The Enthalpy Change of Sodium Chloride Added to Water
Salt dissolves readily in water even though the enthalpy change is positive.
Written by John Brennan
When salt dissolves in water, sodium and chloride ions are pulled apart to form new weak bonds with water molecules. Pulling them apart takes energy, while forming new bonds with the water molecules releases energy. The net amount of energy released or absorbed is called the enthalpy, and it plays a crucial role in determining how much salt can dissolve.
Definition of Enthalpy
In chemistry, enthalpy is defined as the total amount of energy a system contains plus its pressure times its volume. There is no way to measure enthalpy directly, because you can't measure how much energy every individual atom and molecule in the system possesses; there are so many, this is practically impossible. You can, however, measure change in enthalpy, because at constant pressure the change in enthalpy is equal to the amount of heat a process absorbs or releases, and you can measure how much heat is being absorbed or released very easily.
Enthalpy and Bonds
Sodium ions are positively charged while chloride ions are negatively charged, and these opposite charges attract each other, which is what holds a salt crystal together. Pulling them apart takes energy in the same way as pulling apart the north pole of one magnet and the south pole of another. Water molecules, by contrast, do not have a net charge, but they do have one region that has a partial negative charge and another that has a partial positive charge. These regions can align with the sodium and chloride ions so that each ion is surrounded by a small shell of water molecules with positively or negatively charged regions facing inwards depending on the ion. Forming these weak bonds between the ion and the cluster of water molecules around it releases energy.
Enthalpy and Energy
If you add up the total amount of energy required to pull the sodium and chloride ions apart then subtract the energy released when they form new bonds with water molecules, you get the total amount of energy released or absorbed by the dissolving salt; in other words, the change in enthalpy. It turns out that at room temperature and atmospheric pressure this quantity is positive, meaning that more energy is absorbed than is released. For each 58.44 grams (2.06 ounces) of salt that dissolves, 0.717 kilocalories (3 kilojoules) of heat is absorbed, meaning that dissolving salt causes the solution to become colder. The change is so slight you are unlikely to notice it in everyday life.