Question

What is the reaction between ammonia and bleach that makes it unsafe to mix the two?

I think bleach has strong base in it, and I know ammonia can either be an acid or a base, but I don't really have much of an idea of what the reaction between the two is. Does it form some gas?

Answer 1

The most likely reaction is to form chloramine:

`"NH"_3(aq) + "NaOCl"(aq) -> "NH"_2"Cl"(g) + "NaOH"(aq)`

It does look rather awkward, as we might see this and think, "but we tried to react weak base with weak base, didn't we?". [Ammonia is typically a base, and `"OCl"^(-)` is the weak conjugate base of the weak acid `"HOCl"`.]

However, this reputable source states that the above is a "well-known process" for when `"NH"_3` reacts with `"HOCl"` to eventually form `"NCl"_3` in a three-step mechanism.

Note that chloramine, `"NH"_2"Cl"`, supposedly may cause tumors and genetic mutation, and isn't particularly stable, which is why this reaction continues further.

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Based on that source, we can estimate the thermodynamic likelihood of this being a possible reaction. The remaining `DeltaH_f^@` were found from Levine's Physical Chemistry (`"OH"^(-)`), here (`"OCl"^(-)`), and here (`"NH"_3`).

`"NH"_2"Cl": DeltaH_f^@ ~~ "13.74 kcal/mol" = "57.49 kJ/mol"`

`"OH"^(-)(aq)`, `DeltaH_f^@ = -"229.994 kJ/mol"`

`"OCl"^(-)(aq)`, `DeltaH_f^@ = -"118.34 kJ/mol"`

`"NH"_3(g)`, `DeltaH_f^@ = -"45.90 kJ/mol"`

So this is for the reaction:

`"NH"_3(aq) + "OCl"^(-)(aq)-> "NH"_2"Cl"(g) + "OH"^(-)(aq)`

This gives:

`DeltaH_(rxn)^@ = ["57.49 kJ/mol" + (-"229.994 kJ/mol")] - [-"45.90 kJ/mol" + (-"118.34 kJ/mol")]`

`= -"8.26 kJ/mol"`

This reaction forms a gas from aqueous reactants, so the entropy change must be positive.

Hence, the change in Gibbs' free energy is:

`DeltaG_(rxn)^@ = DeltaH_(rxn)^@ - TDeltaS_(rxn)^@`

`= (-) - (+)(+)`

`= (-)`

and this reaction is therefore spontaneous at `"300 K"`.