12.3:
溶液のエンタルピー
A subscription to JoVE is required to view this content. Sign in or start your free trial.
Dissolving a solute in a solution is either an exothermic or an endothermic process. When sodium hydroxide dissolves in water, heat is transferred from the solution to the surrounding water causing the temperature of the water to increase. This is an exothermic process. In endothermic processes, such as dissolving ammonium chloride in water, heat is absorbed by the solution causing the temperature of the water to decrease. At constant pressure, the heat released or absorbed is called the enthalpy change. Solution formation has three steps, each associated with a corresponding enthalpy change. Step one is the separation of the solute particles. This requires an input of energy to overcome the attractive forces between the solute particles. Step two is the separation of the solvent particles. This is also an endothermic step since energy is required to disrupt the attractive forces between the solvent particles. Step three occurs when the solute and solvent particles mix. This step is exothermic because the attractive interactions between solute particles and solvent particles release energy. For a stepwise process, Hess’s Law states that the net enthalpy change is the sum of the enthalpy changes in each step. The sign of the net enthalpy depends on the magnitudes of the enthalpies of the components. If the sum of the component enthalpies is less than the enthalpy of mixing, the net enthalpy change is negative, and the dissolution process is exothermic. If the sum of the component enthalpies is greater than the enthalpy of mixing, the enthalpy change is positive, and the dissolution process is endothermic. If the two are equal, heat is neither released nor absorbed. Solution formation is different from a chemical reaction. When a solute is dissolved in a solvent, the change is physical. Upon evaporating the solution, the solute can be recovered. On the contrary, a chemical reaction alters the properties of the reactants. When copper hydroxide is dissolved in hydrochloric acid, evaporating the solution will not return copper hydroxide. Instead, we will obtain the product, copper chloride.
12.3:
溶液のエンタルピー
自然に溶液が生成することを保証するものではないが、有利な2つの基準があります。
- 系の内部エネルギーの減少 (熱化学の前の章で説明したように、発熱性の変化を示す)
- 系内の物質の分散の増加（熱力学の後の章で学ぶように、系のエントロピーの増加を示す）
溶解の過程では、熱が吸収されたり発生したりすることで、内部エネルギーの変化が起こります。また、溶媒中と溶質分子から均一な溶液が形成されるとき、物質の分散は増加します。
発熱性の溶解プロセスでは自発的な溶液形成が有利になりますが、それ以外にも溶液形成が起こる場合があります。実際、多くの可溶性化合物は熱を放出すると溶解しますが、一部は吸熱的に溶解します。硝酸アンモニウム（NH4NO3）はその一例で、怪我の治療用のインスタントコールドパックを作るのに使われています。水を入れた薄手のビニール袋を、固体のNH4NO3を入れた大きな袋の中に封入します。小さい方の袋を破ると、NH4NO3の溶液が形成され、周囲（パックを当てた負傷部位）の熱を吸収して冷湿布となり、腫れを抑えることができます。このような吸熱性溶解は、溶質種を分離するために、溶質が溶解したときに回収されるよりも大きなエネルギーを必要とします。それにもかかわらず、溶液の形成に伴う無秩序性の増加により、自発的に起こります。
上記の文章は以下から引用しました。Openstax, Chemistry 2e, Section 11.1: The Dissolution Process.
Suggested Reading
- Sugihara, Gohsuke, and Mihoko Hisatomi. "Enthalpy–entropy compensation phenomenon observed for different surfactants in aqueous solution." Journal of colloid and interface science 219, no. 1 (1999): 31-36.
- Wilhelm, Emmerich, David Raal, Jan Thoen, Jean-Pierre Grolier, Dmitry Zaitsau, Claudio Cerdeiriña, Takayoshi Kimura et al. Enthalpy and Internal Energy: Liquids, Solutions and Vapours. Royal Society of Chemistry, (2017).
- Shen, Xinghai, Hongcheng Gao, and Xiangyang Wang. "What makes the solubilization of water in reversed micelles exothermic or endothermic? A titration calorimetry investigation." Physical Chemistry Chemical Physics 1, no. 3 (1999): 463-469.