Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Chem AP, High school help needed

  1. Sep 4, 2006 #1
    I am in Chem AP in high school and it is rediculously tough. Although, I try to study the book by myself, but I always have questions. Too many to ask the teacher. I was wondering if you guys could clear those out. Thx a lot.
    1. I read in the book that H20 is acid. But I always thought water is neutral.
    2. Since acid are electron donator, shouldnt they all have negative charge on top?
    3. Are Alkane, Alkene, and Alkyne, neutral, acid, or base?
    4. I know whenever something burns, oxygen is present on the other side of the equation, is this same for heated?(ex. ethanol is burned completely in air)
    5. "Aqueous solutions of oxalic acid and excess potassium hydroxide" I am suppose to write balanced ionic equations for the following reactions. First of all, how would it matter to me if know weather oxalic acid is aqueous or not? Second, what does excess imply here? I remember hearing in class that excess means that there is enough potassium hydroxide to complete the whole reaction. But what difference does it make in writing out the equation? Btw, here is the answer to this question: H2C2O4 + OH- = H2O + C2O4 2-(this is an ionic equation therefore potassium was crossed out)

    Note: THe only questions I post here are the ones my friends don't know and I couldn't find in google. Thanks a lot for helping me out guys.
  2. jcsd
  3. Sep 4, 2006 #2
    Pure water is neutral. However, minerals and such in natural spring water can make it either acidic or basic.
  4. Sep 4, 2006 #3
    alright, question number 1 is out, lets hit the rest guys.
  5. Sep 4, 2006 #4


    User Avatar
    Science Advisor
    Homework Helper

    Hi and welcome to the forums

    1)Water is amphiprotic, it can be regard as an acid and a base.

    2)It doesn't have to have a negative charge, as in a formal charge, rather, an electron density.

    3)It depends on the specific species in question.

    4)I'm not quite sure what you're referring to here.

    5)The aqueous denotation indicates that the reaction will pertain to an acid base reaction in water; thus the situation can be related to water equilibrium concepts. With an excess amount of the strong base, the reactions will be regarded as complete, or that they will go to completion. That is, you won't need to be concerned about an equilibrium, and the reaction arrow will be that of a single one, from the reactants to products.
  6. Sep 4, 2006 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    No, they are not. Recheck the definition.

    This question is meaningless. pH is defined only for a solution.

    The question is unclear, but my best guess is "no". When a reaction is described as involving the "heating" of a reactant, it does not necessarily imply combustion/burning.

    Please inspect your answer to see that it is NOT correct. Neither the charge nor the number of H and O atoms is balanced.
  7. Sep 4, 2006 #6
    5. thats because it is the unbalanced version, my teacher told us, we don't need to balance ionic equation.(not that we can't) They only test weather we know how to set it up.
    2. Sorry, they are electron acceptor, but still, then they should all have plus charge and all bases should have negative. But thats not true. Why not?
  8. Sep 4, 2006 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If you did take the effort to set it up yourself and then balance it, you will find two possible solutions. One of these corresponds to an excess base (it would be similar to the equation posted above) and the other would correspond to a situation with excess acid.

    Just spend the couple of minutes it takes to do it right and you can fully understand what I mean. But if you balance it and don't understand, I'd still be inclined to explain further.

    It appears that you are asking for an explanation of the moral of a story without first reading the story. You need to understand what it means to be an electron acceptor. Go back to the chapter in your text where you found this sentence. Hopefully, it didn't just appear out of the blue. The section where this sentence was found most likely had a explanation of Lewis Acids and Bases. Read that again, and if there's something that still bothers you, come back with a question.
  9. Sep 4, 2006 #8
    To be simplistic, acids tend to donate their H+, which is why it's alright to keep them "neutral", because once they do their donation, they'd become negative and hence proving the point of electron accepter.
    Like: (My acid-base stuff is a bit rusty so correct me if I'm mistaken)
    [tex]HCl + H_{2}O \rightleftharpoons Cl^{-} + H_{3}O^+[/tex]
    Which should make a little more sense. Do you see how it donated a proton? (i.e. an [tex]H^+[/tex]?)
    I'm using the Bronsted-Lowry definition. (I think)
  10. Sep 4, 2006 #9


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    How does leaving behind a negatively charged ion prove that something is an electron acceptor?
    Yes, which is restricted to a far smaller set of acids/bases than the Lewis definition quoted by the OP and hence, can not explain why Na+ (for instance) is a Lewis Acid.
  11. Sep 5, 2006 #10
    Alright, I got it, acid is a hydrogen donator and after it donates it, it accepts electrons. So both are both donator and acceptor, it just depends on weather you are talking about H+ or electron. Correct me if I am wrong.
  12. Sep 5, 2006 #11
    I am confused about this one more thing...
    What does it mean when something is concentrated, diluted, and in excess amount?(Gokul, I still didn't get it) My friend told me though that excess means all the hydrogen are available for reaction and diluted means only 1 is. If you can see from what perspective he is talking about, could you explain to me that way? What does concentrated mean btw? Here is an example: a strip of copper metal is added to a concentrated solution of sulfuric acid.
  13. Sep 6, 2006 #12


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    This is the unbalanced equation you wrote down in the OP:

    H2C2O4 + (OH)- = H2O + (C2O4)2-

    I'm now going to balance this in two different ways and write down two essentially equivalent equations.

    1. H2C2O4 + 2(OH-) = 2H2O + (C2O4)2-

    2. H2C2O4 = 2(H+) + (C2O4)2-

    The first suggests that in equilibrium there are a number of OH- ions floating around; the second suggests that there are a number of H+ ions in the solution. Which of these two cases will result of you added an "excess of base"?
    Last edited: Sep 6, 2006
  14. Sep 6, 2006 #13
    Can you show me the equation that results if potassium hydroxide wasn't in excess. I want to see the clear diff. b/w those. From where I see right now, there was just one OH- and it became two b/c of balance. How would the balanced version be any diff. from this?(the one w/o excess) Here it is again, pasted from above: "Aqueous solutions of oxalic acid and excess potassium hydroxide"
  15. Sep 6, 2006 #14


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The first equation is used if KOH is in excess (and hence there are OH- ions in equilibrium). The second equation applies to the case where H2C2O4 is in excess (and hence there are H+ ions in equilibrium).

    Before you get to ionic equations, you should have dealt with limiting/excess reagents in stoichiometry problems. This is simply an extension of that basic concept. If one of the reactants is in excess of the amount required to react with the other reactants, there will be some amount of this reactant that survives unreacted.
  16. Sep 9, 2006 #15
    you are right, my bases were weak, now that I have my ground strong, I barely will have any questions, and I could ask those to the teacher, thx.
  17. Sep 9, 2006 #16
    Nvm the last post, lol.(you were right though) While I was rereading the Oxidation-Reduction chapter, I had the following questions.
    1. Can charged molecules or atomes exist in redox equations because here is how my understading about these have developed. If atoms and molecules are already charged, then there is no exchange of electrons going on, they just mix up like puzzles. But if they are both neutral, then one gives up electrons, another accepts it and thats how they both become charged and redox reaction occurs. Tell me if my understanding is wrong.
    Last edited: Sep 9, 2006
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook