Some questions

[Bill Nye Guy]

Hey all, my chem. teacher decided to give us some advanced questions for us to think about for next class period. We havent talked about them yet and i have had trouble with getting the answers. Any help on the following questions would be great.

 

A.) Assign oxidation numbers to all atoms in the following compounds.

a.) (NH4)HPO4

b.) CO

c.) XeOF4

 

B.) Specify which of the following are oxidation-reduction reactions and identify the reducing agent, the substance that is oxidized, the oxidizing agent and the substance being reduced.

a.) O3 (g) + NO(g) --> O2(g) + NO2(g)

b.) Cr2O72-(aq) + 2OH-(ag) --> 2CrO42-(aq) + H2O(l)

c.) 2CuCl(aq)--> CuCl2(aq) + Cu(s)

[woelen]

Question (A). Some elements have well-known oxidation numbers in their compounds. Oxygen usually has oxidation number -2 in its compounds, except for peroxides and superoxides. Fluorine ALWAYS has oxidation number -1, chlorine always has oxidation state -1, except in some chlorine/oxygen-compounds and chlorine/fluorine-compounds. Hydrogen almost always has oxidation number +1, except in some metallic hydrides.

In their elemental form, all elements have oxidation state 0.

 

Now we are going to solving (A). The first compound is (NH4)2HPO4. This is an ionic compound, consisting of ions NH4(+) and HPO4(2-). I do the trick for NH4(+), then you should be able to do for HPO4(2-). For NH4(+), total charge is +1.

 

Now we have ox(N) + 4*ox(H) = +1, here ox(X) is the oxidation number of element X.

ox(H) = +1 ===> ox(N) = -3.

Write down the equation for ox(P), ox(O), ox(H) and determine ox(P).

 

For compound (b) it is really simply:

 

ox© + ox(O) = 0 (we have a molecule, not an ion, so total charge equals 0).

 

For compound © it is not much more difficult than for compound (b)

 

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Now for question (B). Again, using the well-known values for O, H, etc. determine the ox(X) for the element N in equation (a), for the element Cr in equation (2) and for element Cu in equation (3).

 

Only for the reactions, where oxidation states are different, one has a redox reaction. E.g. for HCl + NaOH --> H2O + NaCl, you see that it is not a redox reaction. Oxidation states are the same left and right of the arrow. E.g. for 2HCl + O3 ---> H2O + Cl2 + O2 one has a redox reaction. Chlorine is going from -1 to 0 and one of the oxygen atoms is going from 0 to -2.

 

Keep in mind, some compounds can at the same time be oxidizer and reductor. This may seem very surprising, but it is not uncommon.