Which of the following reactions are redox reactions?

A) 4Li (s) + O2 (g) →2Li2O (s)

B) Mg (s) + Fe2 + (aq) →Mg2 + (aq) + Fe (s)

C) Pb (NO3) 2 (aq) + Na2SO4 (aq) →PbSO4 (s) + 2NaNO3 (aq)

D) HBr (aq) + KOH (aq) →H2O (l) + KBr (aq)

B) Mg (s) + Fe2 + (aq) →Mg2 + (aq) + Fe (s) is redox reaction.

Explanation:

A Redox reaction is a reaction in which two simultaneous effects occur, the oxidation of a reducing agent and the reduction of an oxidizing agent.

The oxidizing agent is that chemical element that tends to capture these electrons, leaving a state of oxidation lower than it had, suffering a reduction.

The reducing agent is that chemical element that supplies electrons to the medium, increases its oxidation state, that is, it oxidizes.

because:

A) 4Li (s) + O2 (g) →2Li2O (s) is a combination reaction

B) Mg (s) + Fe2 + (aq) →Mg2 + (aq) + Fe (s) is a redox reaction

C) Pb (NO3) 2 (aq) + Na2SO4 (aq) →PbSO4 (s) + 2NaNO3 (aq) is a Ion-exchange reaction

D) HBr (aq) + KOH (aq) →H2O (l) + KBr (aq) is an acid-base reaction