- Chemical reaction concept
- Characteristics of chemical reactions
- Kinetics
- Conservation of dough
- Physical changes and / or changes of state
- Color variation
- Release of gases
- Temperature changes
- Parts of a chemical reaction
- Reagents and products
- Reaction media
- Vessels or reactors
- Types of chemical reactions
- - Oxidation-reduction (redox)
- Copper oxidation
- Iron and cobalt
- Iodine and manganese
- Combustion
- - Synthesis
- Ionic compounds
- Coordination
- - Decomposition
- Class volcano
- - Displacement
- Of hydrogen
- Of metals and halogens
- - Gas formation
- - Metathesis or double displacement
- Precipitation
- Base acid
- Examples of chemical reactions
- Displacement
- Neutralization
- Double scroll
- Redox
- Solved chemical reactions exercises
- - Exercise 1
- - Exercise 2
- - Exercise 3
- References
The chemical reactions are the subject suffers changes in the arrangement of their atoms, and when two substances are different compounds or contact. Changes arise in the process that can be seen immediately; such as an increase in temperature, cooling, gas formation, flashing or precipitation of a solid.
The most common chemical reactions often go unnoticed in everyday life; thousands of them are carried out in our bodies. Others, however, are more visible, since we can make them in the kitchen by selecting the correct utensils and ingredients; for example, mixing baking soda with vinegar, melting sugar in water, or acidifying red cabbage juice.
The reaction of baking soda and vinegar is an example of a recurring chemical reaction in cooking. Source: Kate Ter Haar (https://www.flickr.com/photos/katerha/5703151566)
In laboratories, chemical reactions become more usual and common; they all occur inside beakers, or Erlenmeyer flasks. If they share something in common, it is that none of them is simple, since they hide collisions, link breaks, mechanisms, link formation, energy and kinetic aspects.
There are chemical reactions so striking that hobbyists and scientists, knowing the toxicology of reagents and some safety measures, reproduce them on large scales in fascinating demonstration events.
Chemical reaction concept
Chemical reactions take place when a bond (ionic or covalent) is broken, so that another is formed in its place; two atoms or a set of them stop interacting strongly to originate new molecules. Thanks to this, the chemical properties of a compound, its reactivity, stability, with what it reacts can be determined.
In addition to being responsible for the chemical reactions that matter constantly transforms, without its atoms being affected, they explain the emergence of compounds as we know them.
Energy is required for bonds to break, and when bonds are formed it is released. If the energy absorbed is greater than that released, the reaction is said to be endothermic; we have a cooling of the surroundings. Whereas if the heat released is higher than the absorbed, then it will be an exothermic reaction; the surroundings are heated.
Characteristics of chemical reactions
Kinetics
Molecules in theory must collide with each other, carrying with them enough kinetic energy to promote the breaking of a bond. If their collisions are slow or inefficient, the chemical reaction is kinetically affected. This can happen either by the physical states of the substances, or by the geometry or structure of the same.
Thus, in a reaction, matter is transformed by absorbing or releasing heat, at the same time that it undergoes collisions that favor the formation of products; the most important components of any chemical reaction.
Conservation of dough
Due to the law of conservation of mass, the total mass of the assembly remains constant after a chemical reaction. Thus, the sum of the individual masses of each substance is equal to the mass of the result obtained.
Physical changes and / or changes of state
The occurrence of a chemical reaction can be accompanied by a change in the state of the components; that is, a variation in the solid, liquid or gaseous state of the material.
However, not all state changes involve a chemical reaction. For example: if water evaporates due to the effect of heat, the water vapor produced after this change of state is still water.
Color variation
Among the physical attributes that result from a chemical reaction, the change in the color of the reagents versus the color of the final product stands out.
This phenomenon is noticeable when observing the chemical reaction of metals with oxygen: when a metal oxidizes, it changes its characteristic color (gold or silver, as the case may be), to turn a reddish-orange hue, known as rust.
Release of gases
This characteristic is manifested as a bubbling or with the emission of particular odors.
Generally, bubbles appear as a consequence of subjecting a liquid to high temperatures, which incites an increase in the kinetic energy of the molecules that are part of the reaction.
Temperature changes
In case heat is a catalyst of the chemical reaction, a change in temperature will be induced in the final product. Hence, the input and output of heat in the process can also be a characteristic of chemical reactions.
