Why do some solutions have temperature changes when they are dissolved?

The process of solution formation is accompanied by changes in energy, volume, and sometimes color. Dissolution is a special physicochemical change that is divided into two processes. One is the discrete discrete of solute molecules or ions, a process that requires endothermic heat to overcome the intermolecular attraction while increasing the volume; The second is the combination of solvent molecules and solute molecules, which is an exothermic process with a decrease in volume.

The whole process is a combination of two aspects.

Substances are dissolved in water and usually go 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. When the heat released is greater than the heat absorbed, the temperature of the solution will rise, such as concentrated sulfuric acid, sodium hydroxide, calcium oxide (quicklime), etc.; When the heat released is less than the heat absorbed, the temperature of the solution will decrease, such as ammonium nitrate, etc.; When the heat emitted is equal to the heat absorbed, the temperature of the solution does not change, such as salt, sucrose.

Is it a solid dissolution, right? The dissolution temperature of the solid will change, because the chemical bond is broken, but it is not obvious, such as CAO, NaOH, etc.

Some dissolve and exothermic, such as sodium hydroxide, acid, and some endothermy, such as ammonium chloride, anhydrous magnesium sulfate.

It becomes an ionic exothermic state, but some of it is exothermic violently.

Solubility. First of all, it is related to the properties of solutes and solvents, and secondly, it is related to external conditions, the solubility of gas becomes larger with the increase of air pressure and decreases with the increase of temperature, most of the solubility of solids increases with the increase of temperature, a few increase with the increase of temperature but the amplitude is very small, and a very small number of them become smaller with the increase of temperature, and the solubility of solids has little relationship with the air pressure.

Let's talk about it in detail.

Factors influencing the size of the solubility of solids.

The nature of the solute, the solvent itself. The solute and solvent are different at the same temperature, and the solubility is different.

The temperature is also an important factor affecting the solubility. The solubility of solid substances varies with temperature. The solubility of most solid substances increases with increasing temperature; A small number of substances (e.g., sodium chloride.

The solubility is very little affected by temperature; There are also very few substances (such as hydrated lime.

The solubility decreases with increasing temperature.

Speaking of dissolving balance, I guess this brother is going to take the exam.

Increase the concentration of reactants or decrease the concentration of products, and the equilibrium is carried out in the direction of the positive reaction. Reduce the concentration of reactants or increase the concentration of products, and the equilibrium is carried out in the direction of the reverse reaction.

with chemical equilibrium.

It's the same thing there.

During the exam, you should pay attention to the conditions given in the questions, the reaction between various ions, and judge which way the equilibrium moves according to the principle, and the problem can be solved. Common questions are about the relationship between ion concentrations. Look hard, no problem!

The solubility increases with the increase of temperature.

Introduction to solubility:

1. At a certain temperature, the mass of the solute dissolved by a solid substance when it reaches saturation in 100g of solvent is called the solubility of the substance in this solvent. The solubility of a substance is a physical property.

2. The solubility of a gas usually refers to the number of volumes of the gas (whose pressure is 1 standard atmosphere) dissolved in 1 volume of water at a certain temperature. It is also commonly used as a unit of "g 100g solvent" (naturally the volume can also be used).

3. Under a certain temperature source and pressure, the maximum amount of a substance dissolved in a certain amount of solvent. It is generally expressed in grams of dissolvable substances in 100 grams of solvent. The solubility of a substance in a certain solvent is mainly determined by the properties of the solvent and solute.

Factors influencing solubility:

Whether a substance is dissolved or not, the size of the solubility of the solubility is determined by the intrinsic cleavage of the substance (referring to the solvent and solute) on the one hand; On the other hand, it is also related to external conditions such as temperature, pressure, solvent type, etc. Under the same conditions, some substances are easily soluble and others are difficult to dissolve, i.e., different substances have different solubility in the same solvent.

The ability of a substance to dissolve in another substance is often referred to as solubility. For example, sugars are easily soluble in water, while fats and fats are insoluble in water because they are different in their solubility over water. Solubility is a quantitative representation of solubility.

