Enthalpy Changes in Chemical Processes

Steps:

• Define each enthalpy change: combustion (reaction with oxygen releasing heat), hydration (ions gaining water molecules), solution (solute dissolving in solvent).
• Assess ΔH sign: combustion usually negative but can vary in rare cases; hydration always releases energy; solution can be positive or negative.
• Compare to question: identify process where ΔH is invariably negative.
• Conclude hydration fits as always exothermic.

Why B is correct:

• Enthalpy of hydration is defined as the exothermic process where gaseous ions form hydrated ions in water, releasing energy (ΔH < 0) per ionic bonding principles.

Why the others are wrong:

• A. Combustion is typically exothermic (ΔH < 0) but not always, as some reactions with oxygen absorb heat.
• C. Solution enthalpy varies; dissolution can be endothermic (ΔH > 0, e.g., NH₄NO₃ in water) or exothermic.

Final answer: B