How to tell if a substance is endothermic or exothermic when dissolved

It is endothermy. 1. The concept of melting (melting) and solidification.

The process of transformation of the state of matter. The process of transforming a substance from a solid state to a liquid state is called melting (melting), and the process of changing from a liquid state to a solid state is called solidification.

2. Conditions of melting (melting) and solidification.

Melting: Reaching the melting point, continuing to absorb heat. (Amorphous has no melting point) solidification: Reach the freezing point and continue to exothermic. (Amorphous without freezing point) 3. Melting of crystals and amorphous.

The properties of crystals are different from those of amorphous melting. When the heated crystal reaches a certain temperature, the temperature of the crystal stops rising, and the heat provided when the heating continues is used to change it from a solid state to a liquid state. This temperature is called the melting point; This heat is called the heat of melting.

The freezing point temperature of a substance changing from a liquid state to a solid state is the same as the melting point of the same substance under the same conditions. At the melting point (or freezing point), liquid and solid state coexist. Amorphous can continuously transition from solid to liquid within a certain temperature range, that is, there is a temperature range from a solid state to a liquid state (called the softening temperature), so the amorphous has no definite melting temperature, that is, there is no melting point.

4. The melting point of the element.

The melting point of an element is a periodic function of the atomic number (see Atoms). The melting point of inorganic substances is generally higher and increases with the increase of valency; The melting point of organic compounds is generally low, and the melting point of congeners is related to their molecular weight. The melting point is also related to the pressure of the environment.

If the volume decreases during solidification, the melting point rises with the increase of pressure; In contrast, if the volume expands during solidification, the melting point decreases as the pressure increases. The melting point at one atmospheric pressure is called the normal melting point.

Simple, judging which bond energy of the product and reactant is higher or lower can determine whether it is endothermic or exothermic.

The substance dissolves in water and usually passes through.

Two processes. One is the diffusion process of solute molecules (or ions), which is a physical process that requires the absorption of heat; The other is the process of forming solvent (hydration) molecules (or hydrated ions) by the interaction of solute molecules (or ions) and solvent (water) molecules, which is a chemical process that emits heat.

So just compare the relative sizes of the two processes.

However, high schools generally do not require comparisons, just remember some examples.

It can vary according to the temperature of the solution.

The temperature rises. It is exothermic.

The temperature is lowered. It is endothermy.

Exothermic: sodium hydroxide, potassium hydroxide, quicklime (liquid: concentrated sulphuric acid).

Endothermic: ammonium and nitrate.

Basically unchanged: NACL

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 solute - molecules or ions have to overcome their own attraction to each other to leave the solute; On the other hand, the dissolved solute needs to diffuse into the entire solvent, and these processes require energy, so when the substance is dissolved, it must absorb 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 (hydrates if the solvent is water) with the solvent molecules, and heat is released in this process.

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 emitted for different solutes, when there is more endothermic than exothermic, for example, when potassium nitrate is dissolved in water, because it is unstable in binding with water molecules, it absorbs more heat than it emits heat, it is endothermic. When dissolved, the temperature of the solution decreases, on the contrary, when the heat release is more than the heat absorption, for example, when the concentrated sulfuric acid and sodium hydroxide solids are dissolved in water, because it and the water molecules form mutually stable compounds, the heat released is more than the heat absorbed, 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:

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The temperature of the solution decreases;

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increased temperature of the solution;

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There was no significant change in the temperature of the solution.

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

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.