The problem on my test was to balance a reaction using the half reaction method. The formula is H3AsO4 + Zn –> AsH3 + Zn+2 . I put the oxidation number of H as -1 because it is bonded exclusively to a metal (As). So that makes the oxidation number of As +3 correct? The teacher put it wrong and corrected it to an oxidation number of -3 for As. Who is right?
3 Answers
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If you remember which is more electronegative, then it is easy. Since As is more electronegative than H, thereforre we cannot put ON of H as negative and of As as positive. However, if you do not remember which is more electronegative, then read the following:
H has oxidation number -1 when it combines with metal provided the metal is not a transition element. As is a transition element.It can have a range of oxidation numbers. Therefore its oxidation number should be calculated from the oxidation numbers of other elements in a compound.
In H3AsO4, put oxidation number of H = +1, of O = -2. Then you will find that the O.N. of As = +5
In AsH3, put O.N. of H=+1. Then O.N. of As = -3
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SO4 is a polyatomic ion, meaning entire all of those atoms have a cost at the same time as you positioned them jointly. SO4 has a cost of -2 and you have have been given 3 of them, so the traditional fee is -6. even inspite of the undeniable fact that the total fee of the molecule Fe2(SO4)3 is 0, meaning that the two irons (Fe) cancel it out with an excellent 6 fee. So if 2 irons have a confident 6 cost, then one ought to have an excellent 3 fee. this suggests it incredibly is iron(III) (i.E., the oxidation quantity is 3). So the compound may be referred to as ferric sulfate.
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According to one electronegativity scale, the electronegativity of As is 2.18 and H is 2.20. The more electronegative element is assigned the negative oxidation number.
See the electronegativity scale at: http://en.wikipedia.org/wiki/Electronegativity
