When balancing a half reaction by adding water and oxonium ions or hydroxide ions, how do we know that the balanced half reaction will occur?

Question

Let's take an example. Balance the following redox reaction under acidic conditions: $\ce{Zn + BrO3- -> Zn^2+ + Br-}$.

The half-reactions are:

$\ce{Zn -> Zn^2+ + 2 e-}$ (oxidation half-reaction and already balanced)

$\ce{BrO3- + 6 e- -> Br-}$ ($\ce{Br}$ is reduced but reaction is not balanced)

When balancing the second half-reaction by adding $\ce{H2O}$ and $\ce{H+}$, it becomes

$\ce{BrO3- + 6 H+ + 6 e- -> Br- + 3 H2O}$.

I understand that the underlying idea is that we can add $\ce{H2O}$ and $\ce{H+}$ since both are at least relatively abundant in an acidic solution. However, when adding $\ce{H2O}$ and $\ce{H+}$ we modify the reaction. How can you be sure that this modified reaction will occur?

Reference for chem equation/expression formatting: Notation basics / Formatting of math/chem expressions /
upright vs italic // For more: Math SE MathJax tutorial. // Not to be applied in CH SE titles. — Poutnik, Apr 13 '23 at 10:11
We do not modify the reaction, we fix the reaction, as not fixed one does not occur. Without nuclear reactions, there is law of element atom conservation. It is an equation after all. It means atom counts for all elements must be equal for both sides. This count balance is required but not sufficient condition for a formal reaction being a real reaction. — Poutnik, Apr 13 '23 at 10:20
Yes, the non-fixed reaction does not occur. And, yes, it is an equation so we can add atoms as long as we obey the "law of element atom conservation", as you said. However, what says that the "new" reaction will occur? — UserE, Apr 13 '23 at 10:21
Knowledge of chemistry. There are no formal rules for that. $\ce{He + 3 F2 -> HeF6}$ is formally correct, but otherwise nonsense. — Poutnik, Apr 13 '23 at 10:22
OK! Can you provide some resources or anything for me to understand why that is? I don't want to "just accept it". What is that "knowledge of chemistry" you are talking about? Thanks! — UserE, Apr 13 '23 at 10:24
See also CH SE resources // Rules No 1 - there are no shortcuts in learning chemistry. You either have to understand it, to remember it or to search it out. — Poutnik, Apr 13 '23 at 10:25
Yes, I checked that. It is incredibly broad. Could you point to what this knowledge is called (something similar to "acid-base neutralization reaction")? Thank you! — UserE, Apr 13 '23 at 10:26

Poutnik · Answer 1 · 2023-04-15T06:10:43.903

1

There are three basic rules in enumeration of redox half reactions.

Two are consequence of the fundamental conservation laws:

Conservation of counts of element atoms
Conservation of total charge

One rule is a mental helper:

For the reduction half-reaction where an oxoanion loses its oxygen, imagine formally there are temporarily released anions $\ce{O^2-}$, which are much stronger base than $\ce{OH-}$. They immediately react:
In acidic solutions: $\ce{O^2-(aq) + 2 H+(aq) -> H2O}$
In any solutions: $\ce{O^2-(aq) + H2O -> 2 OH-(aq)}$

Note that a formally correct equation for a chemical reaction is not sufficient condition for such a reaction to occur. This has to be addressed by applying particular knowledge of chemistry of involved reagents.

For more, see also CH SE learning resources

edited Apr 15 '23 at 06:10

answered Apr 13 '23 at 12:10

Poutnik

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An alternative "mental helper". In acid, $\ce{O = H2O - 2H^+}$. In base, $\ce{O = 2OH^- - H2O}$ and $\ce{H = H2O - OH^-}$. – user55119 Apr 13 '23 at 14:21
@user55119 You must mean $\ce{O^2- = H2O - 2 H+}$ and $\ce{O^2- = 2 OH- - H2O}$ - But a good one. – Poutnik Apr 13 '23 at 14:24
When you need oxygen (O) or hydrogen (H) to balance an equation, these expressions are what I use. Balancing the overall charge in the half reaction comes later. – user55119 Apr 13 '23 at 15:27

When balancing a half reaction by adding water and oxonium ions or hydroxide ions, how do we know that the balanced half reaction will occur?

1 Answers1