Question

Which of the following equations represents an exothermic reaction? (Select all that apply.)

NH3(g) + 12.0 kcal ½N2(g) + 3/2 H2(g)
C(graphite) C(diamond), H = - 0.45 kcal
C + 2S CS2, H = 27,550 cal
CH4 + 2O2 CO2 + 2H2O + 212,800 cal
2H2O 2H2 + O2, H = +58 kcal

Answer 1

C (graphite) → C (diamond), ΔH = - 0.45 kcal

CH4 + 2O2 → CO2 + 2H2O + 212,800 cal

Step-by-step explanation:

exothermic reactions are characterized by releasing heat to the surroundings; the enthalpy change is negative, and the system temperature increases.

NH3 (g) + 12.0 kcal → ½N2 (g) + 3/2 H2 (g): it is not exothermic.

C (graphite) → C (diamond), ΔH = - 0.45 kcal: the reaction is exothermic because ΔH is negative

C + 2S → CS2, ΔH = 27,550 cal: it is not exothermic because the enthalpy change is positive

CH4 + 2O2 → CO2 + 2H2O + 212,800 cal: it is exothermic because it releases heat.

2H2O → 2H2 + O2, ΔH = +58 kcal: it is not exothermic because ΔH is positive.

Answer 2

CH₄ + 2O₂ ⟶ CO₂ + 2H₂O + 212 800 cal

Step-by-step explanation:

CH₄ + 2O₂ ⟶ CO₂ + 2H₂O + 212 800 cal

The heat is on the right-hand side of the equation, so the reaction is releasing heat.

The reaction is exothermic.

A is wrong. Your sign of ΔH is wrong, because graphite is the more stable form of carbon. We must add energy to graphite to convert it to diamond, so the process is endothermic.

B and D are wrong, because a ΔH > 0 indicates an endothermic reaction.