Hello,
I had a question about the "net balanced reaction" for a reaction mechanism. If there are two steps in the reaction mechanism, and the intermediate is a substrate for the second step but not the limiting reagent, won't there be some amount of this intermediate left in the very end? I'm having difficulty with this because intermediates, by definition, don't appear in the net balanced equation and should not be present at the end of a reaction.
Also, does complete dissociation indicate complete solubility? For example, are strong acids and bases also completely soluble in water?
And what kind of bond exists between metal hydroxides? An ionic or coordinate covalent bond? How can you distinguish between the two types of bonds when given a compound in general?
Thank you very much!
So let's think about this. If there are two steps in a reaction mechanism, say 2NO2--> NO3 + NO. Then, NO3 + CO---> NO2 + CO2. In this reaction, NO3 is the intermediate substrate for the second reaction. If you are given 0.5 mol of CO and 1 mol of NO3, then yes you would have excess NO3. Just like any other reaction, the limiting reagent is used up completely. Now, look at the equation for the formation of the NO2 and CO2. You start out with 2 mol NO2 and it forms NO3 and NO. Then NO3 reacts with CO to form NO2 and CO2. Because NO3 appears in a product and reactant side, it does not appear in the net equation. This is a convention when you are trying to understand the overall picture (net). So you put all of your reactants together and all of products together and cancel what appears on both sides. 2NO2 +NO3 +CO---> NO3 +NO +NO2 +CO2. Since NO3 appears on both sides of the equation, it will cancel. Also, note that NO2 appears twice on reactants and once on products, meaning 1 mol NO2 cancels. Thus, the net equation is simply NO2 +CO---> NO +CO2. This makes sense and is indeed the "net reaction." However, the net equation says nothing about the intermediates that are formed. For instance, looking at the net equation above suggests that CO abstracts oxygen from NO2 to yield CO2 and NO. However, the mechanism shows that NO3 and NO first form, and CO abstracts oxygen from NO3, not NO2. The net equation is simply a way to see the big picture of what you started with, and what you ultimately end up with.
B. Complete dissociation indicates complete solubility in solution. Strong acids have larger dissociation constants than weak acids. Consider the reaction: HA-->H + A (cation and anion). Since Ka is a direct measure of [Products]/[Reactants], and Ka is large, then the denominator must be small in comparison to the numerator. (for example: 100/1 > 90/10 ) This means that the amount of products is large compared to the reactants, so H (cation) and A (anion) are present in a lot higher concentration than HA. In strong acids, this dissociation essentially goes to completion, and the concentration of reactants goes to zero as the concentration of product increases to the original concentration of reactant. (ice table type stuff) So.... all this means that strong acids completely dissociate in water and are completely soluble. However, often times, solubility is discussed more in terms of precipitation reactions than acid base equilibria.
C. So, ionic bonds are electronic attractions that occur between atoms of opposite charge ( Coloumb's Force etc etc). Ionic bonds can occur between metals and hydroxides. For example, Na+ OH- bind together ionically. They are NOT sharing electrons. They bind together due to the force defined as F=kq1q2/r^2. Coordinate covalent bonds occur between lewis acids and bases. In general, if you have some metal hydroxide in which the metal is a transition metal and contains empty d-orbitals, then it can act as a lewis acid and accept an electron pair from an electron donor. For example, Fe3+ has empty d orbitals. OH- has a lone pair electrons on the oxygen atom. 3 OH- can COORDINATE to the lewis acid and donate electrons forming the Fe(OH)3 complex. Similar reactions coordinations take place in grignard reactions with carbonyl compounds.
Hope this helps.
