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Thread: Basic Chemistry Stuff(2)

  1. #1 Basic Chemistry Stuff(2) 
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    I am taking an Earth Science course now and currently reviewing some basic chemistry knowledge needed for further study on earh mineral and rocks. But I haven't touched chemistry for almost 2 years that I start to forget things, can someone help and clarify some basic chemistry stuff? I would appreciate! :wink:

    1) The smallest unit of a covalent compound is molecule. But what is a formula unit? Is it the smallest unit of an ionic compound that still retains its chemical and physical properties?

    2) Why is a solution considered to be a mixture instead of a compound? Is dissolving a chemical reaction?

    3) For examle, predict the chemical formula of carbon chemically combining with oxygen, C2O4 is obtained which must be reduced to its lowest ratio, CO2, but why something such as C6H12O6 cannot be reduced to CH2O? How can I know when a chemical formula is to reduce and when not to?


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    Quote Originally Posted by kingwinner
    1) The smallest unit of a covalent compound is molecule. But what is a formula unit? Is it the smallest unit of an ionic compound that still retains its chemical and physical properties?
    Formula unit comes into action when we have crystals; take the crystal salt its formula unit is NaCl. The answer to your second question I do not have (maybe I can look it up, if I have time). I'm not sure one mol seperate NaCl have de same dissociationenergy then one mol NaCl in a crystal. Maybe I am way of here?


    Quote Originally Posted by kingwinner

    2) Why is a solution considered to be a mixture instead of a compound? Is dissolving a chemical reaction?
    How can a solution not be mixture?
    a solution is not the soluble form of a compound. When you have pure (which is impossible to obtain but just as an example) ice after melting it it will not be a solution of water.
    So a solution is a mixtures because you have the solvent (the compound that bring the solute into solution) which dissolves the solute.
    I.e.: water = solvent en salt = solute (you obtain a salt solution in water = mixture).
    In my opinion dissolving is a chemical reaction because its changes the molecular composition of the originals molecules.
    Quote Originally Posted by kingwinner

    For examle, predict the chemical formula of carbon chemically combining with oxygen, C2O4 is obtained which must be reduced to its lowest ratio, CO2, but why something such as C6H12O6 cannot be reduced to CH2O? How can I know when a chemical formula is to reduce and when not to?
    This one I don’t understand!


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    1) The word “covalent” means to share electrons (e-) by doing this the compound remains neutral. All compounds that share e- are called molecules, and a molecule consists of two or more atoms sharing e-.(can not be more redundant). Examples: C6H12O6 glucose, O2 oxygen as gas.

    The ionic bond is formed by the attraction of opposite charges. Their bond is not shared like the molecule, instead one atom attracts an electron more than the other (plainly speaking) making the whole compound charged and they are called formula units.

    All compounds have fixed composition by mass ex. a molecule of glucose will always be made of six carbon atoms twelve H and six O.

    2). A solution is a homogenous mixture, this means that all of its components are physically distributed and there are no visible divisions, it looks like one. The prefix “homo” means one. I underlined physically because a solution doesn’t undergo a reaction therefore a solution can not be a compound.

    The process of dissolving is not related to a chemical reaction because a new compound is not formed. In basic terms, for the process of dissolving you need the liquid (solvent) and a solid (solute). The solvent surrounds the solute in an electrical attraction. Example: water is a solvent that is partially charged. One side of the molecule is positive and the other is negative. When it comes in contact with a solute like table salt NaCl, which is an ionic compound, the salt dissociates (separates) into Na+ and Cl- Sodium and Chloride ions respectivly. Now water, like any solvent, is able to surround the ions and dissolve them.
    This is some representation.
    +H2 O- +H2O-
    O- +H2O- +H2
    +H2O- Na+ -OH2+ -O+H2 Cl- +H2O-
    O- +H2
    +H2 +H2O- O- +H2O-

    Water is an interesting compound. It is a molecule and has ionic properties because it is partially charged. Oxygen’s nucleus is strong. It strongly attracts the electrons of hydrogen which makes it partially negative and the hydrogen’s nucleus are exposed making them partially positive. O-
    +H H+


    3). This question is answered with the law of definite composition. It states that a compound is composed of the same elements with a fixed proportion or percent by mass. The mass of an element is multiplied by the number it appears in the compound and then divided by the total mass of the compound. Example: calcium carbonate CaCO3
    Ca = 40.0 total mass of compound 100.0
    C = 12.0
    O = 16.0 (3)
    the proportion is 40% Ca, 12% C, 48% O.

    In the case of glucose C6H12O6 the proportion or percent is 40% C, 6%H, 54%O. Even though CH2O has the same proportion as glucose the lowest ratio possible is C6H12O6.
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    sorry the molecular representations didn't come out as I thought
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    Quote Originally Posted by guilleermo
    The ionic bond is formed by the attraction of opposite charges. Their bond is not shared like the molecule, instead one atom attracts an electron more than the other (plainly speaking) making the whole compound charged and they are called formula units.
    Knowing this; dissolving is indeed a physical transition and not a chemical reaction.
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    Quote Originally Posted by Him
    Knowing this; dissolving is indeed a physical transition and not a chemical reaction.
    I disagree - dissolving can be an example of a chemical reaction, especially if you have an ionic solid dissolving. I agree that dissolving something like a diatomic gas probably wouldn't be a chemical reaction.
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    My basic chemistry knowledge (especially he inorganic) has went to back of head, so a little bit to far).

