If reaction between acid and metal always give salt and H2????

1 Answer
Truong-Son N.
Jun 1, 2018

No.

Counterexample below...

Consider the half-reactions for concentrated nitric acid reacting with magnesium metal:

#2(cancel(e^(-)) + "H"^(+)(aq) + "HNO"_3("conc.") -> "NO"_2(g) + "H"_2"O"(l))#
#ul("Mg"(s) -> "Mg"^(2+)(aq) + cancel(2e^(-)))#
#color(blue)(2"H"^(+)(aq) + "Mg"(s) + 2"HNO"_3("conc.") -> "Mg"^(2+)(aq) + 2"NO"_2(g) + 2"H"_2"O"(l))#

Here, we can see that no #"H"_2(g)# forms... On the other hand, if the nitric acid was dilute... then a simpler redox occurs:

#2"HNO"_3("dil.") + cancel(2e^(-)) -> "H"_2(g) + 2"NO"_3^(-)("dil.")#
#ul("Mg"(s) -> "Mg"^(2+)(aq) + cancel(2e^(-)))#
#"Mg"(s) + 2"HNO"_3("dil.") -> "H"_2(g) + "Mg"("NO"_3)_2(aq)#

...and this would be the usual single-displacement reaction.

This is because at higher nitric acid / nitrate concentration, the reduction of nitrate to nitrogen dioxide is favored over the reduction of a proton to dihydrogen gas. More of something = that reacts more favorably.

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