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What is a chemical reaction? definition, signs, and problems related

We will start here a topic about chemical reaction definitions, one of the most important basic concepts and most used terms is the chemical reaction, the basic routine that chemicals used to create new materials that may be more beneficial or most dangerous than their Ancestors.

Exact Chemical reaction definition in chemistry is: 
To define chemical reaction may we use Simple definition
general chemistry, thermodynamics, basic, MCQ, problem, chemical, chemistry, reactions, signs, evidence, exothermic, endothermic, definition, a, It is a conversion and transformation of initial substances or molecules with certain properties into new final substances or molecules that differ in their physical and chemical properties and has a different chemical identity, and the whole process completed under special factors of temperature, pressure, and addition of third substance that we may be call it reagent and the final process accompanied by increase or decrease in the system energy.

Or we can define the chemical reaction as: it is a regeneration of new chemical substances from old or different substances and that what chemist like to call it a synthesis process.

After definition of chemical reaction we will explain how reactants and products are involved?
What is affected when a chemical reaction occurred? Chemical reaction is followed by a change in bonding, energy, and mass change but according to a rules by the chemist called Lavoisier who put the law of conservation of mass, or principle of mass conservation, which states that for any system closed to all transfers of matter and energy (both of which have mass), the mass of the system must remain constant over time, as system mass cannot change quantity if it is not added or removed. Hence, the quantity of mass is "conserved" over time. The law implies that mass can neither be created nor destroyed, although it may be rearranged in space, or the entities associated with it may be changed in form. 

And applied for the chemical reactions as the following:
“Atoms of the object can not be created or destroyed, but can be moved around and changed into different molecules.”This law says that when a chemical reaction makes two different atoms into a new product, the mass of the reactants (chemicals by chemical reaction) is the same as the mass of products (making the new complex, the product). However through nuclear reactions (fusion / fission), mass can be converted in the field of energy.”

In this chapter I will discuss the change mentioned:
Bonds rearrangement process to the chemical molecular bonds of reactants atoms to form new molecular bonds between atoms of final products of the reaction, in which reactant bonds will be broken and new bonds created of the products 

Energy change: according to the matter and energy save principal: the total energy of reactants side must be equal to the total energy of the products side. Chemical reaction that gives off heat called exothermic reaction, and a chemical process that absorbs energy is called endothermic reaction, where Exothermic. "Exo" means out, so exothermic means heat is given off from the system or reaction, and “Endo-” meant In so that Endothermic means energy is wanted in the system. 

How to recognize Exothermic or Endothermic reaction in a written equation? 
By two methods:
First: there is a delta H represents the heat content or enthalpy for a given reaction equation, delta H value is (- = exo, + = endo).
Second: generally exothermic reactions tend to form bonding, and endothermic reactions tend to breaking bonds. 

Mass change: according to principal of preserve the matter the total mass of the reactants must be equal the final mass of the products after completion of the reaction, so that reactants atoms end up to atoms of products and no atoms are destroyed, and all of these a complied with the law of conservation of mass mentioned above.

Factors affect chemical reaction speed, it is a concern of what chemist call it rate of chemical reaction… later more detailed information and illustrated equations on how to calculate the rate of the chemical reaction and use it o determine the products type and speed of overall steps of any chemical reaction.

How to write a reaction in chemistry? Chemists like to write chemical reaction in the following formula:
Initial molecules that interact with each others are called reactants
Final molecules that produced by the chemical reaction are called products
The place which the reactant converted to product is called reactor
The third substance that may be used to redirect the reaction to any direction chemists call it reagent.
The whole process is called a chemical reaction
Chemists write the whole things in code like called chemical equation

How chemists write a chemical reaction by equations in chemistry? Chemists write chemical equation by symbols to facilitate the study of chemistry, so that the chemical equation is the tool of chemists to represent the reaction in a simple method like that:
Reactants in the left side of the chemical equation
Product in the right side of the chemical equation
In between both sides we draw down an arrow with a head, its direction tell us what the right and left directions of the reaction
The reagents or any factor like the temperature of the system or if there is applied pressure is likely to be written above the arrow of the chemical equation
Example of writing equations for any chemical reaction
A + B ---- AB
where A, B are reactant molecules/atoms
AB is a formed product

