CHEMISTRY

Chemical Reactions

Chemical reactions are the process of arranging atoms and bonds when chemical substances come into contact. Chemical properties change by altering the arrangement of the atoms in that substance.

The chemicals that change are the reactants and the new substances that are formed or appear are the products:

style size 14px bold REACTANTS bold bold right arrow PRODUCTS end style

The chemical equations show what are the reactants, the products and other participants as catalysts and energy. The reactants are noted to the left of the arrow pointing to the products, in this way:

bold CH subscript bold 4 open parentheses bold g close parenthesis end subscript bold more bold 2 bold subscript 2 open parentheses bold g close parentheses end subscript bold right arrow bold CO subscript bold 2 open parentheses bold g close parentheses end subscript bold more bold 2 bold H subscript bold 2 bold or subscript open parentheses bold g close parenthesis end subscript

This is read as follows: a molecule of methane gas reacts with 2 molecules of oxygen to form a molecule of gaseous carbon dioxide and two molecules of water vapor.

Oxygen and methane are the reactants, carbon dioxide and water are products.

Characteristics of chemical reactions

  • They can be reversible or irreversible: If the products become reactants, the reaction is said to be reversible. If, on the other hand, when the products do not re-form the reactants that gave them their origin, we talk about irreversible reactions. For example, when sugar is transformed into caramel when heated, no matter how much we cool the caramel, it is impossible for it to turn into sugar again.
  • The atoms do not change: in a chemical reaction, the atoms remain the same, what changes is the way they are united between them.
  • Most of the reactions that occur in organisms take place in aqueous solutions, that is, the substances are dissolved in the water.
  • They can be simple, they require only one step for the reactants to become products. Other reactions are more complex, so they have several steps between reactants and products. Between the steps, intermediate compounds can be formed, which are neither reactants nor products.
  • The amount of atoms in the reactants is equal to the number of atoms in the products.

Balance the chemical reaction equation

In a chemical reaction the number of atoms does not change : if there were 10 hydrogen atoms in the reactants, there must be 10 hydrogen atoms in the products.

When we check that between reactants and products the amount of atoms are equal, we are talking about balancing the reaction equation . For example:

style size 18px bold H subscript open bold parenthesis g close parenthesis end subscript bold more bold or subscript open parentheses bold g close parentheses end subscript bold right arrow bold H subscript bold 2 bold O subscript open bold parenthesis l close parentheses end subscript end style

Hydrogen and oxygen are diatomic molecules, that is,

style size 18px bold H subscript bold 2 subscript open parentheses g close parenthesis end subscript bold more bold B subscript 2 subscript open parentheses g close parenthesis end subscript bold right arrow bold H subscript bold 2 bold OR subscript open bold parenthesis l close parentheses end subscript end style

But on one side there are 2 oxygen atoms and on the other side of the arrow there is a single atom of oxygen. We fix this by placing a 2 in front of the water:

style size 18px bold H subscript bold 2 subscript open parenthesis bold g close parenthesis end subscript bold more bold B subscript bold subscript open parentheses g close parenthesis end subscript bold right arrow bold 2 bold subscript bold 2 subscript or subscript open bold parentheses l close parentheses end subscript end style

Now we have two oxygen atoms on the left and two oxygen atoms on the right. We place a 2 in front of the H 2 on the left and the equation is balanced:

style size 18px bold 2 bold H subscript bold 2 subscript open parentheses bold g close parenthesis end subscript bold more bold subscript 2 subscript open parentheses bold g close parenthesis end subscript bold right arrow bold 2 bold subscript bold 2 subscript open parenthesis bold l close parentheses end subscript end style

How do chemical reactions occur?

Chemical reactions occur due to the collision or collision between reactants.

For the reactions to take place, the reactants must collide. Energy is required to break the bonds between atoms and when new unions are formed, energy is released.

The theory of collision states that the collisions between molecules can provide the energy necessary to break the bonds between atoms and in turn generate the energy to form new unions

An exothermic reaction is one where thermal energy is released, that is, the reactants have more energy than the products. Exo = exterior, releases energy.

