Chemical reactions It is the breaking of the chemical bonds between the molecules of the reactants, to produce new bonds, in the molecules of the substances resulting from the reaction, which leads to the formation of different new substances, in their chemical and physical properties together.

Chemical reactions It includes changing the arrangement of atoms in chemical molecules, and in such an interaction we witness the union of some molecules in other ways to form a form of a larger or more complex compound, or the dissolution of compounds to form smaller molecules, or the rearrangement of atoms in the compound. Chemical reactions usually involve the breaking or forming of chemical bonds.

Chemical reactions play a major role in the metabolism of living organisms and in the photosynthesis of plants that provide us with food and oxygen. And respiration is also a chemical reaction. Combustion is a chemical reaction, including oxidation reactions, that takes place in the internal combustion engine that powers cars.

• Oxidation-reduction reactions
• combustion reaction.

patterns of interactions[عدل]

Chemical reactions can be classified in different ways, depending on a specific aspect of the interaction, on the basis of which the division is made, or on the basis of the chemical branch within which it falls. Some examples of terms used to describe common types of interactions:

• Isomerisation, in which a chemical compound undergoes a rearrangement of its structure without changing its chemical composition, that is, the substance molecule only changes its shape (see stereoisomerism).
• Direct union Combination reaction or synthesis, in which two or more chemical compounds merge to form one complex chemical compound.

(2h₂ (gas) + O₂ (gas) → 2H₂O (liq

In this reaction hydrogen and oxygen react to produce water. This reaction is intense if the ratio of hydrogen to oxygen is 1:2, respectively, and that mixture is called an explosive mixture and is accompanied by the diffusion of great heat (exothermic reaction). At the same time, this type of reaction is called an irreversible reaction because it only goes in one direction, from left to right.

• Chemical decomposition: or analysis: in which the chemical compound is broken down into smaller compounds or dismantled into its constituent elements. Let us assume here the case of water analysis:

(2h₂O (liquid) → 2H₂ (gas) + O₂(gas

We saw above that the reaction of oxygen and hydrogen is usually an irreversible reaction and proceeds from left to right and is accompanied by a large diffusion of heat since it is an exothermic reaction. But we can drive the reaction in the opposite direction, as we see in our case here, which is the decomposition of water into its elements oxygen and hydrogen, and we can do this by doing work from the outside with an electrical source. Electric energy supplies the water with the amount of energy equivalent to what is produced by the reaction of oxygen and hydrogen when they unite to produce water, thus we overcome the cohesion of water and direct the reaction in the reverse direction. In such interactions, the system must be supplied with energy or heat from the outside in order for the reaction to proceed in its opposite direction (according to the second law of thermodynamics).

• Single displacement reaction: in which one element of a chemical compound is replaced by another, more reactive element:

(2Na(cr) + 2HCl(aq) → 2NaCl(aq) + H₂ (gas

In this reaction sodium (solid) reacts with hydrochloric acid (liquid), producing sodium chloride and releasing hydrogen gas. This reaction is irreversible because the hydrogen gas, once formed, separates and leaves solution.

• A double displacement reaction or coupling substitution, in which two chemical compounds in an aqueous solution (usually in an ionic form) exchange elements or ions of different compounds:

(NaCl (aq) + AgNO₃ (aq) → NaNO₃ (aq) + AgCl (s

In this reaction sodium replaces the chlorine atom with the nitrate molecule, NO₃ It becomes the “salt” of sodium nitrate, and at the same time the silver ion combines with the chlorine ion to form the “silver chloride” salt.

• Combustion: in which a combustible substance combines with an oxidizing element to produce heat and an oxidizing compound:

C₁₀H₈ (g) + 12O₂ (g) → 10CO₂ (g) + 4H₂O (l)

The combustion reaction is familiar to us, as we know the combustion of wood in the air or the combustion of natural gas, in which carbon unites with oxygen and produces heat and carbon dioxide.

Some branches of chemistry regard any small changes in chemical conformation as a type of reaction, while others regard it as merely a physical change.

Other classifications of chemical reaction:

• organic interactions

• ionic reaction
• radical reaction (chemical radicals)
• carbene reaction

Reactions can also be classified according to the direction of reaction:

• Complete reactions (i.e. all reactants are converted into products after a certain time, long or short).
• Reversible reactions (they do not take place to the end, and part of the reactants along with the products are present in the reaction vessel no matter how long it takes)

chemical balance[عدل]

A chemical reaction that takes place in a homogeneous medium (liquid only, solid only, or gas only) can go in opposite directions. When two substances react with each other to form a third compound, this third compound usually exists in a dissociated state composed of the two reactants. The reaction usually proceeds in the two opposite directions, and a “competition” occurs between the materials entering the interaction and the materials produced, and each direction is characterized by its own reaction rate that depends on the properties of the materials. Since reaction rates also depend on the concentration of each of the substances, they also change with time. The speeds of the two directions of the reaction approach over time when the two speeds or rates are equal. Then the concentration of each of the substances in the mixture does not change and we reach a state of equilibrium called chemical equilibrium.

The location of equilibrium depends on the properties of the reactants, as well as on temperature and pressure, and is determined by what is called free energy. We often use the differential derivative of the free enthalpy or the free enthalpy differential of the reaction, which must become equal to zero at the chemical equilibrium.

