though, the number of neutrons an atom has is equal to the number of protons (this is not a hard and fast rule).

    In this case, oxygen has eight neutrons (8 protons = 8 neutrons).

    It terms of the question I am asking, the key phrase is "with reference to chemical potential energy and/or electron kinetic energy". Below is an example of the type of explanation I am looking for, but instead of talking about the formation of an oxygen anion, I would like to hear one about water formation from H2 and O2.

    I must emphasis, below I have written an example of the type of explaination I am looking for.

    Potential energy is "the energy due to the position of matter".

    Chemical potential energy refers to the energy associated with the distance between the particles of an atom.

    Oxygen by itself has a neutral charge. It also wants to be in a more stable state. That is, a state of lower energy.

    Imagine the oxygen obtains two extra electrons (forming an anion). The difference in charge (the atom is now slightly more negatively charged then positive), causes the electron cloud to be attracted more strongly to the nucleus. This will decrease the overall size of the atom. This will also mean a decrease in the distance between atomic particles. This means less chemical potential energy in the atom. Therefore, it is in a more stable form.

    Once again, the key phrase in the question I am asking is: "with reference to chemical potential energy".

    Thanks for any input.
    Last edited by Guidelines; November 6th, 2012 at 12:56 AM.
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  7. #6  
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    I might be misinterpreting your last post, but I think you are suggesting that anions are smaller than their neutral parent atom. (Imagine the oxygen obtains two extra electrons...) This isn't correct because when an atom gains electrons forming an anion the overall size of the atom increases due to electron-electron repulsion.
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  8. #7  
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    OneAtATime,

    You are correct, it is cations that decrease in size. However, my original question still stands, which I will restate in a new way (as I am still not getting the answers I am looking for - I appreciate you trying anyway).

    Total Energy = Kinetic Energy + Potential Energy.

    Atoms are trying to become more stable (less energy).

    In the formation of water from H and O, how does the total energy of the atoms decrease? This can be either due to kinetic energy, or potential energy (for example chemical potential energy).

    In the case of an Oxygen anion, as you have correctly pointed out OneAtATime, the atom would grow larger due to electron-electron repulsion. This would increase the chemical potential energy though (due to increased distance between the atomic particles), so it would make the atom less stable. So why does Oxygen have such a willingness to become an anion? Do the extra electrons decrease kinetic energy somehow (for example the speed of the electrons slow down for some reason)?

    If both potential energy and kinetic energy increase, then the energy of an atom should increase. However, more energy = less favoured.

    Thank you
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  9. #8  
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    " Oxygen by itself, is not strong enough to completely rip an electron from Hydrogen to make it an anion"

    Looks like a logical error, when you rip an electron you ionize so you make a cation not an anion.
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  10. #9  
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    That is an error in punctuation and/or sentence structure.

    I am refering to Oxygen ripping an electron from hydrogen to make oxygen an anion.

    You could also look at it as oxygen forming a hydrogen cation.

    My original question still stands.

    Also, for clarification, I am hoping for an explanation, "with reference to chemical potential energy and/or electron kinetic energy", regarding the formation of water from Hydrogen and Oxygen. So it is only possible for me to make errors in the way I ask the question. It is also possible for my logic to be in error when I give examples of the explanation I am looking for, however, this is not relevant. What I have written are examples - in the hope this will ensure that someone will post an answer that "references chemical potential energy and/or electron kinetic energy".

    Whether my examples are right or wrong, they include the words "chemical potential energy and/or electron kinetic energy". It is my sincere hope that future replies will also include a reference to "chemical potential energy and/or electron kinetic energy".
    Last edited by Guidelines; November 6th, 2012 at 12:54 AM.
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  11. #10  
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    Oh I get it, you may be confusing 2 things, the homolitical and heterolitical cleavage energies. For example when you have an acid like COOH, we refer to the OH bond to be weak because the H can easely detach, but that is an heterolitical detachement, that is the oxygen becomes an anion. If you acually tried to achieve an homolitical detachement of the OH bond, that is, to form a H radical and a O radical, it is one of the hardest bonds to split!

    So when you say that H2O loses an H to form OH- that's ok! No problem it's heterolitical. If you said H2O loses an H to form OH. and H. I would argue with you over it but if you say OH- and H+ no problem. That's easy to do. It happends all the time, even in pure water with a 10^-7 rate, which also gives the pH of 7.

    Another example, if you try to split N2, how is it easier, to form 2x N radical or a N+ and N-? The radical is very simple, in fact those bonds are frequently used in cancer medicine because it's easy to split, HOMOLITICALLY. If you try to split heterolitically you'll never succeed. If you try to obtain N+ cation...you may require to power down the city for a while so you can do it on a thumb sized sample lol. Why? Because the ionization energy of N is very high. For more details look into quantum chemistry.

    Not sure if this answers your question.
    question for you likes this.
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  12. #11  
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    by the way, the lowering of potential energy is only one part of what makes a process spontaneous, so you can't assume that just because one process lowers potential energy more than another it will be favored.
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  13. #12  
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    Quote Originally Posted by hermanntrude View Post
    by the way, the lowering of potential energy is only one part of what makes a process spontaneous, so you can't assume that just because one process lowers potential energy more than another it will be favored.
    Cool story bro 8]
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