chem不明(redox reaction)

Thehalf equation when dilute nitric acid is used as an oxidizing agent:
4H^+(aq) + NO3^-(aq) + 3e^- → NO(g)+ 2H2O(l)

The 3e^- in the equation can be explained in either of the following two ways:

The first way:
There are 4 H, 1 N and 3 O on both sides of the equation, and the materials arebalanced. However, if the "3e^-" is absent, the charges are notbalanced as there are totally 3 positive charges (+4 -1 = +3) on the left handside, while the right hand side is neutral. Therefore, "3e^-" isadded to the left hand side in order to make both sides to be neutral, and the charges are said to bebalanced.

The second way:
The oxidation number of N decreases from +5 (in NO3^-) to +2 (inNO). The decrease in oxidation number is 3. 3 negatively-charged electrons canfacilitate such decrease in oxidation number.

可能你未學redox half equation 所以唔明

等我以最簡單講

NO3- 會變NO(g) 係 reduction (唔明咩黎唔緊要住)

最重要係

NO3- ----------------> NO
不平衡,有冇留意左邊3個O, 右邊1個O
所以要加2個O係右邊,會用H2O 唔係用O2!

NO3- ----------------> NO +2H2O
跟住有右邊有4個H, 左邊0個, 再平衡

NO3- + 4H+ --------------------------> NO +2H2O

你問既問題呢家解
因為右邊你見唔到正OR負, 即0
左邊你會見到 一個負, 4個正, 即 +3

由於要平衡所以要加3e-

then 1+4-3 =0
左右相同

2011-06-07 15:07:49 補充：
--------------------------------

4H+ AND NO3- 加在一起不是1+,
然後要加e1- 去平衡嗎???
------------------------------
NO3- + 4H+ --------------------------> NO +2H2O

個NO3- 係負一, 因為如果你記得NO3係ION 係負一既
所以係1+4-3 =0
只睇ion前既數 e.g. 4H+ = 4個+ ; NO3- =1個-
所以係加3e-
---------------------------------------
希望解答到你

An oxidizing agent undergoes reduction in which the oxidizing agent gains electrons.

Oxidation number of O in NO3^- = -2
So by setting up equation, N + 3(-2) = -1
=> Oxidation number of N = +5
Oxidation number of N in NO = +2
Since this is a reduction reaction, one mole of NO3^- tends to lose 3 moles of electrons to produce one mole of NO gas.

2011-05-31 22:18:37 補充：
sorry for my typo, you're right. It should gain electrons instead

As you say, this is one HALF equation from a redox reaction.

This half equation only show the reduction part of the redox reaction.

If you write down the other part of the reaction(oxidation part), then you will soon find out where the electrons come from.