Question

Classify each reaction as endothermic or exothermic.

H2 + Br2 → 2HBr, ΔH = -73 kJ
2NH3 → N2 + 3H2, ΔH = 92 kJ
CH4 + 2O2 → CO2 + 2H2O, ΔH = -890 kJ
2HCl → H2 + Cl2, ΔH = 184 kJ

Answer 1

Reactions :

1.$\bold{H_{2}\:+\:Br_{2}\longrightarrow\:2HBr}$   ΔH = -73 kJ

2.$\bold{2NH_{3}\longrightarrow\:N_{2}\:+\:3H_{2}}$, ΔH = 92 kJ

3.$\bold{CH_{4}\:+\:2O_{2}\longrightarrow\:CO_{2} + 2H_{2}O}$, ΔH = -890 kJ

4.$\bold{2HCl\:\longrightarrow\:H_{2}\:+\:Cl_{2}}$, ΔH = 184 kJ

Criteria for a reaction to be classified into exothermic or endothermic :

• Increase in enthalpy (+ΔH) => EndoThermic reaction.
• Decrease in enthalpy (-ΔH) => ExoThermic Reaction

So, According to criteria,

Reaction 1 is Exothermic. (∵ΔH = -73 kJ)

Reaction 2 is Endothermic. (∵ΔH = 92 kJ)

Reaction 3 is Exothermic. (∵ΔH = -890 kJ)

Reaction 4 is Endothermic. (∵ΔH = 184 kJ)

Thanks!