A change is spontaneous if it leads to greater randomness and chaos in the universe as a whole. We can determine the direction of change by the distribution of energy.

When a change occurs, the total energy remains constant but is parcelled out differently. For example, heat will flow from a hot body to a cooler one, but we cannot observe the opposite even though it does not violate the conservation of energy rule. Some things happen spontaneously, others do not. However, when the same reactants react to produce liquid water, the enthalpy change is -571.5 kJ.ĢH 2 (g) +O 2 (g) → 2H 2O (g) ∆H= -483.7 kJĢH 2 (g) +O 2 (g) → 2H 2O (l) ∆H= -571.7 kJ What is Entropy? For instance, when the oxygen and hydrogen gases react to produce water vapour, the enthalpy change is -483.7 kJ. Moreover, enthalpy change depends on the phase of the reactants. The change in enthalpy between any pair of reactants and products is independent of the path between them. If the value is positive, then the reaction is endothermic.įigure 01: Relationship Between Enthalpy Change and Phase Change If this value is negative, then the reaction is exothermic. We can obtain the enthalpy change (∆H) for a reaction in a given temperature and pressure by subtracting the enthalpy of reactants from the enthalpy of products. Therefore, we need to measure the change in enthalpy during a reaction. However, we cannot measure the enthalpy of molecules. When a reaction takes place, it may absorb or evolve heat, and if we carry out the reaction at constant pressure, we call it the enthalpy of the reaction. Side by Side Comparison – Enthalpy vs Entropy in Tabular Form

Both enthalpy and entropy are thermodynamic state functions. Enthalpy and entropy are two terms describing the reactions taking place in a system and the surrounding. At any time, the part we are going to study is the system, and the rest is surrounding. The key difference between enthalpy and entropy is that enthalpy is the heat transfer taking place in a constant pressure whereas entropy gives an idea of the randomness of a system.įor the study purposes in chemistry, we divide the universe into two as a system and surrounding.

Entropy and enthalpy