Chemical Bonding

  1. 1. What test would determine whether a solid substance contains ionic bonds?

    and

    2.

    a) Explain why graphite is soft and has a high melting point.
    b) Explain why diamond is hard and has a high melting point.
    c) Why is diamond a poor conductor of an electric current and graphite a good conductor?
     
  2. jcsd
  3. 2a. Graphite has strong covalent bonds in one plane, but extremely weak bonds (pi bonds) connecting each plane. The hybridization in this case is sp2. This makes it extremely weak since those bonds can be broken with extraordinary ease.

    2b. Diamond has strong covalent bonds in all directions. Each carbon atom is tetrahedrally (sp3) cordinated to another carbon atom, which is a very thermodynamically stable structure.

    2c. In diamond, there is a large bandgap between the HOMO and the LUMO. Remember that conduction is the excitation of electrons into an excited state -- that is, into a conduction band. In graphite, there is an extremely small bandgap between the HOMO and the LUMO. This means that it's easy to get electrons into a conduction band. The existence of pi bonds also means that there are delocalized electrons.
     
  4. Well you need to give in a LARGE amount of energy to break an ionic bond, as compared to a covalent bond. This is because of the coulombic forces of attraction between an anion and a cation. Further, every cation is surrounded by several anions (and in a crystal lattice, this leads to even stronger forces).

    So essentially you need a chemical test that can utilize the abovementioned characteristics. I am presently unaware of any process except solvation (or hydration) which can be used effectively to determine whether a substance has ionic bonds. If you are looking for an experimental technique however, you can determine the structure through spectroscopy. Alternatively you can theoretically compute the net ionic character in a bond in order to make a useful argument about the nature of the bond.

    Remember that a solid ionic substance is made up of a very strong lattice structure which won't easily yield to a chemical test (normally) and you will be better off dissolving it in a polar solvent first...

    Cheers
    Vivek
     
  5. Most ionic compounds are readily soluble in water to different amounts. I'd put a few flakes in 10-20 ml of water and heat it to near boiling if it did not dissolve at RT.

    High MP is due to various factors, one of which is covalent bonding. Ostwald gave a good summary.

    Diamond is normally a poor conductor for the reasons stated by Ostwald, but Diamond can be a good electrical conductor but only under special conditions.
     
  6. Yes but what about Question 1?
     
  7. Carbon bonds with 4 other carbon atoms. This is what happens in Diamonds, all the bonds are used, therefore it is strong and hard and has a high boiling point to break the bonds.

    Graphite has carbon atoms that join in threes, therefore there is a free atom, which also the graphite to be soft but it has a high boiling point because it still has 3 bonds to break.

    The Bob (2004 ©)
     
  8. To find out a structure of a compound you need to find out what is in it. Then you can consult a valency chart and see if the elements need to bond ionically or covalently.

    Hope my posts helped.

    The Bob (2004 ©)
     
  9. He wants a test to determine if the compound is ionic or covalent. So I guess we need a chemical test or a convincing physical test...
     
  10. Brake a compound into it's elements and the bonding is easy to find.

    The Bob (2004 ©)
     
  11. Arent almost all solid compounds at room temp. ionic?
     
  12. Not necessarily, think of the 1000s of organic compounds alone that are solids at room temperature.
     
  13. GCT

    GCT 1,769
    Science Advisor
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    Although not all ionic compounds dissolve to an appreciable extent in water, however almost all of them (perhaps all of them) dissolve to some extent and thus a solution of water and a specific ionic compound will conduct electricity.

    This conclusion of mine is somewhat tentative.

    -------
    Online help with chemistry
    http://groups.msn.com/GeneralChemistryHomework/_whatsnew.msnw
     
  14. chem_tr

    chem_tr 613
    Science Advisor
    Gold Member

    Dear Chemrookie,

    You asked about "What test would determine whether a solid substance contains ionic bonds?", I wanted to contribute to this one only; the others are already well explained by the members.

    If a solid substance contains ionic bonds, the substance can be polarized, and conducts electricity. This one is correct, as generalchemtutor explained. I think dimethylsulphoxide or dimethylformamide, with greater dielectrical constants, will help more than water, as some ionic substances may not be very soluble in aqueous solution. As an alternative, a DMSO+DMF+water mixture will be helpful due to synergistic effect. Please note that my idea depends on solubility tests; other means of analysis are also present.

