Question

Can someone explain the anticlockwise rule for electrode potentials?

All I find online are people asking about the anticlockwise rules in terms of questions they get.

Answer 1

See below.

Explanation:

The anticlockwise rule is a quick way to determine whether a redox reaction will occur between two species.

Here's how it works.

Example

Will metallic `"Ag"` react with an aqueous solution of `"Cu"^"2+"`?

Solution

Step 1. Write the standard reduction half-reactions for `"Ag"` and `"Cu"`

Make sure that you write the half-reaction with the more negative ( less positive) potential first. This gives

`"Cu"^"2+""(aq)" + 2"e"^"-" ⇌ "Cu(s)"; E^@ = "+0.34 V"`
`"Ag"^"+""(aq)" + "e"^"-" ⇌ "Ag(s)"; color(white)(m)E^@ = "+0.80 V"`

Step 2. Use the anticlockwise rule

Draw anticlockwise curved arrows (the blue arrows in the diagram below) anticlockwise above and below your equations.

The `bb(color(blue)("anticlockwise rule")` states that the half-reactions that will occur are those that follow anticlockwise paths.

Step 3. Determine the equation for the reaction that will occur

We must reverse the top equation.

`"Cu(s)" ⇌ "Cu"^"2+""(aq)" + 2"e"^"-" ;color(white)(mmmmmml) E^@ =color(white)(l) "-0.34 V"`
`ul("2Ag"^"+""(aq)" + 2"e"^"-" ⇌ "2Ag(s)";color(white)(mmmmmll) E^@ = "+0.80 V")`
`"Cu(s) + 2Ag"^"+" ⇌ "Cu"^"2+""(aq)" + "2Ag(s)";color(white)(l) E^@ = "+0.46 V"`

The rule predicts that `"Cu"` will react with `"Ag"^"+""` to form `"Ag"` and `"Cu"^"2+"`.

The other conclusion is that `"Ag"` will not react with `"Cu"^"2+"`.