F.5 CHEM REDOX

group I metal ions almost can NEVER be the oxidizing agent. forget about it.

nitrate cannot be oxidizing agent unless there's (concentrated) strong acid.
see the equation:
NO3(-) + H(+) + e- ------> NO2 + H2O
NO3(-) + 3H(+) + 3e- ------> NO + 2H2O

so it's apparent. iron(III) is the oxidizing agent.


always be alert to commonly-redox involved ions, like bromide, iodide, sulphite and iron(III).
but also be alert that existence of them doesn't necessarily mean there's redox. they can be involved in other reactions.
bromide, iodide ----> silver ions
sulphite ----> acids
iron(III) ----> hydroxide ions


2011-01-06 09:34:59 補充：
同樣地, 除非是直接並單獨使用濃硫酸, 否則都不會行
2H2SO4 + 2e(-) ------> SO4(2-) + SO2 + 2H2O
這條路.

2011-01-06 09:45:13 補充：
至於Fe(II) 是RA 還是OA, 這在題見中很容易看出來:
Fe(II) 極少做OA, 除非是那些metal displacement reaction, 即遇著Zn, Mg, Ca 之類金屬;
然後確定反應中沒有OH(-), 不是precipitation,
就有99% 機會是RA 了.

2011-01-06 09:45:23 補充：
另外, Fe(II) 不會做disproportionation: 3Fe(2+) ------> Fe + 2Fe(3+)
反而有機會做coproportionation: Fe + 2Fe(3+) ------> 3Fe(2+)

而MnO4(-) 十成十是OA, 課程中似乎沒有提過其他反應.

2011-01-06 09:49:17 補充：
WInG, 你打錯字?
MnO4(-) + 8H(+) + 5e(-) ------> Mn(2+) + 4H2O
Mn 由+7 跌落+2, 佢先係gain electron;

Fe(2+) ------> Fe(3+) + e(-)
Fe 由+2 升上+3, 佢先係lose electron.

RA=reducing agent OA= Oxidizing agent






其實一開始我都覺得好難, 不過做得多就OK.


First of all, METAL 係會比COVALENT COMPOUND OR 其實NONMETAL 更容易FORM CATION, 咁METAL FORM CATION就係要放電子 由此METAL 好多時候一定係REDUCING AGENT 姐係行OXIDATION (E.G)



Mg---->Mg2+ + 2e-



其實你只要記熟ELECTROCHEMICAL SERIES 最勁個5種的RA 同OA就OK 當你一見到Mn04 同Cr207就一定佢地係OXIDIZING AGENT 因為佢地有COLOR CHANGE, 老師好鐘意問呢樣野



仲有, 你果條FeSO4 +KMnO4- 咁你一睇到有MN04 你就先知道佢一定係OA THEN RA 就一定係另外個COMPOUND 到(UNLESS DISPORPORTION) 咁另外個COMPOUND冰個係RA? 你知道既然MN04係LOSE E- 咁另外一個一定要係GAIN E- LA= = 咁GAIN E- 味一定係FE2+ LO 無錯, FE, FE2+ 同FE3+ D TEACHER好鐘意考 因為佢可以做冰樣都得 不過其實都唔難分 如果個題目你斷定左係FE 同埋佢果D FD有可能係RA OR OA 的話 你就要睇下佢係METAL 定CATION, 如果一開始咁ON居 俾個FE你 咁你實知佢一定係RA用黎 LOSE E-LA 而如果係FE3+ 就一定係OA 因為FE3+如果就LOSE ELECTRON 就會去左FE4+(無可能)到FE2+, 點分呢就要睇你另外一個COMPOUND 如果你另外一個COMPPUND係OA 好似你果條咁 咁FE2+就會係RA LA IN MY OPINION, FE2+千奇唔好一開始就下定論 睇左另外果個先 先決定佢係咩野



仲有一定要記好ELECTROCHEMICAL SERIES we only use stronger oxidizing agent and REDUCING AGENT to write our equation











希望幫到你:)










WInG