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

Question

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?

Wyatt Atkins · Accepted Answer

Chloramine formation is the most likely reaction to occur:

#"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.

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"#

Here is the response, then:

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

This results in:

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

#= -"8.26 kJ/mol"#

Since the aqueous reactants of this reaction are converted into a gas, a positive entropy change is required.

Gibbs' free energy changes as a result, and it is:

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

#= (-) - (+)(+)#

#= (-)#

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

Caleb Adams · Answer

undefined Ammonia ($NH_3$) and bleach ($NaClO$) react to produce toxic gases, namely chloramine ($NH_2Cl$) and chlorine gas ($Cl_2$), which can cause respiratory irritation, coughing, chest pain, and in higher concentrations, more serious health effects, such as lung damage or even death. Because of the potential risks involved in the release of these toxic gases, mixing ammonia and bleach is strongly discouraged.