Qxi.xli
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why is the halide ions easier to discharge than hydroxide ions, and hydroxide easier than all other negative ions?

if someone could briefly explain this to me would be greatly appreciated, I'm recapping it and am getting confused...thanks!!
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charco
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(Original post by vix.xvi)
why is the halide ions easier to discharge than hydroxide ions, and hydroxide easier than all other negative ions?

if someone could briefly explain this to me would be greatly appreciated, I'm recapping it and am getting confused...thanks!!
These are reactions just like any other and, as such, are governed by the laws of thermodynamics and kinetics.

The overall driving force behind every reaction is its effect on the universal entropy. However, a full explanation is probably too complicated for your level, so suffice to say the reaction:

2H+(aq) + 2e ==> H2(g)

is thermodynamically (energetically) more favourable (easier) than:

4OH-(aq) - 4e ==> 2H2O(l) + O2(g)

This reaction, in turn is easier than:

2Cl-(aq) - 2e ==> Cl2(g)

Wait a minute (I hear you cry), I thought that the last two were the other way round?

This is where kinetics come into play.

The actual concentration of hydroxide ions in an aqueous solution is very low (about 1 x 10-7 mol dm-3), whereas in a solution containing chloride ions the concentration is usually much higher. This tips the balance in favour of the chlroride reaction.

If you dilute the chloride solution down below about 0.01 mol dm-3 then oxygen gas will be preferentially released.

There is no clear cut-off, rather the chloride reaction rate steadily decreases as the solution concentration decreases. This is what we would expect with kinetics (rate of reaction).
Last edited by charco; 9 months ago
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Qxi.xli
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Report Thread starter 9 months ago
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(Original post by charco)
These are reactions just like any other and, as such, are governed by the laws of thermodynamics and kinetics.

The overall driving force behind every reaction is its effect on the universal entropy. However, a full explanation is probably too complicated for your level, so suffice to say the reaction:

2H+(aq) + 2e ==> H2(g)

is thermodynamically (energetically) more favourable (easier) than:

4OH-(aq) - 4e ==> 2H2O(l) + O2(g)

This reaction, in turn is easier than:

2Cl-(aq) - 2e ==> Cl2(g)

Wait a minute (I hear you cry), I thought that the last two were the other way round?

This is where kinetics come into play.

The actual concentration of hydroxide ions in an aqueous solution is very low (about 1 x 10-7 mol dm-3), whereas in a solution containing chloride ions the concentration is usually much higher. This tips the balance in favour of the chlroride reaction.

If you dilute the chloride solution down below about 0.01 mol dm-3 then oxygen gas will be preferentially released.

There is no clear cut-off, rather the chloride reaction rate steadily decreases as the solution concentration decreases. This is what we would expect with kinetics (rate of reaction).
ahh kk thanks sooo much!! i appreciate it
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