Which of the following chemical equations represents an exothermic reaction at 298 K?

• A. 6CO_2(g) + 6H_2O(l) ightarrow C_6H_{12}O_6(s) + 6O_2(g) \,\;\Delta H = +2801\;\text{kJ}
• B. 2H_2(g) + O_2(g) ightarrow 2H_2O(l) \,\;\Delta H = -572\;\text{kJ}
• C. CaCO_3(s) ightarrow CaO(s) + CO_2(g) \,\;\Delta H = +178\;\text{kJ}
• D. N_2(g) + O_2(g) ightarrow 2NO(g) \,\;\Delta H = +180\;\text{kJ}

Answer: (B)

Verified step by step guidance

1. Understand that an exothermic reaction releases heat to the surroundings, which means the enthalpy change (\$\Delta H\$) for the reaction is negative.
2. Look at the given chemical equations and their associated enthalpy changes (\$\Delta H\$ values).
3. Identify the sign of each \$\Delta H\$: if it is positive, the reaction is endothermic (absorbs heat); if it is negative, the reaction is exothermic (releases heat).
4. From the list, find the reaction with a negative \$\Delta H\$ value, indicating it is exothermic.
5. Confirm that the reaction with \$\Delta H = -572\;\text{kJ}\$ is exothermic, while the others with positive \$\Delta H\$ values are endothermic.