To determine which reaction is exothermic, we need to understand the definitions of exothermic and endothermic reactions:
- Exothermic reactions release energy to the surroundings, usually in the form of heat. This means they have a negative energy change (ΔH < 0), and energy is listed as a product.
- Endothermic reactions absorb energy from the surroundings. This means they have a positive energy change (ΔH > 0), and energy is listed as a reactant.
Now, let’s analyze each reaction given:
1. Reaction 1: \( CH_4 + 2 O_2 \rightarrow CO_2 + 2 H_2O + 891 \, \text{kJ} \)
- In this reaction, 891 kJ of energy is released, as indicated by the energy term being on the product side and positive. This indicates that the reaction is exothermic.
2. Reaction 2: \( NH_4NO_3 + H_2O + 25 \, \text{kJ} \rightarrow NH_4^+(aq) + NO_3^-(aq) \)
- Here, 25 kJ of energy is required on the reactant side, indicating that the reaction absorbs energy. This means the reaction is endothermic.
3. Reaction 3: \( 2 H_2O + 286 \, \text{kJ} \rightarrow 2 H_2 + O_2 \)
- Similarly, 286 kJ of energy is required on the reactant side. This indicates an absorption of energy, classifying the reaction as endothermic.
4. Reaction 4: \( N_2 + O_2 + 181 \, \text{kJ} \rightarrow 2 NO \)
- Here, 181 kJ of energy is listed on the reactant side, indicating that this reaction also absorbs energy. Thus, it is endothermic.
From our analysis, only the first reaction releases energy, making it exothermic. Thus, the exothermic reaction is:
[tex]\[
CH_4 + 2 O_2 \rightarrow CO_2 + 2 H_2O + 891 \, \text{kJ}
\][/tex]
Therefore, the answer is:
The first reaction is exothermic.