An endothermic reaction is one where thermal energy is absorbed, that is, the products have more energy than the reactants. Endo = inside, energy enters.

When we cook we are facing an endothermic reaction.

Types of chemical reactions

Depending on what happens between reactants and products, the chemical reactions can be the following:

Precipitation reactions

When sodium chloride is added to a silver nitrate solution, the water-insoluble silver chloride drops.

When two solutions are mixed and a solid substance is formed that does not dissolve, we are in the presence of a precipitation reaction. The solid that forms is called a precipitate .

Precipitation is the process by which when a salt is formed from its soluble ions, it falls because it is not soluble in water.

Example

When AgNO 3 aqueous silver nitrate is added to a solution of KCl potassium chloride, silver chloride AgCl is formed which precipitates as a white solid. The reaction is as follows:

style size 18px AgNO subscript 3 open normal parenthesis l close parenthesis end subscript plus KCl subscript open normal parenthesis l close parenthesis end subscript right arrow AgCl subscript open normal parenthesis s close parenthesis end subscript plus KNO subscript 3 open normal parenthesis l close parentheses end subscript end style

We can know if a compound is soluble or not in water by following these rules:

Rules for predicting the solubility of compounds
SolubleInsoluble
All compounds containing ammonium NH + or alkali metal cationsAll carbonates, sulphites and phosphates (except those formed with NH + ammonium and alkali metal cations)
All nitrates and acetatesAll hydroxides (except those formed with NH 4+ ammonium and alkali metal cations)
All chlorides, bromides and iodides (except those formed with Ag, Pb, Hg)All sulfides (except those formed with NH +ammonium and alkali metal and alkaline earth metal cations)
All sulfates (except those formed with Ag, Pb, Hg (I), Ba, Sr, CaAll oxides (except those that are formed with Ca, Ba, and alkali metal cations)
Alkali metals: Lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), Francio (Fr); alkaline earth metals: Beryllium (Be), Magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra).

Acid-base or neutralization reactions

As the base is added to an acid, the pH increases.

An acid is a substance that increases the concentration of hydrogen ions (H + ) in a solution when some of its hydrogen atoms are dissociated. A base provides hydroxyl ions OH  or another negatively charged ion that reacts with H + in solution, lowering the H + concentration .

When an acid and a base react, a double displacement or neutralization reaction occurs because water forms. Example:

style size 18px normal H subscript 2 SO subscript 4 open parentheses ac close parenthesis end subscript plus 2 NaOH subscript open parentheses ac close parentheses end subscript right arrow Na subscript 2 SO subscript 4 open parentheses ac close parenthesis end subscript plus 2 normal H subscript 2 normal Or subscript open normal parenthesis l close parentheses end subscript end style

This reads: a molecule of sulfuric acid (H 2 SO 4 ) reacts with two molecules of sodium hydroxide (NaOH), forming a molecule of sodium sulfate (Na 2 SO 4 ) and two molecules of water.

Oxide-reduction reactions

In the reactions of oxide-reduction or also called reactions redox there is transfer of electrons between the reactantes. This transforms the oxidation states of the elements or compounds. The corrosion of iron tools is a classic example of an oxidation-reduction reaction.

Oxidation refers to the loss of electrons, reduction refers to the gain of electrons. An oxidizing agent is the species or substance that oxidizes another species, reducing in the process. A reducing agent is the species that reduces another species, oxidizing in the process.

style size 18px bold H subscript bold 2 bold S more bold bold Cl subscript bold 2 bold arrow bold 2 bold HCL bold more bold S style

In this case, hydrogen sulfide (H 2 S) is the reducing agent while chlorine (Cl 2 ) is the oxidizing agent.

