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, energy is on the reactant - side, indicating energy is absorbed (endothermic reactions).

Answer:

CH₄ + 2O₂ → CO₂ + 2H₂O + 891 kJ