What are signs of chemical reaction progression?
How can we know that a chemical reaction is occurring?
All these asked questions is a problem for many students studying chemistry, but let we go step by step to illustrate how to recognize the signs of a chemical reaction
When chemical reaction occur is followed by a chemical and physical change which chemists use it as the key indicators of a chemical change and evidence of chemical reactions, such observation of heat, odor, color
Heat: may be due to energy change and released heat out the chemical system
Odor: due to evolution of gases and evaporation of some volatile constituents or products
Observed steam/vapor or color if the reaction accompanied by color change
Observed precipitate: which last at the bottom of chemical reactor
Reactions in chemistry thermodynamics: What is meant by spontaneous and non-spontaneous reactions?
The processes which are accompanied with a decrease in Gibb's free energy & increase in entropy are spontaneous, the reactions which do not fullfil these criteria are non- spontaneous

Is Spontaneous chemical reaction Exothermic or endothermic? Exothermic reactions are not supposed to be spontaneous. Spontaneousity is only depending on the gibbs free energy. gibbs free energy is a function of enthalpy and entropy. If a reaction is exothermic but entropy of the reaction is positive, it may not be spontaneous.

If a reaction is endothermic, it can still be spontaneous depending on the entropy of the reaction. If the entropy is high and positive, the reaction can be spontaneous.

That depends on its entropy change. Remember that spontaneous reactions have a DG <0 -="" and="" br="" delta="" dg="DH" tds="" that="">
The first equation is Gibbs free energy.
When G is negative, the reaction is spontaneous. In contrast,
a positive number G is non spontaneous. The interesting thing is that spontaneous reactions can be EITHER exothermic or endothermic. Lets look at this:
GD = HD - TDS where D = delta for change in values.
GD < 0 spontaneous
HD < 0 exothermic; HD > 0 endothermic
lets assume HD has a value of 100. This means TDS would have to be bigger than 100 in order to make GD a negative number. An endothermic reaction which has a positive H can still be exothermic.

Is an exothermic reaction spontaneous? The answer should be: Always.
Is an endothermic reaction spontaneous? This can be either.
So, an exothermic reaction that increases in entropy will always be spontaneous. But one which has a negative entropy change will be spontaneous at lower temperatures but not spontaneous at higher temperatures.

Note the following definitions:
Endergonic

Describing a reaction that absorbs (heat) energy from its environment.

Exergonic
Releasing energy (especially as heat).

Entropy
A thermodynamic property that is the measure of a system’s thermal energy per unit temperature that is unavailable for doing useful work.

ACTIVATION ENERGY Activation energy is the energy required to initiate a reaction. For example, the flame from the fuse of a firecracker provides a small initial amount of energy, after which the explosive reaction proceeds by itself, releasing a considerably larger quantity of energy. A small push given to a stable but top-heavy object may cause it to fall over; the potential energy released during the fall was present in the system all along but could not be realized as long as the object was upright and balanced.

Physical and Chemical Properties of Matter: simply are the Physical and Chemical Changes to Matter

CONCENTRATION
The proportion of a substance in a mixture

Later we will talk further more about chemistry thermodynamics and spontaneous and non-spontaneous reactions. 

MCQ suggestions for chemical reaction definitions Do all chemical reactions release energy?
The answer is: Chemical reactions may release or absorb energy. But no doubt the energy conversion in a reaction occurs. It may, as heat or work.
There is a reaction called fusion taking place in the sun. This is like
two deuterium atoms (an isotope of hydrogen) are combined together and release energy as heat and light, and this energy is much higher than that released in the fission reaction 

What Name Is Given To Chemical Reactions That Give Out Heat? In the following equation: Al + 3HgCl = AlCl3 + 3Hg

Solution is: exothermic 

Are redox reactions endothermic or exothermic? Here is a simple definition of reduction and oxidation:
* Oxidation describes the loss of electrons by a molecule, atom or ion
* Reduction describes the gain of electrons by a molecule, atom or ion
as you can see, are pretty much opposite of any other. Assuming your first statement is true, the reductions would be endothermic as chemically opposite process. However, if my memory serves me correctly, there can be the 4 possibilities (oxidation exothermic, endothermic oxidation, reduction exothermic, endothermic reduction). However, a majority must be exothermic oxidation. (Think of burning things)
But in general you should look always to the reactant side and product side of any given reaction equation, forming bonds release energy so that reactions end up formed products is Exothermic, and breaking bonds will need energy to be done, so that chemical reactions end up products resulted from breaking old bonds between reactants will be endothermic reactions.

Is dissolving is a chemical reaction?
No, dissolving is not a reaction, because in solutions solute particles spread in between the spacing of the solvent molecules, reaction does not occur in the real sense of the chemical reaction, in the sense of the changing nature of articles solvent and solute

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