How to distinguish whether a substance is endothermic or exothermic when dissolved

When a substance dissolves in water, it absorbs a lot of heat. For example, if you dissolve potassium nitrate or ammonium nitrate in water, you will find that the temperature of the solution is significantly reduced.

Other substances will release a large amount of heat when dissolved in water, such as dissolving caustic soda in water, or slowly pouring concentrated sulfuric acid into water, and the temperature of the solution will increase significantly.

When a substance is dissolved, why is there endothermic or exothermic phenomenon?

This is because: the dissolution of matter, on the one hand, is the particle of the solute - molecules or ions have to overcome their own mutual attraction to leave the solute, on the other hand, the dissolved solute has to diffuse into the entire solvent, these processes need to consume energy, so when the substance dissolves, it absorbs heat. This is the reason for the temperature drop during the dissolution process.

If the dissolution process is just simple diffusion, it should all be endothermic, so why is there exothermy? It turns out that in the process of dissolution, the solute particles, molecules or ions, not only separate from each other and disperse into the solvent, but also the solute particles dissolved in the solvent can also form solvates with the solvent molecules (if the solvent is water, hydrates).In this process, heat is released.

Thus, when a substance is dissolved, two processes take place at the same time:

One is the solute particles - molecules or ions leave the solid (liquid) surface and diffuse into the solvent, this process absorbs heat, which is a physical process;

Another process is the particulates of solutes – molecules or ions and solvent molecules that generate solvates and release heat, which is a chemical process.

These two processes are not equal to the heat absorbed and released for different solutes, when there is more heat absorption than heat release, for example, when potassium nitrate is dissolved in water, because it is unstable in combination with water molecules, it absorbs more heat than it emits heat, it is manifested as endothermy, and when it is dissolved, the temperature of the solution decreases. Conversely, when there is more exothermic than endothermy, for example, when concentrated sulfuric acid is dissolved in water, because it and water molecules form mutually stable compounds, and more heat is released than it absorbs, it is manifested as exothermic, so the temperature of the solution increases significantly.

Whether the temperature of a substance dissolved in water increases or decreases depends on the amount of heat absorbed or released by the two processes during the dissolution process. If:

Q suction Q release The temperature of the solution drops.

Q suction Q release The temperature of the solution increases.

There was no significant change in the temperature of the q-suction and q-release solution.

The change in heat during solute dissolution can be measured with an instrument.

The above is a simple dissolution process, and I would also add that it would be if the solute is to be ionized or hydrolyzed in solution (e.g. various electrolytes) and at the same time it will have to be endothermic. For example, quicklime dissolves in water to react with water and release a large amount of heat is a special situation, which cannot be understood only as a dissolution process.

Keep in mind that the dissolution of acids and subtractive acids is mostly exothermic, and the dissolution of salts is mostly endothermic, so it is best to use temperature measurement.

All ammonium salt dissolution is endothermic, hydroxide and acid dissolution is exothermic, and most of the salt heat effects are not obvious.

The order of the questions is this.

In past experiments, it has been found that the temperature of existing substances dissolved in aqueous solutions is significantly reduced. However, the temperature of some substances dissolved in aqueous solutions increases significantly. Later, the microscopic process of dissolving substances in water was deeply studied, and it was found that when solutes diffuse in water, they are generally endothermic (this is calculated according to thermodynamics), and the heat of the substance is out of the above process, and its solute molecules or ions will combine with water to form hydrated molecules or ions, and this hydration process is exothermic.

The sequence is to discover the phenomenon first and then study the mechanism of the process.

According to this law, combined with practice, later generations summed up the laws of heat release and heat absorption of most substances.

When a substance dissolves, heat is released with dissolution.

Just touch the tube with your hand.

Junior Chemistry - Endothermic and Exothermic phenomena during dissolution.

There are two changes in the dissolution process, one is that the molecules (or ions) of the solute diffuse into the water under the action of water molecules, and this process absorbs heat; The other is the diffusion of the solute molecules (or ions) and the interaction of water molecules to produce water and molecules (or water and ions), a process that releases heat.

Temperature change during dissolution:

1. The heat absorbed during the diffusion process >> The heat released during the hydration process, the temperature of the solution decreases, such as: NH NO dissolved in water.

2. The heat absorbed during the diffusion process "The heat released during the hydration process, the temperature of the solution increases, such as: NaOH and concentrated sulfuric acid Yuyu are dissolved in water.

3. The heat absorbed during the diffusion process The heat released during the hydration process, the temperature of the solution is almost unchanged, such as: NaCl is dissolved in water.