Exothermic and Endothermic Reactions

Question

Some solids dissolve by exothermic reaction. Other solids require heat in other to dissolve. What make this difference?

Please give more detail.

Please notice that my question is not what is exothermic reaction.
My question is
1. what make it release heat energy when a solid dissolved,
and
2. what make it gain heat energy when a solid dissolved in the same solvent.

Answer 1

The process of dissolving a solid in a solvent can be divided into two steps :

Step 1 :
The solid lattice of the solute is broken down into separated particles. For example, sodium chloride lattice is broken down into separated sodium ions and chloride ions.
Obviously, energy is needed in step 1. In other words, step 1 is endothermic.

Step 2 :
The separated particles of the solute mix with the molecules of the solvent. Due to the difference in amount, the separated particles of the solute are surrounded by the molecules of solvent. Attractions between the separate particles of the solute and the molecules of the solvent are formed. This step is known as solvation. (If water is used as the solvent, this step is also known as hydration.) In salvation, energy is released. In other words, step 2 is exothermic.

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Whether the dissolving of a solid in a solvent is endothermic or exothermic ?

1. When the energy needed to break the lattice of the solute (in step 1) is more than the energy released in solvaton (in step 2), the process of dissolving is endothermic.

2. When the energy needed to break the lattice of the solute (in step 1) is less than the energy released in solvation (in step 2), the process of dissolving is exothermic.

Answer 2

In thermodynamics, the word exothermic describes a process or reaction that releases energy in the form of heat. Its etymology stems from the Greek prefix ex-, meaning “outside” and the Greek word thermein, meaning “to heat”. The opposite of an exothermic process is an endothermic process.
The concept is frequently applied in physical sciences to e.g. chemical reactions, where chemical bond energy is converted to thermal energy (heat).
Overview
Exothermic refers to a transformation in which a system releases heat to the surroundings:

Q < 0
When the transformation occurs at constant pressure:

∆H < 0
and constant volume:

∆U < 0
In an adiabatic system (i.e. a system that does not give off heat to the surroundings), an exothermic process results in an increase in temperature.[1]

Exothermic processes
Some examples of exothermic processes are:[2]

Freezing of water
Combustion
Respiration


Endothermic reaction




In Chemistry an endothermic reaction is one in which the reactants have less energy than the products, and thus a net input of energy, usually in the form of heat, is required. Endothermic reactions are often described as reactions that "feel cold", and contrast with exothermic reactions, in which heat is released.
Although the process of bond breaking amongst reactants in a chemical process requires an initial input of energy (the activation energy), in the case of an endothermic reaction, the energy released when bonds are formed to create reactants is less than that required to break the bonds in the products; bonding electrons in the products are therefore at a higher energy than the reactants. Heat energy from the material surrounding the reactants is usually what breaks their bonds, so as heat energy is transferred from the surroundings to the reactants, the surroundings get colder. This is often summarized in a chemical equation as follows:

Reactants + Energy → Products

[edit] Examples of endothermic reactions

Reactions within food when cooking.

Photosynthesis reaction

The polymerization of ethene to polythene.

The reduction of silver ions to silver.

Electrolysis. Energy is provided in the form of electricity.

The mixing of barium hydroxide and ammonium thiocyanate causes a powerful endothermic reaction that causes the products to become so cold that the moisture from the air forms a layer of frost on the outer surface of the beaker.

Reactions in an aqueous solution where heat energy is transferred from the water to the reactants. In this way, the temperature of the solution falls.

Answer 3

The strengths of the bonding holding up particles in different solid are not the same.