Parts of a chemical reaction
Reagents and products
Any chemical reaction is represented by an equation of the type:
A + B → C + D
Where A and B are the reactants, while C and D are the products. The equation tells us that the atom or molecule A reacts with B to originate products C and D. This is an irreversible reaction, since the reactants cannot originate again from the products. On the other hand, the reaction below is reversible:
A + B <=> C + D
It is important to emphasize that the mass of the reactants (A + B) must be equal to the mass of the products (C + D). Otherwise, the dough would not be preserved. Likewise, the number of atoms for a given element must be the same before and after the arrow.
Above the arrow some specific specifications of the reaction are indicated: the temperature (Δ), the incidence of ultraviolet radiation (hv), or the catalyst used.
Reaction media
As far as life and the reactions that occur in our bodies are concerned, the reaction medium is aqueous (ac). However, chemical reactions can take place in any liquid medium (ethanol, glacial acetic acid, toluene, tetrahydrofuran, etc.) as long as the reagents are well dissolved.
Vessels or reactors
Controlled chemical reactions take place in a vessel, be it a simple glassware, or in a stainless steel reactor.
Types of chemical reactions
The types of chemical reactions are based on what happens at the molecular level; which bonds are broken and how the atoms end up joining. Likewise, it is taken into consideration whether the species gain or lose electrons; even though in most chemical reactions this occurs.
Here we explain the different types of chemical reactions that exist.
- Oxidation-reduction (redox)
Copper oxidation
In the example of patina an oxidation reaction takes place: metallic copper loses electrons in the presence of oxygen to transform into its corresponding oxide.
4Cu (s) + O 2 (g) => Cu 2 O (s)
Copper (I) oxide continues to oxidize to copper (II) oxide:
2Cu 2 O (s) + O 2 => 4CuO (s)
This type of chemical reaction in which the species increases or reduces their oxidation number (or state) is known as an oxidation and reduction (redox) reaction.
Metallic copper with oxidation state 0, first loses one electron, and then the second (oxidizes), while oxygen remains (reduces):
Cu => Cu + + e -
Cu + => Cu 2+ + e -
O 2 + 2e - => 2O 2-
The gain or loss of electrons can be determined by calculating the oxidation numbers for the atoms in the chemical formulas of their resulting compounds.
For Cu 2 O, it is known that because it is an oxide, it has the O 2- anion, so to keep the charges neutralized, each of the two copper atoms must have a +1 charge. Very similar happens with CuO.
Copper, when oxidized, acquires positive oxidation numbers; and oxygen, to be reduced, negative oxidation numbers.
Iron and cobalt
Additional examples for redox reactions are shown below. In addition, a brief comment will be made and the changes in oxidation numbers will be specified.
FeCl 2 + CoCl 3 => FeCl 3 + CoCl 2
If the oxidation numbers are calculated, it will be noted that those of Cl remain with a constant value of -1; not so, with those of the Faith and Co.
At first glance, iron has been oxidized while cobalt has been reduced. How do you know? Because iron now interacts not with two Cl anions - but with three, the chlorine atom (neutral) being more electronegative than iron and cobalt. On the other hand, the opposite happens to cobalt: it goes from interacting with three Cl - to two of them.
If the above reasoning is not clear, then we proceed to write the chemical equations of the net transfer of electrons:
Fe 2+ => Fe 3+ + e -
Co 3+ + e - => Co 2+
Therefore Fe 2+ is oxidized, while Co 3+ is reduced.
Iodine and manganese
6KMnO 4 + 5KI + 18HCl => 6MnCl 2 + 5KIO 3 + 6KCl + 9H 2 O
The chemical equation above may seem complicated, but it is not. Chlorine (Cl -) and oxygen (O 2-) experience gain or loss of their electrons. Iodine and manganese, yes.
Considering only the compounds with iodine and manganese, we have:
KI => KIO 3 (oxidation number: -1 to +5, loses six electrons)
KMnO 4 => MnCl 2 (oxidation number: +7 to +2, gains five electrons)
The iodine is oxidized, while the manganese is reduced. How to know without doing calculations? Because iodine goes from being with potassium to interacting with three oxygens (more electronegative); and manganese, for its part, loses interactions with oxygen to be with chlorine (less electronegative).
