Classify each of the following as exothermic or endothermic,...

Classify each of the following as exothermic or endothermic, and give the $\Delta H$ for each: a. $\mathrm{CH}_{4}(g)+2 \mathrm{O}_{2}(g) \stackrel{\Delta}{\longrightarrow} \mathrm{CO}_{2}(g)+2 \mathrm{H}_{2} \mathrm{O}(g)+802 \mathrm{~kJ}$ b. $\mathrm{Ca}(\mathrm{OH})_{2}(s)+65.3 \mathrm{~kJ} \longrightarrow \mathrm{CaO}(s)+\mathrm{H}_{2} \mathrm{O}(l)$ c. $2 \mathrm{Al}(s)+\mathrm{Fe}_{2} \mathrm{O}_{3}(s) \longrightarrow \mathrm{Al}_{2} \mathrm{O}_{3}(s)+2 \mathrm{Fe}(l)+850 \mathrm{~kJ}$ The thermite reaction of aluminum and iron(III) oxide produces very high temperatures used to cut or weld railroad tracks.

Classify each of the following as exothermic or endothermic, and give the $\Delta H$ for each:
a. $\mathrm{CH}_{4}(g)+2 \mathrm{O}_{2}(g) \stackrel{\Delta}{\longrightarrow} \mathrm{CO}_{2}(g)+2 \mathrm{H}_{2} \mathrm{O}(g)+802 \mathrm{~kJ}$
b. $\mathrm{Ca}(\mathrm{OH})_{2}(s)+65.3 \mathrm{~kJ} \longrightarrow \mathrm{CaO}(s)+\mathrm{H}_{2} \mathrm{O}(l)$
c. $2 \mathrm{Al}(s)+\mathrm{Fe}_{2} \mathrm{O}_{3}(s) \longrightarrow \mathrm{Al}_{2} \mathrm{O}_{3}(s)+2 \mathrm{Fe}(l)+850 \mathrm{~kJ}$
The thermite reaction of aluminum and iron(III) oxide produces very high temperatures used to cut or weld railroad tracks.

Key Concepts

Thermochemical Equations

Thermochemical equations are chemical equations that include the enthalpy change associated with the reaction. These equations not only convey the stoichiometry of the reactants and products but also indicate whether the reaction is exothermic or endothermic. This integration of energy change information is essential for analyzing reaction energetics and the feasibility of chemical processes.

Enthalpy Change (?H) and Sign Convention

Enthalpy change (?H) is a thermodynamic quantity that indicates the amount of heat absorbed or released during a chemical reaction at constant pressure. The sign of ?H is critical: a negative ?H denotes an exothermic reaction where energy is released, and a positive ?H denotes an endothermic reaction where energy is absorbed. This concept helps chemists understand and predict reaction spontaneity and feasibility.

Exothermic Reaction

An exothermic reaction is one in which energy, typically in the form of heat, is released into the surroundings. This release of energy results in a net negative change in enthalpy (?H), meaning the products are at a lower energy level than the reactants. Exothermic reactions are common in combustion and many oxidation processes, and the energy released can be harnessed for various applications such as heating and welding.

Endothermic Reaction

An endothermic reaction is a process that absorbs energy from its surroundings, resulting in a net positive change in enthalpy (?H). In these reactions, the energy required to break bonds in the reactants exceeds the energy released upon forming bonds in the products, meaning the products are at a higher energy level. Endothermic reactions are often involved in processes such as photosynthesis and certain decomposition reactions.

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