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Carbonate ion 2- charge, how?

  1. May 6, 2009 #1
    Hi, I m 21 and am studying Chemistry in my spare time, I haven't done it since i was 15 so I am only a beginner and i do not understand simple things lol, Here's one such example;

    carbonate ion, CO3^2-, is a polyatomic anion with 1 carbon atom and 3 oxygen atoms.
    The ion has two units of negative charge as there are two more electrons (32) than protons (30).

    Now how is this so? Oxygen ions have a 2- charge, so 3 of them would equal 6- charge,
    and although my book does not say it, i suppose carbon has a 4- charge being a non-metal, but this would equal a 10- charge... So i have two questions really,

    Alpha) Can you explain the carbonate ion to me
    Beta) What, if any, is the charge on Carbon.

    Thanks a lot, i know this is a physics forum but i am having no Physics problems so far (not at QED yet lol)
     
  2. jcsd
  3. May 6, 2009 #2

    Borek

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    Polyatomics are not built of ions, so you can't assume oxygens are -6 and carbon is -4.
     
  4. May 6, 2009 #3

    chemisttree

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    For the detemination of charges for polyatomic ions, you should use the standard oxidation values for each of the atoms that make up the ion. Carbon can either be + or -4. Which do you think it is? Hint: think of the electronegativity of oxygen vs carbon and determine which atom will grab electrons and which will release electrons.

    http://gohs.tvusd.k12.ca.us/TeacherWebs/Science/itaylor/Global/WS/Electronegativity%20chart.pdf [Broken]
     
    Last edited by a moderator: May 4, 2017
  5. May 6, 2009 #4

    Borek

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    At first I wanted to post something similar, but decided not to. This rule is very limited - NO2-, SO32-, ClO2-, ClO3-, ClO4- (not to mention OCl-), MnO4- & MnO42-, HxPyOz acids and so on. List of exclusions is longer than the list of inclusions (not sure if that's in English, but hopefully you will get what I mean)

    I often wonder what is better - to give a very limited "rule" or to say "for now that's just the way it is, you will learn the whys later" :confused:
     
  6. May 6, 2009 #5

    epenguin

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    It just is, but what just is? Isn't it better to say at this level: think first of the (neutral) molecules formed from an element with oxygen. For C that is CO2. All the nonmetallic oxides can react with water to form oxyacids CO2 gives you H2CO3. Think of one of the double bonds of CO2 opening up in the reaction with water (quite slow btw and to a small extent) to give a molecule we can represent H-O-CO-O-H tho usually written H2CO3. The H atoms are not connected directly to the C. These are the ones that can ionise to form CO32- + 2H+. That will happen very little in pure water, but it will all happen nearly completely if you supply OH- ions for the H+ to react with (forming H2O) by e.g. adding sodium hydroxide to the CO2 solution. For other oxides/oxyacids it is similar.

    I don't know whether this explanation is too simple or too advanced for you. :smile:
     
  7. May 6, 2009 #6
    So Carbon can be + or - 4!!! My book does not say this so far, i am on chapter 3 (page106) so it's still early days but it only mentions charge + or - 1, 2, or 3. on individual elements, that's good to know.

    So if it were Co2 it would be balanced and would exist in the world around us (as carbon doixide does) but if it is CO3 (or CO1) it has excess charge so it HAS to be used up in a reaction, i.e. CaCO3.

    I know i still have a lot to do, anyway thanks for the help i'l continue with the book and wont be hesitant to bug you all some more,

    cogitas ergo es? ;)
     
  8. May 7, 2009 #7

    Borek

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    Do they?

    CO doesn't react with water to create H2CO2 that will dissociate to CO22-, so you have to remember CO is an exclusion.

    CO2 reacts with water to give H2CO3 - OK. But then you may expect NO2 to react with water to create H2NO3. It doesn't. So you have to remember NO2 is an exclusion.

    NO? N2O? What about H3PO2 - which oxide does it come from? Exclusion, exclusion, exclusion.

    It is too erratic - it can be applied to some acids, but it leaves other unexplained for no apparent reason - the only reason being "that's just the way it is" - and thus can be confusing.

    What I am aiming at is that I am not sure our philosophy of teaching chemistry is right. Quite too often we say things like

    only to explain the students later that is a BS that we told them earlier, but in the reality the rule is blah blah blah (again, this rule hod only till we tell them it wasn't true either). So in practice we want them first to learn something as a principle/rule, then we want them to unlearn it. They tend to remember what they were told first.
     
    Last edited: May 7, 2009
  9. May 7, 2009 #8

    Borek

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    In organic compounds it can be assigned any oxidation number between -4 and +4 (-4, -3, -2, -1, 0, 1, 2, 3, 4). Don't treat these numbers too seriously.

    No, CO - carbon monoxide - is a neutral molecule. Not very stable, as it rather easily gets oxidized to CO2, but stable enough to be a dangerous poison.
     
  10. May 7, 2009 #9

    chemisttree

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    Right, no one treats these formal oxidation numbers too seriously in organic chemistry. Poster isn't there yet though.

