How can reduction and oxidation occur separately in batteries if they're meant to be one reaction?

Question

According to this page along with many others, oxidation and reduction must occur simultaneously. However, in a battery, it seems like they actually occur separately. Oxidation must first occur to provide the electrons for reduction, and since these electrons cannot pass through the electrolyte barrier they have to go through the circuit, which takes time. This not only staggers the oxidation and reduction reactions, but it also isolates the electrons being transferred, which should be impossible if reduction and oxidation can only happen together. LibreTexts also states that "oxidation and reduction always occur together; it is only mentally that we can separate them", but is that not what is happening in a battery? They are not only physically separated but also it is as if each reaction is its own half-reaction, with electrons being isolated as a product and reactant.

Answer 1

[OP] According to this page along with many others, oxidation and reduction must occur simultaneously. However, in a battery, it seems like they actually occur separately

Simultaneously means at the same time. Separately could mean independently (not the case here) or in different locations (that's the appropriate interpretation).

[OP] Oxidation must first occur to provide the electrons for reduction, and since these electrons cannot pass through the electrolyte barrier they have to go through the circuit, which takes time. This not only staggers the oxidation and reduction reactions, but it also isolates the electrons being transferred, which should be impossible if reduction and oxidation can only happen together.

This is an inaccurate description. The electrons don't have to travel all the way, the wire is filled with electrons already (using a classical view of electrons). On a particular level, oxidation and reduction can occur independently (in fact, if you just have a half-cell, there will be oxidation and reduction events, but they don't result in a net reaction). At a bulk level, they are coordinated and occur virtually simultaneously.

Oxidation or reduction can occur "first". For oxidation, extra electrons can go into the electrode and circuit. For reduction, needed electrons can be taken from the electrode and circuit. Both just react to a certain point because the charge buildup encourages the reverse reactions.

[OP] They are not only physically separated but also it is as if each reaction is its own half-reaction, with electrons being isolated as a product and reactant.

The electrons are the key here. You can't run bulk half reactions because there is no source or sink for the electrons unless you pair oxidation half reaction with reduction half reaction. It's like trying to sell your cup cakes at a bake sale when there are no buyers, or trying to eat cup cakes when there is no one to bake them.

Answer 2

This will probably be too short of an answer, but the ones you have got are good though. However, this is fundamentally true:

IF something is oxidized... something else must be reduced! When a redox reaction occurs, you always end up with one reduced species and one oxidized species. Always. It is true tautologically and physically. Anything else will be absurd.

Now, the confusing bit might be to identify this in battery half cells - but that electron didn't get magically created. It existed already, and it came from somewhere. One suggestion would be that it came from the valence band of your conductor, via the electrode. Taking this electron increased the formal charge of the electron sea by exactly (+I).

Answer 3

Taking electrons away is oxidation. Providing electrons is reduction. As both happen at different places, oxidation and reduction occur separately.

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The general requirement of simultaneous (and often implied localized) oxidation and reduction is based on the principle of no accumulation of excess or deficit of electrons. But this can be managed on different places as well, if charge exchange is managed by other means, like by wire in context of electrochemical cells and circuits.

For the example of the Daniell cell $\ce{Zn|ZnSO4||CuSO4|Cu}$, it can be separated in 3 processes, each being a very formal redox reaction:

\begin{align} \ce{Zn + anode &-> Zn^2+ + anode^2-}\\ \ce{Cu^2+ + cathode &-> Cu + cathode^2+}\\ \ce{anode^2- + cathode^2+ &->[via the wire] anode + cathode} \end{align}

One can this way

$$\ce{red1 + ox2 -> ox1 + red2}$$

avoid formal half-reactions like

\begin{align} \ce{red1 &->[anode] ox1 + n e-}\\ \ce{ox2 + n e- &->[cathode] red2} \end{align}

that are generally more illustrative, unless one has any understanding problem with them.

Answer 4

A balanced chemical or physical process requires conservation of Mass-Energy and Momentum. The balanced chemical equation must demonstrate that. The chemical equation is a STATE FUNCTION; it does not depend on the pathway to get there; the path is the job of nature or the chemist.

An example, lets make 1gram mole of sodium chloride:

Method 1: vaporize the sodium detach the electrons with an electric field and pass them into some chlorine combine the ions.

Method 2: Mix finely divided sodium with chlorine and stand back.

Method 3: Evaporate enough ocean water and recrystallize the salt until enough pure NaCl is isolated.

Method 4: Neutralize appropriate amounts of sodium hydroxide with hydrochloric acid and remove the water.

Each process affords the same result: Na + Cl = NaCl. Each step of the mechanism has its own time constant ranging from attoseconds to millennia [There probably are electrons from the original big Bang still looking for their positrons or protons]. Chemistry tries to make it simple and says that the net equation is equal amounts of oxidation and reduction; Simultaneous? no!

Is it important to know the individual steps and the mechanisms? at a simple level it is enough to know they exist. As a chemist the more one knows or thinks about the better; at least one knows to ask for help.