Posted by Kelly on Monday, May 31, 2010 at 3:24pm.
MnO4- +5e -> Mn2+}*2
C+1 -2e -> C+3 }*5
2MnO4- + 5C6H7O +6H+ -> 2Mn2+ + 5C6H7O2 + 3H2O
Step 1: realize the oxidation of element: (oxidation of H and O not change)
write half reaction
equalize the number of election yieled and accepted by the coefficient ( here e yield is 2 and e accepted is 5)
let the coefficient into the equation
check the equation: if the side of equation has more O: add H+ and the H2O to the other side
the last step: check the charge of both side, if wrong check every step you did again.
for your equation, becaue the carbons on the ring do not change their oxidation state, only carbon of carbonyl group change because aldehyde group is converted into carboxyl group
I think you have to review the following conceps below :
In free elements (that is, in uncombined state), each atom has an oxidation number of zero. Ex. In O2, the oxidation number of each oxygen atom is zero.
2. For ions composed of only one atom, the oxidation number is equal to the charge on the ion. Ex. The oxidation number of Ca2+ is +2.
3. All alkali metals (elements in column 1of the periodic table, with the exception of hydrogen) have an oxidation number of +1. Ex. The oxidation numbers of Li, K, and Na will always be +1.
4. All alkaline earth metals (elements in column 2 of the periodic table) have an oxidation number of +2. Ex. The oxidation number of Ba is +2.
5. The oxidation number of Aluminum (Al) is always +3.
6. The oxidation number of oxygen in most compounds (such as H2O and CO2) is -2. In hydrogen peroxide (H2O2) and peroxide (O22-) oxygen shows a -1 oxidation number.
7. The oxidation number of hydrogen is +1, except when in is bonded to a metal as a negative ion, in which case it is -1. Ex. H2O shows hydrogen as +1. NaH shows hydrogen as -1.
8. When halogens (elements in column 17 on the periodic table) form negative ions, they will have an oxidation number of -1. Ex. NaCl and CaCl2 both show chlorine with a -1 oxidation number.
9. In a neutral molecule, the sum of the oxidation numbers of all of the atoms must be zero. Ex. In H2O, each hydrogen is +1 and the oxygen is -2. So, (2 x +1) + (-2) = 0.
10. In a polyatomic ion, the sum of oxidation numbers of all the elements in the ion must be equal to the net charge of the ion. Ex. In the polyatomic ion known as hydroxide (OH-), the oxygen is -2 and the hydrogen is +1. So, (-2) + (+1) = -1, the same as the charge on the hydroxide ion (OH-)
good luck
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