Which of the following chemical reactions is a redox reaction? NH4OH (aq) + KCl (aq) KOH (aq) + NH4Cl (aq)

Zn (s) + H2SO4 (aq) ZnSO4 (aq) + H2 (l)

H2SO4 (aq) + CaCl2 (aq) CaSO4 (aq)+ HCl (aq)

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

Which of the following is not a characteristic of a hydrogen bond? 1. it is responsible for the unusual physical properties of water. 2. it is weaker than a covalent bond. 3. it is stronger than other dipole-dipole interactions. 4. it can occur when hydrogen is covalently bound to very electronegative elements liks f, cl, br and i.