Question

which reaction is exothermic?
ch4 + 2o2 → co2 + 2h2o + 891 kj
nh4no3 + h2o + 25 kj → nh4+(aq) + no3-(aq)
2h2o + 286 kj → 2h2 + o2
n2 + o2 + 181 kj → 2no

Explanation:

Step1: Define exothermic reaction

An exothermic reaction releases energy.

Step2: Analyze each option

In the reaction CH₄ + 2O₂ → CO₂ + 2H₂O + 891 kJ, energy (891 kJ) is on the product - side, meaning energy is released. In the other reactions (NH₄NO₃ + H₂O + 25 kJ → NH₄⁺(aq) + NO₃⁻(aq), 2H₂O + 286 kJ → 2H₂ + O₂, N₂ + O₂ + 181 kJ → 2NO), energy is on the reactant - side, indicating energy is absorbed (endothermic reactions).

Answer:

CH₄ + 2O₂ → CO₂ + 2H₂O + 891 kJ