In which of the following reactions does the oxidation number of hydrogen change?

• A. HCl + NaOH → NaCl + H2O
• B. CaCO3 → CaO + CO2
• C. 2 H2 + O2 → 2 H2O
• D. 2 Na + 2 H2O → 2 NaOH + H2

Answer: (D)

Verified step by step guidance

1. Recall that the oxidation number of hydrogen is usually +1 when bonded to nonmetals and -1 when bonded to metals (as in hydrides).
2. Analyze each reaction to see if the oxidation number of hydrogen changes by identifying the oxidation states of hydrogen in the reactants and products.
3. For the reaction HCl + NaOH → NaCl + H2O, hydrogen is +1 in HCl and remains +1 in H2O, so no change occurs.
4. For CaCO3 → CaO + CO2, hydrogen is not present, so no change in hydrogen's oxidation number.
5. For 2 H2 + O2 → 2 H2O, hydrogen is 0 in H2 (elemental form) and +1 in H2O, so the oxidation number of hydrogen changes here.
6. For 2 Na + 2 H2O → 2 NaOH + H2, hydrogen is +1 in H2O and 0 in H2 (elemental form), so the oxidation number of hydrogen changes in this reaction as well.