I've always hated redox reactions and electro chemistry... It's really not that difficult – at least not more difficult than anything else within the field of chemistry – but it's just so... messy, so much to take into account. And if you forget just one detail, everything falls into bits and pieces. Anyhow, I plan to write some sort of summary or whatever about it in the following days. This is part one, starting with the basics.
What is electricity?
Electrons passing from one metal to another. Thus electrictity may be used in order to make chemical reactions occur or the other way around : we can use chemical reactions to produce electricity!
Electrochemistry can also be used to meassure pKa-values, actitvity in the brain, etc...
What is the definition of an oxidation?
The loss of electrons!
Let's consider the following two reactions :
2Mg(s) + O2(g) → 2MgO(s) The solid MgO consists of two ions; 2Mg2+(s) + 2O2- (s)
The reaction above describes the transfer of electrons from Mg to O. Mg loses electrons that go to oxygen instead. When Mg(s) looses its electrons it is transformed to Mg2+ ions, and these ions start a relationship with the 2O2- ions. Remember that opposites attracts! The resulting solid is a salt.
The reaction below is essentially the same, Mg loses electrons, but to Cl this time. Still, it is called “oxidation” since electrons are lost, even though no oxygen is involved.
2Mg(s) + Cl2(g) → 2MgCl2(s) The solid MgCl2 consists of two ions; 2Mg2+(s) + 2Cl-(s)
How can you determine wether an element has undergone oxidation or not?
Its charge is increased. This rule does often, but not always, apply. It can be used on anions as well as on uncharged speices. Consider this reaction :
2 NaBr(s) + 2Cl2(g) →2NaCl + Br2(l)
Br is oxidized by Cl in this case. One might say that Cl breaks up the marriage between Na and Br, since Na is more attracted to Cl. That's how I understand things. (My chemistry professor would kill me...) The charge in Br thus goes from (-1) to (0). I will go through the basic rules of “oxidation numbers” or “oxidation states” further on.
Define “reduction”!
This refers to the opposite of an oxidation; if an atom is reduced it gains electrons. In the case above, 2 NaBr(s) + 2Cl2(g) →2NaCl + Br2(l), Cl is reduced, because its chage goes from (0) to (-1). It takes electrons! The same rule applies to oxygen in the MgO formation described above. This touches on something rather important; electrons cannot just get lost through oxidation, they are in fact particles that go somewhere. So : whenever oxidation occur, reduction will occur.
Identify the spices that have been oxidized and reduced in the following reaction!
3 Ag+(aq) + Al(s) → 3Ag(s) + Al3+(aq)
Ag+ gets reduced. Its charge goes from (1+) to (0), it gains electrons that counteracts the positive charge. Al gets oxidized. It loses electrons and goes from (0) to (+3).
Identify the species that have been oxidized or reduced in the following reaction!
2 Cu+ (aq) + I2(s) → 2 Cu2+ + 2I-
Well, the charge of Cu+ increases from (1+) to (2+), so it is oxidized! The charge of I2(s) is lowered from (0) to (-1) so it is reduced.
What is oxidation number, what is oxidation state, what's the difference?
These terms are often used in the same context. But to be precise (or nerdy, rather) – an oxidation number is given to a spieces, depending on wether it is in a molecule or not, what kind of molecule it is, etc. The oxidation number corresponds to a condition, denoted by oxidation state.
So Mg2+ has the oxidation number (2+) which corresponds to the oxidation state (2+).
In short : no real difference, don't worry your pretty little head.
Eum that's all I have (made...) time for today. More to come. Hope this helps anyone.
Inga kommentarer:
Skicka en kommentar