Question

In which reaction does the oxidation number of hydrogen change? In which reaction does the oxidation number of hydrogen change? 2HClO4 (aq) + CaCO3 (s) → Ca(ClO4)2 (aq) + H2O (l) + CO2 (g) CaO (s) + H2O (l) → Ca(OH)2 (s) HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l) 2Na (s) + 2H2O (l) → 2NaOH (aq) + H2 (g) SO2 (g) + H2O (l) → H2SO3 (aq)

Answer 1

Answer: The correct answer is $2Na(s)+2H_2O(l)\rightarrow 2NaOH(aq.)+H_2(g)$

Explanation:

Oxidation number is defined as the number which is given to an atom when it looses or gains electron. When an atom looses electron, it attains a positive oxidation state. When an atom gains electron, it attains a negative oxidation state.

Oxidation state of the atoms in their elemental state is considered as 0. Hydrogen is present as gaseous state.

For the given chemical reactions:

• Reaction 1:  $2HClO_4(aq.)+CaCO_3(s)\rightarrow Ca(ClO_4)_2(aq.)+H_2O(l)+CO_2 (g)$

Oxidation state of hydrogen on reactant side: +1

Oxidation state of hydrogen on product side: +1

Thus, the oxidation state of hydrogen is not changing.

• Reaction 2:  $CaO(s)+H_2O(l)\rightarrow Ca(OH)_2(s)$

Oxidation state of hydrogen on reactant side: +1

Oxidation state of hydrogen on product side: +1

Thus, the oxidation state of hydrogen is not changing.

• Reaction 3:  $HCl(aq.)+NaOH(aq.)\rightarrow NaCl(aq.)+H_2O(l)$

Oxidation state of hydrogen on reactant side: +1

Oxidation state of hydrogen on product side: +1

Thus, the oxidation state of hydrogen is not changing.

• Reaction 4:  $2Na(s)+2H_2O(l)\rightarrow 2NaOH(aq.)+H_2(g)$

Oxidation state of hydrogen on reactant side: +1

Oxidation state of hydrogen on product side: 0

Thus, the oxidation state of hydrogen is changing.

• Reaction 5:  $SO_2(g)+H_2O(l)\rightarrow H_2SO_3(aq.)$

Oxidation state of hydrogen on reactant side: +1

Oxidation state of hydrogen on product side: +1

Thus, the oxidation state of hydrogen is not changing.

Hence, the correct answer is $2Na(s)+2H_2O(l)\rightarrow 2NaOH(aq.)+H_2(g)$

Answer 2

The oxidation number of hydrogen changes in the reaction 2 Na(s) + 2 H₂O(l) → 2 NaOH(aq) + H₂(g).

We want to identify a change in the oxidation number of hydrogen. We will consider that:

• The oxidation number of H in most compounds is +1.
• The oxidation number of H in H₂ is 0.
• The oxidation number of H in metal hydrides is -1.

In which reaction does the oxidation number of hydrogen change?

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

NO. The oxidation number of H in HClO₄ and H₂O is +1.

CaO(s) + H₂O(l) → Ca(OH)₂(s)

NO. The oxidation number of H in H₂O and Ca(OH)₂ is +1.

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

NO. The oxidation number of H in HCl and H₂O is +1.

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

YES. The oxidation number of H in H₂O is +1 whereas in H₂ is 0.

SO₂(g) + H₂O(l) → H₂SO₃(aq)

NO. The oxidation number of H in H₂O and H₂SO₃ is +1.

The oxidation number of hydrogen changes in the reaction 2 Na(s) + 2 H₂O(l) → 2 NaOH(aq) + H₂(g).

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