KI cannot lose six electrons if KMnO 4 gains five; that is why the number of electrons must be balanced in the equation:
5 (KI => KIO 3 + 6e -)
6 (KMnO 4 + 5e - => MnCl 2)
Which results in a net transfer of 30 electrons.
Combustion
Combustion is a vigorous and energetic oxidation in which light and heat are released. Generally, in this type of chemical reaction oxygen participates as an oxidizing or oxidizing agent; while the reducing agent is the fuel, which burns at the end of the day.
Where there is ashes, there was combustion. These are essentially composed of carbon and metallic oxides; although its composition logically depends on what the fuel was. Below are some examples:
C (s) + O 2 (g) => CO 2 (g)
2CO (g) + O 2 (g) => 2CO 2 (g)
C 3 H 8 (g) + 5O 2 (g) => 3CO 2 (g) + 4H 2 O (g)
Each of these equations correspond to complete combustions; that is to say, all the fuel reacts with an excess of oxygen to guarantee its complete transformation.
Likewise, it should be noted that CO 2 and H 2 O are the main gaseous products when carbonaceous bodies burn (such as wood, hydrocarbons and animal tissues). It is inevitable that some carbon allotrope is formed, due to insufficient oxygen, as well as less oxygenated gases such as CO and NO.
- Synthesis
Graphic representation of a synthesis reaction. Source: Gabriel Bolívar.
The image above shows an extremely simple representation. Each triangle is a compound or atom, which join to form a single compound; two triangles form a parallelogram. The masses increase and the physical and chemical properties of the product are, many times, very different from those of its reagents.
For example, the combustion of hydrogen (which is also a redox reaction) produces hydrogen oxide or oxygen hydride; better known as water:
H 2 (g) + O 2 (g) => 2H 2 O (g)
When both gases are mixed, at a high temperature, they burn producing gaseous water. As the temperatures cool, the vapors condense to give liquid water. Several authors consider this synthesis reaction as one of the possible alternatives to substitute fossil fuels in obtaining energy.
The HH and O = O bonds break to form two new single bonds: HOH. Water, as is well known, is a unique substance (beyond the romantic sense), and its properties are quite different from gaseous hydrogen and oxygen.
Ionic compounds
The formation of ionic compounds from their elements is also an example of a synthesis reaction. One of the simplest is the formation of metal halides of groups 1 and 2. For example, the synthesis of calcium bromide:
Ca (s) + Br 2 (l) => CaBr 2 (s)
A general equation for this type of synthesis is:
M (s) + X 2 => MX 2 (s)
Coordination
When the compound formed involves a metallic atom within an electronic geometry, it is then said that it is a complex. In complexes, metals remain attached to ligands by weak covalent bonds, and are formed by coordination reactions.
For example, you have the 3+ complex. This is formed when the Cr 3+ cation is in the presence of the ammonia molecules, NH 3, which act as chromium ligands:
Cr 3+ + 6NH 3 => 3+
The resulting coordination octahedron around the chromium metal center is shown below:
Coordination octahedron for the complex. Source: Gabriel Bolívar.
Note that the 3+ charge on chromium is not neutralized in the complex. Its color is purple, and that is why the octahedron is represented with that color.
Some complexes are more interesting, as in the case of certain enzymes that coordinate iron, zinc and calcium atoms.
- Decomposition
Decomposition is the opposite of synthesis: a compound breaks down into one, two, or three elements or compounds.
For example, we have the following three decompositions:
2HgO (s) => 2Hg (l) + O 2 (g)
2H 2 O 2 (l) => 2H 2 O (l) + O 2 (g)
H 2 CO 3 (aq) => CO 2 (g) + H 2 O (l)
HgO is a reddish solid that, under the action of heat, decomposes into metallic mercury, a black liquid, and oxygen.
Hydrogen peroxide or hydrogen peroxide undergoes decomposition, giving liquid water and oxygen.
And carbonic acid, for its part, decomposes into carbon dioxide and liquid water.
