Which of the following reactions is endothermic?

h2(g) + ½o2(g) → h2o(g), δh = -57.82 kcal
½n2(g) + o2(g) + 8.1 kcal → no2(g)
½n2(g) + 3/2h2(g) → nh3(g) + 11.0 kcal
c(diamond) + o2(g) → co2, δh = -94.50 kcal

2) ½N2(g) + O2(g) + 8.1 kcal → NO2(g)

Explanation:

Chemical reactions in general can be classified as endothermic and exothermic reactions.

Endothermic reactions are accompanied by the absorption of energy. Here, the enthalpy (ΔH) change of the reaction is positive.

Exothermic reactions are accompanied by the release of energy. Here, the enthalpy (ΔH) change of the reaction is negative.

1)H2(g) + ½O2(g) → H2O(g), ΔH = -57.82 kcal

Here, ΔH is negative, hence exothermic

2)½N2(g) + O2(g) + 8.1 kcal → NO2(g)

Here, energy is being absorbed, hence endothermic

3) ½N2(g) + 3/2H2(g) → NH3(g) + 11.0 kcal

Here, energy is released, hence exothermic

4)C(diamond) + O2(g) → CO2, ΔH = -94.50 kcal

Here, ΔH is negative, hence exothermic