Question

Without consulting any tables, determine the \(\Delta H\) for this reaction. Explain your answer. \(\mathrm{H}_{2} \mathrm{O}\left(\ell, 25^{\circ} \mathrm{C}\right) \rightarrow \mathrm{H}_{2} \mathrm{O}\left(\ell, 25^{\circ} \mathrm{C}\right)\)

Short answer

The \(\Delta H\) for this reaction is zero because there is no change occurring.

Step-by-step solution

Understand the Reaction

We are given a reaction where liquid water (\(\mathrm{H}_2\mathrm{O}(\ell)\)) at \(25^{\circ}\)C is undergoing a transformation into itself: liquid water (\(\mathrm{H}_2\mathrm{O}(\ell)\)) at \(25^{\circ}\)C.

Identify State Change

In this reaction, there is no change in state, phase, or any chemical transformation. The substance remains as \(\mathrm{H}_2\mathrm{O}(\ell)\) at the same temperature and pressure.

Evaluate Enthalpy Change

Since there is no change in phase, temperature, or molecular structure, there is no energy change associated with this process. The enthalpy change, \(\Delta H\), from reactants to products is zero.

Key concepts

Chemical Reactions

In the world of chemistry, chemical reactions are processes where substances, known as reactants, transform into new substances called products. These transformations involve breaking and forming chemical bonds, which usually result in changes in properties and energy content. For instance, when hydrogen gas reacts with oxygen gas, they form water, and energy is released as heat. However, not all reactions involve an obvious product formation.
In cases like the given exercise, where liquid water is simply transformed into itself, there is no change in the actual chemical makeup or arrangement. Here, no new bonds are formed or broken, which makes it a unique example demonstrating that not all chemical "reactions" involve noticeable changes. Understanding this nuance is crucial. It shows how sometimes the classification of reactions can simply refer to any process or hypothetical change evaluated for energy variations. Hence, the enthalpy change is zero because there is no energy required or released without any transformation.

State Change

State change refers to the transition of a substance from one phase to another, such as solid to liquid, or liquid to gas. This relies heavily on temperature and pressure conditions. The exercise provided deals with water, a substance capable of existing in all three states: solid (ice), liquid (water), and gas (steam).
In this exercise, liquid water does not actually change its state. It remains liquid throughout the process at a constant temperature of 25°C. Hence, there is no transition from solid to liquid, liquid to gas, or any other transformation of phase that usually involves absorption or release of energy. This reinforces the principle that a state change is synonymous with energy modification in thermodynamics, but without such a change, the energy required, or released is nonexistent, making the enthalpy change zero.

Thermodynamics

Thermodynamics is the study of energy, heat, work, and how they interact in physical systems. It provides a framework for understanding how energy is transformed and conserved. One important aspect of thermodynamics is the study of changes in enthalpy (\(\Delta H\)).
In thermodynamics, enthalpy represents the total heat content of a system. For chemical reactions or physical processes, \(\Delta H\) defines whether a system absorbs or releases energy. However, in the presented exercise, the water remains unchanged both in its chemical constitution and state. Therefore, based on the laws of thermodynamics, namely the conservation of energy, no net energy exchanged means no enthalpy change.
Knowing this aspect of thermodynamics helps explain why a process devoid of actual energetic alterations still constitutes a scenario to evaluate enthalpy, offering insights into the equilibrium of reactions and processes where theoretical changes in state or condition are considered.