The solubility of a solid substance refers to the mass of a substance that dissolves when it reaches saturation in 100 grams of solvent at a certain temperature. In the absence of such an indication, solubility usually refers to the solubility of a substance in water. For example, at 20, the solubility of table salt is 36 grams, and the solubility of potassium chloride is 34 grams.

These data can show that at 20 o'clock, the maximum dissolved amounts of table salt and potassium chloride in 100 g water were 36 g and 34 g, respectively. It also shows that at this temperature, table salt has a stronger solubility in water than potassium chloride.

In the case of gas, the solubility is greatly affected by the pressure in addition to the temperature.

Solubility is temperature and pressure dependent.

For example? I don't quite understand what you're saying.

Not exactly. There are also those that increase in temperature and decrease.

Under certain conditions, the amount of a substance that can be dissolved is limited. Different substances have different dissolving abilities under the same conditions. The solubility of a substance varies with temperature:

The solubility of most solid substances increases with increasing temperature; The solubility of a few substances (e.g., table salt) is less affected by temperature; There are also very few substances (e.g. hydrated lime) whose solubility decreases with increasing temperature.

The solubility product constant and the equilibrium constant of the chemical reaction are similar to ohns and are only related to temperature, the principle is equilibrium, you think about it, let's say barium sulfate. Deposit in Baso4 = (reversible) Ba2+

So42+ equilibrium, if the product of ion concentration reaches the solubility product constant at the fixed temperature of a potato liquid, the forward and reverse reaction rates are equal. If it is greater than this value, it means that there are more ions, the equilibrium will move in reverse, and the precipitation will increase! If the ion concentration is less than this value, the reaction moves in a positive direction and the precipitate dissolves into ions.

Let's analogy with the chemical reaction equilibrium constant and the movement of the equilibrium will help to understand! Let's get rid of the special principle!

Most solids increase in solubility with increasing temperature, such as potassium nitrate.

The solubility of a small number of solids is not affected by temperature, such as table salt (sodium chloride), and the solubility of a very small number of substances decreases with the increase of temperature, such as hydrated lime (calcium hydroxide). Because calcium hydroxide has two hydrates Ca(OH)2·2H2O and Ca(OH)2·12H2O. These two hydrates have a greater solubility and anhydrous calcium hydroxide has a low solubility.

As the temperature increases, these crystalline hydrates gradually change to anhydrous calcium hydroxide, so the solubility of calcium hydroxide decreases with increasing temperature.

The higher the temperature of the gas, the less solubility it has.

The liquid is not very clear.

The solubility product is a function of temperature, and solubility is also related to other factors such as pH value and side reaction factors at a certain temperature.

Take this question as an example.

Hydrochloric acid is added to AGCL to introduce a large number of chloride ions, because the temperature does not change, the solubility product is unchanged, compared with the original equilibrium in the new equilibrium there are more chloride ions, less silver ions, solubility is of course a small ion, silver ions are not as large as the original, so the solubility becomes smaller.

a. Comparison of two insoluble substances in composition and structure, the solubility of insoluble substances with small solubility product must be small, and the solubility of insoluble substances with different composition and structure cannot be directly compared by solubility product, which is correct; b. The ionic product constant kw of water increases with the increase of temperature, indicating that the ionization of water is an endothermic reaction, which is wrong; c. The expression given is the ionization constant expression of ammonia monohydrate, the influencing factor of the ionization constant is temperature, a small amount of ammonium sulfate solid is added to the ammonia water, and the ionization constant remains unchanged, which is wrong; D. Sodium acetate is a strong alkali and weak salt, and the hydrolysis is alkaline, and C (H+) C (OH—) is wrong.

a. When the substance is dissolved, it is often accompanied by endothermic or exothermic phenomenon, and the concentrated sulfuric acid dissolved in water releases a large amount of heat, and the temperature rises, so the option does not meet the topic

b. When the substance is dissolved, it is often accompanied by endothermic or exothermic phenomena, and the sodium hydroxide solid dissolved in water releases a large amount of heat, and the temperature rises, so the option does not meet the topic

c. Calcium oxide reacts with water to release a large amount of cherry plum heat, and the temperature rises, so the option does not meet the topic D. When potassium chloride crystals are dissolved in water, the temperature of the solution does not change significantly, so the selection of the spine Kaichi term is in line with the topic

Therefore, choose D