Combination reactions

The combination reactions involve the union of elements or molecules to form a compound:

style size 18px bold Boldface bold B bold Bold arrow Bold AB style

Example

style size 18px bold 2 bold Na subscript open bold parenthesis s close parenthesis end subscript bold more bold Cl subscript bold 2 open parenthesis bold g close parenthesis end subscript bold right arrow bold 2 bold NaCl subscript open parentheses bold close parenthesis end subscript end style

Two sodium atoms combine with a gaseous chlorine molecule to give 2 molecules of sodium chloride.

Decomposition reactions

By means of electrolysis, water is separated into its components hydrogen and oxygen.

In the case of decomposition reactions, a compound is divided into its components:

style size 18px bold AB bold right arrow bold A bold more bold B end style

Example

style size 18px bold 2 bold H subscript bold 2 bold O subscript open bold parenthesis l close parenthesis end subscript bold right arrow bold 2 bold subscript 2 open parentheses bold g close parenthesis end subscript bold more bold or subscript bold 2 open bold parenthesis g close parentheses end subscript end style

Two water molecules are divided into two molecules of hydrogen and one molecule of oxygen.

The zinc Zn displaces in the silver nitrate the silver Ag that forms species of trees.

In the displacement reactions, one of the reactants substitutes a part of the other reactant, as in the following case:

style size 18px bold AB bold more bold C bold right arrow bold A bold more bold BC end style

Reactant C displaced component A of the AB molecule.

Example

style size 18px bold Cl subscript bold 2 open parenthesis bold g close parenthesis end subscript bold more bold 2 bold NaBr subscript open parentheses bold close parenthesis end subscript bold right arrow bold 2 bold NaCl subscript open parentheses bold s close parenthesis end subscript bold more bold Br subscript bold 2 open parentheses bold g close parentheses end subscript end style

A gaseous chlorine molecule displaces bromine in two molecules of sodium bromide, producing 2 molecules of sodium chloride and one molecule of bromine.

Double displacement reactions

Copper sulfate reacts with sodium carbonate to give sodium sulfate and copper carbonate in a double displacement reaction.

Within the displacement reactions, we can get reactions where the parts of the reactants are exchanged. These are known as double displacement reaction:

style size 18px bold Fe subscript bold 2 bold O subscript bold 3 bold more bold 6 bold HCl bold right arrow bold 2 bold FeCl subscript bold 3 bold bold 3 bold H subscript bold 2 bold O end style

This is read as iron oxide (III) reacts with six molecules of hydrochloric acid and forms two molecules of iron (III) chloride plus three water molecules.

Combustion reactions

A carbon-containing compound reacts with oxygen to produce carbon dioxide, water and heat.

style size 18px bold C subscript bold 3 bold H subscript bold 8 open parentheses bold g close parenthesis end subscript bold more bold 5 bold subscript 2 bold right arrow bold 3 bold subscript 2 open bold parenthesis g close parenthesis end subscript bold more bold 4 bold H subscript bold 2 bold O subscript open parentheses bold g close parentheses end subscript bold more bold heat end style

In this case a propane molecule reacts with five oxygen molecules from the air and produces 3 molecules of carbon dioxide and 4 molecules of water. The most important product of this reaction is heat.

Dismutation reactions

In the dismutation reactions a molecule is reduced and oxidized at the same time. These reactions are rare. The classic example is the dismutation of hydrogen peroxide:

style size 18px bold 2 bold H subscript bold 2 bold O subscript bold 2 open parentheses bold l close parenthesis end subscript bold right arrow bold 2 bold subscript bold 2 subscript open bold parenthesis l close parentheses end subscript bold more bold subscript 2 open parentheses bold g close parentheses end subscript end style

In this reaction the oxygen of H 2 O 2 has an oxidation state equal to -1. In H 2 O, the oxidation state is -2, meaning that it gained an electron and was reduced. In O 2 , the oxidation state is 0, that is, it lost an electron and oxidized.

Examples of chemical reactions

In our daily life we ​​are surrounded by chemical reactions. Some of the most common examples below.

Reaction of vinegar and sodium bicarbonate

The reaction between vinegar and bicarbonate is the most used.

