In which reaction does the oxidation number of hydrogen change? And why?

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A) CaO (s) + H2O (l) → Ca(OH)2 (s)

B) 2HClO4 (aq) + CaCO3 (s) → Ca(ClO4)2 (aq) + H2O (l) + CO2 (g)

C) SO2 (g) + H2O (l) → H2SO3 (aq)

D) 2Na (s) + 2H2O (l) → 2NaOH (aq) + H2 (g)

E) HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)

2 Answers

  • Choice D shows Na donating electrons to the H+ in the water molecule and REDUCING its charge from plus one down to zero.

           Na goes from zero charge as an element to a plus one charge as a Na+ ion  (has been oxidized)
              H+ goes from plus one charge to zero as an element.   (has been reduced)
    Source(s): retired chem teacher USA
  • D

    The oxidation number of H in 2H2O is 1+. In H2, the oxidation number is 0.

    It may be misleading because the reaction is redox, but the hydrogen in reaction D changes oxidation number.

    Source(s): Medic.

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