A "drier" decomposition is that suffered by metallic carbonates:
CaCO 3 (s) => CaO (s) + CO 2 (g)
Class volcano
Ammonium dichromate volcano burning. Source: Наталия
A decomposition reaction that has been used in chemistry classes is the thermal decomposition of ammonium dichromate, (NH 4) 2 Cr 2 O 7. This orange-colored carcinogenic salt (so it must be handled with great care), burns to release a lot of heat and produce a green-colored solid, chromic oxide, Cr 2 O 3:
(NH 4) 2 Cr 2 O 7 (s) => Cr 2 O 3 (s) + 4H 2 O (g) + N 2 (g)
- Displacement
Graphic representation of a displacement reaction. Source: Gabriel Bolívar.
Displacement reactions are a type of redox reaction in which one element displaces another in a compound. The displaced element ends up reducing or gaining electrons.
To simplify the above, the image above is shown. The circles represent an element. It is observed that the lime green circle displaces the blue one, remaining on the outside; but not only that, but the blue circle shrinks in the process, and the lime green one oxidizes.
Of hydrogen
For example, we have the following chemical equations to expose the above explained:
2Al (s) + 6HCl (aq) => AlCl 3 (aq) + 3H 2 (g)
Zr (s) + 2H 2 O (g) => ZrO 2 (s) + 2H 2 (g)
Zn (s) + H 2 SO 4 (aq) => ZnSO 4 (aq) + H 2 (g)
What is the displaced element for these three chemical reactions? Hydrogen, which is reduced to molecular hydrogen, H 2; it goes from an oxidation number of +1 to 0. Note that the metals aluminum, zirconium and zinc can displace the hydrogens of acids and water; while copper, neither silver nor gold, cannot.
Of metals and halogens
Likewise, there are these two additional displacement reactions:
Zn (s) + CuSO 4 (aq) => Cu (s) + ZnSO 4 (aq)
Cl 2 (g) + 2NaI (aq) => 2NaCl (aq) + I 2 (s)
In the first reaction, zinc displaces the less active metal copper; zinc oxidizes while copper is reduced.
In the second reaction, on the other hand, chlorine, an element more reactive than iodine, displaces the latter in the sodium salt. Here it is the other way around: the most reactive element is reduced by oxidizing the displaced element; therefore, chlorine is reduced by oxidizing iodine.
- Gas formation
In the reactions it could be seen that several of them generated gases, and therefore also enter into this type of chemical reaction. Likewise, the reactions of the previous section, that of hydrogen displacement by an active metal, are considered gas formation reactions.
In addition to those already mentioned, metal sulfides, for example, release hydrogen sulfide (which smells like rotten eggs) when hydrochloric acid is added:
Na 2 S (s) + 2HCl (aq) => 2NaCl (aq) + H 2 S (g)
- Metathesis or double displacement
Graphic representation of a double displacement reaction. Source: Gabriel Bolívar.
In the metathesis or double displacement reaction, what occurs is a change of partners without electron transfers; that is, it is not considered a redox reaction. As can be seen in the image above, the green circle breaks the link with the dark blue one to link to the light blue circle.
Precipitation
When the interactions of one of the partners are strong enough to overcome the solvation effect of the liquid, a precipitate is obtained. The following chemical equations represent precipitation reactions:
AgNO 3 (aq) + NaCl (aq) => AgCl (s) + NaNO 3 (aq)
CaCl 2 (aq) + Na 2 CO 3 (aq) => CaCO 3 (s) + 2NaCl (aq)
In the first reaction, Cl - displaces NO 3 - to form silver chloride, AgCl, which is a white precipitate. And in the second reaction, CO 3 2- displaces Cl - to precipitate calcium carbonate.
Base acid
Perhaps the most emblematic of the metathesis reactions is that of acid-base neutralization. Finally, two acid-base reactions are shown as examples:
HCl (aq) + NaOH (aq) => NaCl (aq) + H 2 O (l)
2HCl (aq) + Ba (OH) 2 (aq) => BaCl 2 (aq) + 2H 2 O (l)
The OH - displace the Cl - to form water and chloride salts.
Examples of chemical reactions
Below and below, mention will be made of some chemical reactions with their respective equations and comments.
Displacement
Zn (s) + AgNO 3 (aq) → 2Ag (s) + Zn (NO 3) 2 (aq)
Zinc displaces silver in its nitrate salt: it reduces it from Ag + to Ag. As a result, metallic silver begins to precipitate in the medium, observed under the microscope like silver trees without leaves. On the other hand, nitrate combines with the resulting Zn 2+ ions to form zinc nitrate.