Acetic acid, which we commonly know as vinegar, is a weak acid. Sodium bicarbonate, which is a base, reacts with vinegar in a two-stage reaction. The first stage is a double displacement reaction:

style size 18px bold NaHCO subscript bold 3 open bold parenthesis s close parentheses end subscript bold more bold CH subscript bold 3 bold COOH subscript open parentheses bold ac close parenthesis end subscript bold right arrow bold CH subscript bold 3 bold COONa subscript open bold parenthesis s close parenthesis end subscript bold more bold H subscript bold 2 bold CO subscript bold 3 subscript open parentheses bold ac close parentheses end subscript end style

Na sodium from the bicarbonate is changed to a hydrogen from the vinegar to form sodium acetate and carbonic acid. The carbonic acid is unstable and undergoes a decomposition reaction:

style size 18px bold H subscript bold 2 bold CO subscript bold 3 subscript open parentheses bold ac close parenthesis end subscript bold right arrow bold H subscript bold 2 bold subscript open parentheses l close parentheses end subscript bold more bold subscript B subscript 2 open parentheses bold g close parenthesis end subscript end style

Water and carbon dioxide are produced in this reaction. The bubbles we see in this reaction is carbon dioxide in gaseous form.

Reaction of antacids

The hydrochloric acid (red) reacts with some antacid tablets and the solution is neutralized (green).

Hydrochloric acid found in the stomach can sometimes cause discomfort or ulcers. To neutralize it, there are different compounds that are used as “antacids” that are actually bases. Examples of the neutralization reactions of HCl with various “antacids” follow below:

style size 18px bold When you open parentheses bold OH close parenthesis subscript bold 3 subscript open bold parenthesis s close parenthesis end subscript bold more bold 3 HCl subscript open parentheses bold ac close parenthesis end subscript bold right arrow bold bold subscript subscript 3 open bold parenthesis ac close parenthesis end subscript bold more bold 3 bold H subscript bold 2 bold O subscript open bold parenthesis l close parentheses end subscript end style

Aluminum hydroxide plus hydrochloric acid forms aluminum chloride and water.

style size 18px bold CaCO subscript bold 3 open bold parenthesis s close parenthesis end subscript bold more bold 2 bold HCl subscript open parentheses bold ac close parenthesis end subscript bold right arrow bold CaCl subscript bold 2 open parentheses bold ac close parenthesis end subscript bold more bold H subscript bold 2 bold OR subscript open bold parenthesis l close parentheses end subscript bold more bold CO subscript bold 2 open parentheses bold g close parentheses end subscript end style

Calcium carbonate plus hydrochloric acid forms calcium chloride, water and carbon dioxide.

style size 18px bold MgCO subscript bold 3 open parentheses s close parenthesis end subscript bold more bold 2 bold HCl subscript open parenthesis bold ac close parenthesis end subscript bold right arrow bold MgCl subscript bold 2 open parentheses bold ac close parentheses end subscript bold more bold H subscript bold 2 bold OR subscript open bold parenthesis l close parentheses end subscript bold more bold CO subscript bold 2 open parentheses bold g close parentheses end subscript end style

Magnesium carbonate plus hydrochloric acid forms magnesium chloride, water and carbon dioxide.

style size 18px bold Mg open bold parenthesis OH close parenthesis subscript bold 2 open bold parenthesis s close parenthesis end subscript bold more bold 2 HCl subscript open parentheses bold ac close parenthesis end subscript bold right arrow bold type MgCl subscript bold 2 open bold parenthesis ac close parenthesis end subscript bold more bold 2 bold H subscript bold 2 bold O subscript open bold parenthesis l close parentheses end subscript end style

Magnesium hydroxide plus chlorine forms magnesium chloride and water.