    Regards
    chem_tr
     
  15. Gokul43201

    Gokul43201 11,141
    Staff Emeritus
    Science Advisor
    Gold Member

    Solubility is a simple way to go. Use solubility in water, dil. HCl (other polar solvents) as positive results and solubility in acetone, methanol, CS2 (non-polar solvents) as negative results.
     
  16. Thanks.

    (Having issues with table..hope you see)

    I have two tables here..and have to answer questions based on the tables. I'll try this, but not sure.

    Element Melting Point Boiling Point Molar Volune at STP (L)
    A -189 -186 22.4
    B -100 -35 22.4
    C -7 58 2.6 * 10-2
    D -30 2240 1.2 * 10-2
    E 44 280 1.7 * 10-2
    F 660 2450 1.0 * 10-2
    G 3410 5930 1.0 * 10-2

    (Melting Point/Boiling Point are in degreescelcius.)

    Which elements are likley to be metals?

    E, F and G?

    Which are gases at STP?

    A and B?

    Which are most likley to be non-metallic solid?

    D?

    Which elements are liquid at 200degreescelcius?

    E?

    Which elements are likley to conduct well in the solid form?

    A and B?


    and another table:

    Solubility in Water (kg/kg H2O, 25degreescelcius) Melting Point
    NaF 0.042 988
    NaCl 0.357 801
    NaBR 1.16 755
    Nai 1.84 651​

    Explain the decreasing melting point from NaF to Nai

    Explain the increasing solubility in the same order.

    Cesium flouride has a melting point of 682degreesC and a solubility of 3.67 kg/kg H2O at 25degC. Explain the difference between the properties of CsF and NaF.

    Thanks. These tables deal with the same type of thing as the questions above.
     
    Last edited: Aug 20, 2004
  17. What is STP? Standard Temperature and Pressure means 273 K (zero degrees centigrade) and 1 atm pressure. If Boiling Point > 273 K do you think it will be a gas at STP or a gas? Or will it be a liquid.

    Solubility isn't a simple thing to explain since there are two seemingly unrelated (but actually related) factors: Hydration Energy and Lattice Energy. Hydration (or Solvation) Energy depends on charge density: smaller the size and larger the charge, larger is the Hydration Energy. Greater Hydration Energy also means efficient solvation (dissolution in water called hydration in your case).

    Now according to Fajan's Rules, the criteria for covalency are:

    1. Cation should be small.
    2. Anion should be large.
    3. There should be a high charge on both.

    This means that the decreasing order of covalency is: NaI > NaBr > NaCl > NaF. Here factors 1 and 3 are constant and only the size of the anion affects covalency. This explains melting point differences (ionic compounds have larger melting points than corresponding covalent compounds).

    I would think that ionic compounds have greater lattice energy as compared to covalent compounds of similar molecular weight. If you think so, then NaF should be most soluble in water, contrary to the data that you have given. So I think this conclusion is a wrong one. The problem lies in the fact the lattice energy increases with increase in hydration energy due to the similar factors they individually depend on. The key is to compare not individual lattice or hydration energies but the sum (Lattice + Hydration) energy. If the hydration energy offsets the energy required to break the lattice in solvation, the compound is soluble. If not, it is less (or not) soluble.

    So I would like to believe that in the sodium halide group, NaI having maximum covalent character (and least melting point therefore) has greatest solubility. Why? I am not sure we can answer this convincingly without knowing the energies, but since you're given none there's got to be a valid explanation for solubility. Solubility rules don't help us much. I am waiting for someone to let us know....

    Cheers
    Vivek
     
  18. thanks..I see.

    anybody have anything else to add..or correct? (in regards to to the tables)
     
  19. Well I think the solubility problem is still unresolved isn't it ChemRookie?
     
  20. yeah, not resolved..just thanked you for posting what you did know and what you thought.

    does anybody have any idea?
     
  21. Hello ChemRookie. I was referring to the fact that solubilities need to be explained more accurately than I have done in my post. Yesterday, I read a section in "New Concise Inorganic Chemistry" by JD Lee, on Lattice Energy and Hydration Energy of halides of lithium, cesium, etc (the most common examples). On discussing the details with a learned chemist, I reached the conclusion that solubilities cannot be predicted with ease unless one of the ions is very large--in which case, the effect of hydration energy may be used for some prediction--as otherwise, lattice energy and hydration both are either too high or too low and any qualititative statements cannot be made easily. I would therefore recommend that you get hold of JD Lee and read the particular section to be sure...

    Hope that helps...

    Cheers
    Vivek
     
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