Final answer:
Reactions (b) 2SO₂ + O₂ → 2SO₃ and (d) C₂H₂ + 2H₂ → C₂H₆ are redox reactions due to changes in oxidation states of sulfur and carbon, respectively.
Step-by-step explanation:
To determine which reactions are redox reactions, we must look for changes in oxidation states of the substances involved. A redox reaction involves the transfer of electrons between substances, causing an increase in oxidation state (oxidation) or a decrease in oxidation state (reduction).
- (a) Pb(NO₃)₂ + MgCl₂ → Mg(NO₃)₂ + PbCl₂ is a double replacement reaction, not a redox reaction, as there is no change in oxidation state of Pb, Mg, Cl, or N.
- (b) 2SO₂ + O₂ → 2SO₃ is a redox reaction. Sulfur is oxidized from +4 in SO₂ to +6 in SO₃, and oxygen is reduced.
- (c) CaCl₂ + Na₂CO₃ → 2NaCl + CaCO₃ is a double replacement reaction, not redox, since there is no change in oxidation states.
- (d) C₂H₂ + 2H₂ → C₂H₆ is a redox reaction. Carbon is reduced as its oxidation state decreases from -1 in C₂H₂ to -3 in C₂H₆.
- (f) HCl + NaOH → H₂O + NaCl is an acid-base neutralization reaction, not redox, as no change occurs in the oxidation states of the elements involved.
So, reactions (b) and (d) are redox reactions.