<u>Answer:</u> The correct answer is ![2Na(s)+2H_2O(l)\rightarrow 2NaOH(aq.)+H_2(g)](https://tex.z-dn.net/?f=2Na%28s%29%2B2H_2O%28l%29%5Crightarrow%202NaOH%28aq.%29%2BH_2%28g%29)
<u>Explanation:</u>
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:
- <u>Reaction 1:</u>
![2HClO_4(aq.)+CaCO_3(s)\rightarrow Ca(ClO_4)_2(aq.)+H_2O(l)+CO_2 (g)](https://tex.z-dn.net/?f=2HClO_4%28aq.%29%2BCaCO_3%28s%29%5Crightarrow%20Ca%28ClO_4%29_2%28aq.%29%2BH_2O%28l%29%2BCO_2%20%28g%29)
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.
- <u>Reaction 2:</u>
![CaO(s)+H_2O(l)\rightarrow Ca(OH)_2(s)](https://tex.z-dn.net/?f=CaO%28s%29%2BH_2O%28l%29%5Crightarrow%20Ca%28OH%29_2%28s%29)
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.
- <u>Reaction 3:</u>
![HCl(aq.)+NaOH(aq.)\rightarrow NaCl(aq.)+H_2O(l)](https://tex.z-dn.net/?f=HCl%28aq.%29%2BNaOH%28aq.%29%5Crightarrow%20NaCl%28aq.%29%2BH_2O%28l%29)
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.
- <u>Reaction 4:</u>
![2Na(s)+2H_2O(l)\rightarrow 2NaOH(aq.)+H_2(g)](https://tex.z-dn.net/?f=2Na%28s%29%2B2H_2O%28l%29%5Crightarrow%202NaOH%28aq.%29%2BH_2%28g%29)
Oxidation state of hydrogen on reactant side: +1
Oxidation state of hydrogen on product side: 0
Thus, the oxidation state of hydrogen is changing.
- <u>Reaction 5:</u>
![SO_2(g)+H_2O(l)\rightarrow H_2SO_3(aq.)](https://tex.z-dn.net/?f=SO_2%28g%29%2BH_2O%28l%29%5Crightarrow%20H_2SO_3%28aq.%29)
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)](https://tex.z-dn.net/?f=2Na%28s%29%2B2H_2O%28l%29%5Crightarrow%202NaOH%28aq.%29%2BH_2%28g%29)