# Conjugate Theory and Redox

I'm trying to apply conjugate theory, which I can apply very well to acids/bases, to redox. Can you verify my logic, which I have broken down below?

1) I know that solid sodium is a strong reducer.

2) I know this because solid sodium when placed in water reacts violently, forming, among other products, sodium ion.

3) Reducers are oxidized. Reducers lose electrons. Correct; $\ce{Na^{+}}$ is the product of placing solid sodium in water.

4) So in this half-reaction: $\ce{Na<=>Na^{+} +e^-}$, sodium is the reducer. $\ce{Na^{+}}$ is the conjugate oxidizer.

5) Oxidizers are reduced. Oxidizers gain electrons.

6) Because sodium is a strong reducer, which means it has a strong tendency to be oxidized, or a strong tendency to lose electrons, its conjugate oxidizer must be weak. Oxidizers are reduced; oxidizers gain electrons. The potential for $\ce{Na^{+}}$ to gain the electron that $\ce{Na}$ just lost must be small, or $\ce{Na}$ would not be a good reducer.

7) Therefore, a strong reducer's conjugate oxidizer must be weak.

8) Likewise, a strong oxidizer's conjugate reducer must be weak.

9) Likewise, a weak reducer's conjugate oxidizer must be strong.

10) Likewise, a weak oxidizer's conjugate reducer must be strong.

Also, if anyone could point me to a good reference on gaining insight into redox, tha would be great!

And while we're on the topic of redox, what's the mechanism for this reaction? Are both bonds cleaved homolytically? I see how the nucleophilic chlorines attack the electrophilic hydrogens but how's the bond broken?

$\ce{H_2 + Cl_2 ->2HCl}$

## 1 Answer

Your question covers two different topics:

• Redox properties can be well described with redox potentials attributed to a given reaction. It is essentially the same concept that you are talking about: we assign a number (conveniently it is an actual measurable potential) to an electrode reaction which includes an oxidized and reduced form of the species we are talking about. This number, the redox potential, can show you how strong oxidizer/reducer is the corresponding corresponding form, and just as you guess, strong oxidizers generally have a weak reducer pair.

• The $\ce{HCl}$ formation reaction is generally a radical chain reaction, triggered e.g. by light, and not a redox reaction. The key here is to split the $\ce{Cl2}$ (e.g photo-chemically), then the obtained free radicals induce a chain-reaction with elementary steps like $\ce{Cl + H2 -> HCl + H}$ or $\ce{H + Cl2 -> HCl + Cl}$ The two gas mixed is more or less inert in total darkness.

• I didn't know that about the HCl reaction. It was a picture in the redox chapter of an introductory level text I consulted. Thank you though for clarifying. Also by radical formation I assume that you mean the Cl2 must be split homolytically then! – Dissenter Jun 4 '14 at 3:59
• The formation of Hydrogenchloride from the elements is still a redox reaction, even if its mechanism is a radical one. In one of the reactions $$\ce{H. +Cl2 <=> [HCl2]. <=> HCl + Cl.}$$ The oxidation state of hydrogen is increased while the oxidation state of chlorine is decreased. – Martin - マーチン Jun 4 '14 at 4:05
• You are right. Thank you for the clarification and cleaning up the answer! – Greg Jun 5 '14 at 5:28