The chemist Châtelier expressed the dependence of the rate of reaction on pressure in what is called the principle of Le Châtelier, whereby a system under high pressure tries to make the effect of pressure as low as possible.

In that case, the reaction products have reached a maximum, as by increasing the production of products, the speed of the reverse reaction increases, and the two reaction speeds are equal in both directions when equilibrium is reached. In chemistry, we use a method to increase the production of products by withdrawing (or harvesting) part of them from the reactant mixture, whereby we change the equilibrium mode, or by increasing the pressure on the reactants or raising the temperature of the system.

Chemical reaction thermodynamics[عدل]

The course of a chemical reaction must follow the laws of thermodynamics. It determines the extent to which the reaction proceeds, for example, from left to right, as substances involved in the interaction unite with each other, producing products for the reaction. For the reaction to proceed, the free enthalpy must decrease during the reaction. The free enthalpy consists of two different state functions, enthalpy and entropy. The two values ​​are related to each other in the general equation for free enthalpy.
^[1]

$\displaystyle \Delta G=\Delta H-T\cdot \Delta S$

Where:

G: free enthalpy,

H: enthalpy,

T: temperature,

S: entropy,

Δ: the change in each value, called “delta”.

A chemical reaction can be an exothermic reaction, in which Δ isH The sign is negative and becomes free energy that is purified in the form of heat generated by the reaction. The loss of energy released from the reaction can also result in the product forming a regular crystalline structure characterized by low entropy. An example of an exothermic reaction in which entropy decreases is salt precipitation reactions and crystallization reactions (formation of crystals), in which regular or crystalline products are formed from the interaction of gases or liquids that are not characterized by regular structures.

There are also endothermic reactions and they proceed by absorbing heat from the surrounding atmosphere, or in the laboratory we supply heat from the outside by heating it. These interactions proceed when their entropy increases, which means the entropy of the system. It may be increasing the entropy of the system by gaseous products, as gases have large entropy.

Because entropy depends on temperature and increases with temperature, entropy-determined reactions such as dissociation proceed actively with increasing temperature. Reactions that depend on energy, such as crystallization, are activated by decreasing the temperature. The reaction direction can be chosen by changing the temperature.

Example :

We will study a state of chemical equilibrium known as the Boudward equilibrium:

$\displaystyle \mathrm CO_2+C\rightleftharpoons 2\ CO ;\quad \Delta H=+172,45\ \mathrm kJ\cdot mol ^-1$

This is the reaction of carbon dioxide with carbon that produces carbon monoxide, and it is a reaction that absorbs heat, that is, it needs heat from the outside in order to proceed. Therefore, the chemical equilibrium state for the reaction is at a low temperature on the carbon dioxide side. And by raising the temperature to 800 degrees Celsius and higher than that, the production of carbon monoxide begins to increase with the temperature, due to the increase in entropy.
^[2]

We can also study a chemical reaction in terms of the change in the internal energy of the system. The internal energy of a system is described as a function of entropy, volume change, and chemical potential. The chemical potential depends on the chemical activity of all the reactants, that is, the substances involved in the reaction and the substances resulting from the reaction.
^[3]

$\displaystyle\mathrmdU=T\,\mathrmdSp\,\mathrmdV+\mu\,\mathrmdn\!$

Where:

U: internal energy,

S: entropy,

p: Pressure,

m: chemical potential (or chemical potential),

n: material quantity,

d: differential coefficient.

The equation gives the change of state functions of the substances before and after the reaction.

Classify chemical reactions according to their speed[عدل]

1. Reactions that take place in a very short time, an explosive reaction, such as: the explosion of gunpowder, and the explosion of a mixture of hydrogen and oxygen..

2. Relatively slow rate reactions, such as: the reaction of oils with caustic soda.

3. Very slow reactions that need several months, such as: the interaction of air with iron to form iron rust

4. Very, very slow reactions that need thousands or millions of years, such as: the formation of oil in the ground

Factors affecting reaction speed[عدل]

1 Basic factors (all transformations need them):

-تاثير درجة الحرارة 
-تاثير سطح التلامس 
-تاثير التركيب (المزيج)

2 Secondary factors (needed by some transformations):

-الضغط 
-الوسيط

-time

-concentration

The dependence of the reaction rate on temperature[عدل]

1- In 1884, the Dutch chemist formulated an approximate rule for the dependence of the rate of a chemical reaction on temperature, called the Vant Hoff rule.

2- In 1888, the Swedish chemist Arrhenius formulated his equation called the Arrhenius equation to depend the speed of a chemical reaction on temperature. The Arrhenius equation is more accurate than the Vant Hoff rule, as it takes activation energy into account.

See also[عدل]

• specific detector
• chemical reactor
• Arrhenius equation.
• exothermic reaction.
• Heat absorbing reaction
• reversible reaction.
• product (chemistry)
• thermite reaction
• An oxidation-reduction reaction
• shorthand.
• Vant Hoff base.
• Arrhenius chart.
• nuclear reaction
• Pharaoh snake
• The donor-receiver principle

Reference[عدل]

external links[عدل]

• Chemical control
• GCSE Interactions

chemical reaction On sister projects:
	Pictures and audio from Commons.

• Science portal
• Chemistry portal
• energy portal

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