Neutralization
CaCO 3 (s) + 2HCl (aq) → CaCl 2 (aq) + H 2 O (l) + CO 2 (g)
Hydrochloric acid neutralizes the calcium carbonate salt to produce a salt, calcium chloride, water, and carbon dioxide. CO 2 bubbles up and is detected in the water. This bubbling is also obtained by adding HCl to the chalk or egg shells, rich in CaCO 3.
NH 3 (g) + HCl (g) → NH 4 Cl (s)
In this second reaction, the HCl vapors neutralize the gaseous ammonia. The ammonium chloride salt, NH 4 Cl, forms as a whitish smoke (lower image), as it contains very fine particles suspended in the air.
Ammonium chloride formation reaction. Source: Adam Rędzikowski
Double scroll
AgNO 3 (aq) + NaCl (aq) → AgCl (s) + NaNO 3 (aq)
In a double displacement reaction there is an exchange of "partners". Silver changes partners with sodium. The result is that the new salt, silver chloride, AgCl, precipitates as a milky solid.
Redox
Heat, sound, and blue light are released in the Barking Dog chemical reaction. Source: Maxim Bilovitskiy via Wikipedia.
There are countless redox reactions. One of the most impressive is that of Barkin Dog:
8 N 2 O (g) + 4 CS 2 (l) → S 8 (s) + 4 CO 2 (g) + 8 N 2 (g)
The energy released when the three stable products are formed is so great that a bluish flash is produced (upper image) and a resounding increase in pressure caused by the gases produced (CO 2 and N 2).
And also, all this is accompanied by a very loud sound similar to the barking of a dog. The sulfur produced, S 8, coats the internal walls of the tube in yellow.
Which species is reduced and which one is oxidized? As a general rule, the elements have oxidation number 0. Therefore, the sulfur and nitrogen in the products must be the species that gained or lost electrons.
Sulfur oxidized (lost electrons), as it had oxidation number -2 in CS 2 (C 4+ S 2 2-):
S 2- → S 0 + 2e -
While nitrogen was reduced (gained electrons), because it had oxidation number +1 in the N 2 O (N 2 + O 2-):
2N + + 2e → N 0
Solved chemical reactions exercises
- Exercise 1
What salt precipitates in the following reaction in aqueous medium?
Na 2 S (aq) + FeSO 4 (aq) → ¿?
As a general rule, all sulfides, with the exception of those formed with alkali metals and ammonium, precipitate in aqueous medium. There is a double displacement: iron binds to sulfur, and sodium to sulfate:
Na 2 S (aq) + FeSO 4 (aq) → FeS (s) + Na 2 SO 4 (aq)
- Exercise 2
What products will we get from the following reaction?
Cu (NO 3) 2 + Ca (OH) 2 → ¿?
Calcium hydroxide is not very soluble in water; but the addition of copper nitrate helps to solubilize it because it reacts to form its corresponding hydroxide:
Cu (NO 3) 2 (aq) + Ca (OH) 2 (aq) → Cu (OH) 2 (s) + Ca (NO 3) 2 (aq)
Cu (OH) 2 is instantly recognizable as a blue precipitate.
- Exercise 3
What salt will be produced in the next neutralization reaction?
Al (OH) 3 (s) + 3HCl (aq) →?
Aluminum hydroxide behaves like a base by reacting with hydrochloric acid. In an acid-base (Bronsted-Lowry) neutralization reaction, water always forms, so the other product must be aluminum chloride, AlCl 3:
Al (OH) 3 (s) + 3HCl (aq) → AlCl 3 (aq) + 3H 2 O
This time the AlCl 3 does not precipitate because it is a salt (to some extent) soluble in water.
References
- Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
- Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
- Ana Zita. (November 18, 2019). Chemical reactions. Recovered from: todamateria.com
- Kashyap Vyas. (January 23, 2018). 19 Cool Chemical Reactions that Prove Science is Fascinating. Recovered from: interestingengineering.com
- BeautifulChemistry.net (nd). Reaction. Recovered from: beautifulchemistry.net
- Wikipedia. (2019). Chemical reaction. Recovered from: en.wikipedia.org