    If we consider bio-molecules dissolving must be regarded as a chemical reaction. If a protein is dissolved in water different acid/base reactions occurs. Covalent bonds are broken, so as consequence of dissolving different chemical reactions happen.

    However (and I am in dark here because isn’t my territory anymore)
    Quote Originally Posted by guilleermo
    The ionic bond is formed by the attraction of opposite charges. Their bond is not shared like the molecule.
    I considered this statement as that the Na+ and CL- do not share any electrons and the ionic bond is just formed because of + - attraction. In this case dissolving is only a separation of two charged items away from each other (a physical transformation).
    (If I did understand guileermo correct but he can correct if he wants)

    Quote Originally Posted by guilleermo
    The process of dissolving is not related to a chemical reaction because a new compound is not formed.
    he stated that the chemical composition of Na+ and Cl- are the same in solution or in crystal). If this is correct I also conclude that disvolving (of salts) isn't a chemical reaction.
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    I concur with Him's assessment and would add this point. The individual atoms and molecules have never heard of chemistry and physics. Chemistry and physics are human imposed, artificial classification systems. At times the distinction between physics and chemistry can become meaningless.

    Chemical reactions are going on at the level of the individual molecule and atom. The Na+ ion does not sit there in isolation. Remember that water is a polar molecule. The negative portion of the molecule is attracted to the Na+ ion. I suspect that we get occasional dissociation of water molecules so that the hydroxide ion associates with the Na+ ion.

    But on average - remember all of this is a dynamic equilibrium - no compound is formed, so on a macro scale this remains physics and not chemistry.
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    There are two major points that need to be made here regarding dissolving ionic salts in water.

    First, dissolving an ionic salt in water doesn't simply involve electrostatic interactions; there can also be significant covalent interactions that contribute to the dissolving. The cation in a salt can form covalent bonds of various strengths with the lone pair electrons of a water molecule's oxygen atom when they dissolve, so it's not accurate to characterize dissolving as a purely electrostatic process.

    Second, dissolving ionic salts does involve forming a new compound. In a solid salt crystal of, say, NaCl, you have positive sodium atoms electrostatically bonded to negative chlorine atoms. When you dissolve NaCl in water, you produce two new substances - a hydrated sodium atom and a hydrated chlorine atom. The chemical equation would look something like: NaCl + 12H2O --> Na(+)(H2O)6 + Cl(-)(H2O)6. Salts like NaCl will dissolve in water because a Na(+)(H2O)6 and Cl(-)(H2O)6 molecule are thermodynamically favorable to a NaCl molecule. If this new substance wasn't forming, the salt wouldn't dissolve. These hydrated ion complexes are new compounds that have different chemical properties from your original NaCl crystal. Indeed, you can calculate whether or not a salt will be soluble in water by calculating the enthalpy and entropy of the original salt molecules vs. the hydrated ion complexes that would form when the salt dissolves.
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    Enthalpy and entropy are no values purely for chemical reactions; thermodynamics does also described other changes in the field of physics. I still do not see which covalent bounds are formed in the process of dissolving lets say NaCl.
    Furthermore the structure Na+ surrounded with water is indeed happening; but also electrostatic in my opinion and do not change Na in a chemical manner (it is not a new molecule).
    But as I said earlier; I wasn’t sure the chemical nature of ions is the same in solution or in solid. And I still am not. But also am not convinced of the opposite.
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    Quote Originally Posted by Him
    Enthalpy and entropy are no values purely for chemical reactions; thermodynamics does also described other changes in the field of physics.
    True enough, but every reaction has an entropy and enthalpy change associated with it; you can calculate if a reaction will be spontaneous or not by calculating the entropy and enthalpy of the reactants and products and seeing which will be more thermodynamically favorable.
    I still do not see which covalent bounds are formed in the process of dissolving lets say NaCl.
    There wouldn't be much covalent bonding with NaCl, but other salts like FeCl2 will undergo covalent bonding between the lone pair on the water's oxygen atom and the Fe2+ atom. All transition metal salts will do this to some degree.
    Furthermore the structure Na+ surrounded with water is indeed happening; but also electrostatic in my opinion and do not change Na in a chemical manner (it is not a new molecule).
    The same could be said for reacting NaF with LiCl to produce NaCl and LiF; but I think most people would consider that to be a chemical reaction.
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    Quote Originally Posted by Scifor Refugee
    FeCl2 will undergo covalent bonding between the lone pair on the water's oxygen atom and the Fe2+ atom. All transition metal salts will do this to some degree
    Such processes being a consequence of dissolving are indeed chemical reactions. So in summary: dissolving of certain salt result in chemical reactions, while dissolving of others like NaCl can be regarded as a physical transition. Everybody agrees?

    Quote Originally Posted by Him
    Furthermore the structure Na+ surrounded with water is indeed happening; but also electrostatic in my opinion and do not change Na in a chemical manner (it is not a new molecule).
    Quote Originally Posted by Scifor Refugee
    The same could be said for reacting NaF with LiCl to produce NaCl and LiF; but I think most people would consider that to be a chemical reaction.
    LiF and LiCl do have distinct chemical properties and are regarded as different molecules. So I do not understand the comparison.
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    Quote Originally Posted by Him
    LiF and LiCl do have distinct chemical properties and are regarded as different molecules. So I do not understand the comparison.
    You seemed to be arguing that the hydrated ion complexes that result when a salt dissolves don’t count as “new molecules” simply because they are only held together by electrostatics. My point was that many things that one definitely would consider to be distinct molecules are held together by electrostatics, so why should hydrated ion complexes be any different? Hydrated and anhydrous ions definitely do have different chemical properties.
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