[galpinj]

I was watching some videos on the lemon battery, and found an incongruity that I haven't been able to solve.

In the lemon battery, a zinc anode is connected by a wire to a copper cathode. Zince oxidizes and the two electrons travel over the wire to the copper cathode where they interact with the electrolyte  (H⁺ ions in the acidic lemon solution) to produce hydrogen gas (H₂). Given that the oxidation of zinc is spontaneous at standard conditions, and that the reduction potential of H⁺ is zero, why do we need a copper cathode at all? Why doesn't the redox reaction occur directly between the zinc anode and the H⁺ that surrounds it? I see no reason why electrons need travel to the copper cathode, especially when it plays no part in the overall redox reaction.

One final, slightly related question. In other, more formal, galvanic cells, the two half-cells are often connected by a salt bridge. Some sources I have looked at mention that the ions  in solution can move up the salt bridge and into the other half-cell; I have always been under the assumption that ions in the salt bridge themselves moves to remove polarity and that they interact with the ionic solutions of each half cell, but the ions in the half cells don't themselves move up into the salt-bridge and across. For example, in the following galvanic cell, Zn(s) l ZnSO₄ ll CuSO₄ l Cu(s) (with KCl salt bridge), would Zn²⁺ move across the salt bridge and into the CuSO₄ half cell? Likewise, would SO₄^2- move across the salt bridge and into the ZnSO₄ solution? I contend that only the K⁺ and Cl^- ions will move in this galvanic cell, as these ions will interact and remove the growing charge before Zn²⁺ or SO₄^2- can make it across.

I'm curious to know what everyone thinks; all input is highly appreciated

[AWK]

Juice vesicles of the endocarp work as salt bridge.

[Enthalpy]

Given that the oxidation of zinc is spontaneous at standard conditions [...]

Is it? At least under rain, zinc resists corrosion, that's why steel gets plated with zinc as a protection.

And even if the acid corrodes zinc relentlessly, this wouldn't prevent current from circulating through the outside circuit. Some self-discharge occurs at any battery, just slower at the good ones, and batteries give a useful current while self-discharge occurs too.

Many battery compositions are known to give a lower voltage when the user begins to draw current, and the voltage rises again once some passivation layer is spoiled enough.

[Arkcon]

Juice vesicles of the endocarp work as salt bridge.

Also,not every battery has a salt bridge:  http://www.chemicalforums.com/index.php?topic=84354.msg305697#msg305697

[mjc123]

Quote from: galpinj on Yesterday at 01:21:20 PM

Given that the oxidation of zinc is spontaneous at standard conditions [...]



Is it? At least under rain, zinc resists corrosion, that's why steel gets plated with zinc as a protection.

But if "standard conditions" means 1M H⁺...

[Borek]

Given that the oxidation of zinc is spontaneous at standard conditions [...]

Is it? At least under rain, zinc resists corrosion, that's why steel gets plated with zinc as a protection.

That's not entirely correct. As far as I remember and understand the situation, Zn coat doesn't stop the corrosion - it slows the corrosion of the steel down, with Zn acting as a sacrificial anode.

[Burner]

why do we need a copper cathode at all?

One of the reasons is to produce current(Can you power up anything by connecting it to a piece of zinc dipped in acid?). The lemon battery have no use if the e^- are directly transferred from Zn to H⁺ as no electrons pass through the external circuit and the load.

One final, slightly related question. In other, more formal, galvanic cells, the two half-cells are often connected by a salt bridge. [...]

As Arckon said, not all battery has a salt bridge. See the attached image below: Two metal electrodes are connected by wires and be dipped into a beaker of electrolyte, and a simple chemical cell is formed. This kind of battery have lots of disadvantages, for example the electrodes will be used up and gradually dissolves, and the direct reaction between electrolytes and electrodes, but it can produce electricity for some time.

Given that the oxidation of zinc is spontaneous at standard conditions [...]

Is it? At least under rain, zinc resists corrosion, that's why steel gets plated with zinc as a protection.

That's not entirely correct. As far as I remember and understand the situation, Zn coat doesn't stop the corrosion - it slows the corrosion of the steel down, with Zn acting as a sacrificial anode.

https://en.wikipedia.org/wiki/Plating#Zinc_plating

[Arkcon]

Why doesn't the redox reaction occur directly between the zinc anode and the H+ that surrounds it? I see no reason why electrons need travel to the copper cathode, especially when it plays no part in the overall redox reaction.

I'm sorry I didn't see this sooner, the "wall of text" can hide things when its asked as a question.  Copper is most definitely part of the redox reaction -- that's the whole name: reduction:oxidation.  Something is oxidized, and something is reduced.  You can get a red-ox with a zinc bar in a solution of copper sulfate.  Check your text book.

[P]

the endocarp

Is that a fish out of water or maybe the extinction of a fresh water fish? :-D

[Enthalpy]

[...]As far as I remember and understand the situation, Zn coat doesn't stop the corrosion - it slows the corrosion of the steel down, with Zn acting as a sacrificial anode.

I've read both explanations, even within the same text. Given that the usual zinc layer is very thin, much thinner for instance than the amount of iron corroded away in 20+ years, I consider that the zinc oxide layer is the most efficient protection mechanism.

Some books add that zinc acts additionally as a sacrificial anode where scratches expose the bare iron - I'm pleased with that.

[galpinj]

Why doesn't the redox reaction occur directly between the zinc anode and the H+ that surrounds it? I see no reason why electrons need travel to the copper cathode, especially when it plays no part in the overall redox reaction.

I'm sorry I didn't see this sooner, the "wall of text" can hide things when its asked as a question.  Copper is most definitely part of the redox reaction -- that's the whole name: reduction:oxidation.  Something is oxidized, and something is reduced.  You can get a red-ox with a zinc bar in a solution of copper sulfate.  Check your text book.

But isn't the reduction and oxidation reaction taking place between zince (loses electrons) and H⁺ in the lemon juice (gains electrons). The copper is neither gaining nor losing electrons, so I don't see why electrons would even pass through the wire to the copper cathode when they could simply pass to the hydronium ions in the surrounding liquid.

[Arkcon]

That not what I was taught in college.  What would be the reaction between a zinc bar and copper sulfate solution?  A similar pair of reactions occurs between the two cells connected by a salt bridge and a conducting wire.

[Arkcon]

Consider thus experiment: Do two zinc wires make a current in a lemon?  Or do you need two electrodes of dissimilar metals and why?

[galpinj]

That not what I was taught in college.  What would be the reaction between a zinc bar and copper sulfate solution?  A similar pair of reactions occurs between the two cells connected by a salt bridge and a conducting wire.

https://www.youtube.com/watch?v=GhbuhT1GDpI

In this video Hank Green states that it is the hydrogen gas and zinc that are reacting together, and that the copper plate is not reacting at all. I believe that in a CuSO₄ solution, the redox reaction is taking place between zinc and copper (Cu²⁺), which would be spontaneous; however, in the lemon battery there is no Cu²⁺ in the lemon, only hydronium ions. As such, I can't understand the value of the copper plate in our lemon battery experiment.

[Arkcon]

I'm sorry, my current internet pipe is too narrow for YouTube.  Please write out the zinc hydrogen reaction, in chemical shorthand, with products, like you'd find in a text book.