Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 12.1 Enthalpy vs feasibility-Study Notes - New Syllabus

12.1 Limitations of Enthalpy in Predicting Feasibility of Reactions

Enthalpy change (\( \mathrm{\Delta H} \)) describes the heat energy absorbed or released during a reaction. Although exothermic reactions (\( \mathrm{\Delta H < 0} \)) release energy and are often assumed to occur spontaneously, this is not always sufficient to predict whether a reaction will take place. Importantly, some endothermic reactions (\( \mathrm{\Delta H > 0} \)) can also occur spontaneously at room temperature. This shows that enthalpy alone does not determine the feasibility of a reaction.

Key Idea

Enthalpy change alone does not control whether a reaction is spontaneous.

If enthalpy were the only factor:

• All exothermic reactions would occur spontaneously.
• All endothermic reactions would not occur spontaneously.

However, experimental observations show this is not true.

Evidence from Endothermic Reactions

• Some endothermic reactions occur spontaneously at room temperature.
• For example, certain salts dissolve in water and cause a temperature decrease, yet the process still occurs.
• This indicates that factors other than enthalpy must influence feasibility.

Implication

• A reaction can be spontaneous even if it absorbs heat from the surroundings.
• Therefore, energy change alone is insufficient to predict whether a reaction will occur.

Need for Additional Factor

Since enthalpy alone cannot explain spontaneity, another factor must be considered. This leads to the concept of entropy, which accounts for the degree of disorder in a system and plays a key role in determining whether a reaction is feasible.