    At the poster's level of study, these oxidation numbers are fairly handy. There are exceptions of course, but I doubt they outnumer the "rule" . I was taught that oxygen almost always is -2 and that to which it is bonded can vary (peroxides were handled separately). For carbon this is +/- 4 in polyatomic ions. Phosphourous, sulfur, halogens and nitrogen were taught to be common exceptions to the "standard oxidation" values. I believe that the method taught to me to understand the exceptions was to draw the Lewis Dot structure. It seemed to be helpful to do it that way. Perhaps Sponsoredwalk will soon post some questions regarding dot structures?
     
  11. May 7, 2009 #10

    chemisttree

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    Check the periodic table in your book. It might be on the inside of either the front or back cover. A good introductory Chemistry text should have common oxidation values for the elements in the periodic table. My CRC has them. Nitrogen is shown to have values of 5,4,3,2,1,-1,-2 and -3 but -3 is the most common among those when nitrogen is combined with metals.
     
  12. May 7, 2009 #11
    So these values are informal, are they literally a stepping stone to the hard stuff?
    And you say Nitrogen can have many values, i must say that's cool and i can't wait to find out why although it is quite daunting to learn from a beginner's point of view :(

    I am going as slow as possible to ensure i grasp everything and miss nothing so i'll be posting many times and as you see i will continue to question things and ask when they don't make sense. Now I hate to accept this but if these numbers are so "come and go" should i increase my pace until later chapters where i will find out what actually matters as opposed to worrying about Ionic compounds, idk because i found another problem related to this that doesn't add up;

    hydrogen phosphate ion: HPO4^2-,
    Hydrogen + or - 1,
    Phosphorus 3-.
    4 Oxygen's = 8-,

    my logic brings me to HPO4^10- if Hydroogen were to be +

    Also,
    Hydrogen sulfate ion: HSO4^-
    Hydrogen: + or - 1,
    4 Oxygen's: 8-,
    Sulfur: 2-,
    HSO4^9-

    Obviously this is wrong, unless, like Nitrogen, the numbers magically do quantum leaps to any value lol...
    can i be saved????

    (I would start a new thread but the question is so closely related to charges)

    www.wikipedia.org/wiki/Snuff ;)
     
  13. May 8, 2009 #12

    Borek

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    P +5, S +6. You will get these numbers looking at the oxides (P2O5, SO3) that give both acids when reacting with water. Oxides are neutral, oxygen is -2 almost always, so central atoms have to be +something.

    Have I told you to not treat these numbers too seriously? :rofl:
     
  14. May 9, 2009 #13

    epenguin

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    I'd agree chemistry is the hardest thing to teach.
    Well, that's my excuse for not knowing it very well! :shy:
    What would you recommend me to read to accede to the higher-level BS you mention? What do you teach? :smile:
    I am not sure that a bit of learning and unlearning is not a good or nec. thing.
    Remembering what they were told first is better than remembering nothing.
    In mathematics would you pitch students into what mathematicians regard as fully rigorous from the start, which seems to have happened with the result of students not knowing what it was about or the point of it even when they could do the excercises?


    I am not fully sure of this. Maybe mine was a simplified overstatement. Maybe if I say instead of 'react' by 'formally can think of as product of reaction of the oxide with water'? That was really to the point of where the OP was stuck.

    + Maybe if I said for the 'normal' valencies/oxidation states predicted by octet rules etc. - for N and P +3 and +5?

    Actually chemistry seems like learning verbs in foreign languages, there are regular and irregular conjugations/declensions; fortunately the regular ones take you farther in chemistry than in languages. The student can be given a certain predictive power that the above valencies fit into, but nitrogen has a lot of irregular verbs that can be rationalised but hardly predicted. And would it be fair to say that most of the unpredictable irregulars are concentrated in the first long row of the periodic table where atoms and valency electrons are more on top of each other?

    And even then I am not sure that even all your exclusions really are.
    Can you not regard CO as the anhydride of formic acid? After all the reverse (dehydration) reaction does happen and is even a practical lab. method for making CO.
    I had never heard of hydrides of N2O till yesterday but apparently nitroxyl HNO and hyponitrous acid H2N2O2 exist even have applications.
    For H3PO2 I said the oxides generated oxyacids not that every acid came from an oxide, anyway that is sort of coming from a hydride isn't it?
     
    Last edited: May 10, 2009
  15. May 10, 2009 #14

    Borek

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    I don't teach, so I should either sit silent, or pretend I know everything just from looking. The reality lies somewehre in between.

    Quite often it is, sometimes it is not. I wonder if giving "half rules" - that only hide fact that you still have to remember a lot - is better than simply stating - "these you have to remember, till you will be able to learn why".

    It is a matter of selection of what is told and what is done. It is also a matter of selecting examples.

    I was never a full time teacher, but I did a lot of tutoring. Could be it was my bad luck, but I have spent about half my time helping students to understand alien concepts (of - say - mole and reduction), other half - helping them understand that misconceptions learnt earlier is what stops them from understanding new material. I don't know the cure, but I can see - now and then - the same problems behind questions posted on forums here and there.

    Now that I think about it, perhaps that will help to nail the problem. Even then, that means only a few acids out of tens (if not hundreds) can be explained. This is a "half rule" - "remember to which ones you can apply partial explanation to calculate charge" vs "remember charges".

    You are right when it comes to chemistry, but you are wrong and the pedagogical side. In the context of the original problem (calculation of anion charge) - these are exclusions. Student looks at the acid formula and tries to apply simplified rule - and it doesn't work, or gives erratic results. At this stage reasons are less important.
     
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