The basic structure of a battery is made up of plates of lead oxide PbO 2 and lead Pb, bathed in aqueous sulfuric acid H 2 SO 4 . The reaction that occurs in the battery is as follows:

style size 18px bold PbO subscript bold 2 bold more bold 4 bold H raised to bold more bold more bold SO subscript bold 4 raised to bold equal boldface 2 bold e raised to bold minus bold arrow right bold Pbso subscript bold 4 boldface more bold 2 bold H subscript bold 2 bold O end style

style size 18px bold Pb bold more bold SO subscript bold 4 raised to bold equal bold right arrow bold PbSO subscript bold 4 bold more bold 2 bold e raised to bold minus end style

Lead acts as a reducing agent while lead oxide acts as an oxidizing agent.

Acceleration of chemical reactions

A chemical reaction can be made faster in several ways:

  • Increasing the temperature: the speed of the reactants increases and they are more likely to collide with each other.
  • Increasing the concentration of the reactants: as there is a higher concentration of the reactants, the probability of a reaction is greater.
  • Using catalysts: these are substances that accelerate chemical reactions, without being part of the reactants or products. Living organisms depend on these types of substances, better known as enzymes, for chemical reactions to take place in cells.

Problems of chemical reactions solved

1. Based on the rules of solubility, predict which of the following substances are soluble in water:

Aluminum nitrate

Aluminum nitrate (soluble)

Magnesium chloride

Magnesium chloride (soluble)

Rubidium sulfate

rubidium sulfate (insoluble)

Nickel hydroxide (II)

Nickel (II) hydroxide (soluble)

Lead sulphide

lead (II) sulfide (insoluble)

Magnesium hydroxide

magnesium hydroxide (insoluble)

Iron phosphate (III)

iron (III) phosphate (insoluble)

2. When the following substances are mixed, what precipitate will be formed?

one) style size 14px bold FeSO subscript bold 4 open parentheses bold ac close parenthesis end subscript bold more bold KCl subscript open parentheses bold ac close parenthesis end subscript bold right arrow end style

No precipitate is formed, both the iron chloride FeCl 3 and the potassium sulphate K 2 SO 4 are soluble in water

style size 14px bold Fe subscript bold 2 open parentheses bold SO subscript bold 4 close parentheses subscript bold 3 open parentheses bold ac close parenthesis end subscript bold bold 6 bold KCl subscript open parentheses bold ac close parenthesis end subscript bold right arrow bold FeCl subscript bold 3 bold more bold 3 bold K subscript bold 2 bold SO subscript bold 4 end style

two) style size 14px bold Bold brackets left bold NOT subscript bold 3 bold parenthesis subscript bold 3 open parentheses bold ac close parentheses end subscript bold more bold Ba open parentheses bold OH close parentheses subscript bold 2 open parentheses bold ac close parenthesis end subscript bold arrow right end style

The precipitate that forms is the aluminum hydroxide Al (OH) 3

style size 14px bold 2 bold Bold brackets left bold NO subscript bold 3 bold parenthesis right subscript bold 3 open parentheses bold ac close parenthesis end subscript bold bold 3 bold B open parentheses bold close parenthesis subscript bold 2 open bold parenthesis ac close parentheses end subscript bold right arrow bold 2 bold Bold parenthesis left bold bold OH parenthesis right subscript bold 3 bold more bold 3 bold bold left parenthesis NO subscript bold 3 bold parenthesis right subscript bold 2 end style

3) style size 14px bold CaCl subscript bold 2 open parentheses bold ac close parenthesis end subscript bold more bold Na subscript bold 2 bold SO subscript bold 4 open parentheses bold ac close parenthesis end subscript bold right arrow end style

The precipitate that forms is calcium sultat CaSO 4

style size 14px bold CaCl subscript bold 2 open parentheses bold ac close parentheses end subscript bold more bold Na subscript bold 2 bold SO subscript bold 4 open parentheses bold ac close parentheses end subscript bold right arrow bold CaSO subscript bold 4 open bold parenthesis s close parentheses end subscript bold more bold 2 bold NaCl subscript open parentheses bold ac close parentheses end subscript end style

4) style size 14px bold K subscript bold 2 bold S subscript open parentheses bold ac close parenthesis end subscript bold more bold Neither open parenthesis bold NO subscript bold 3 close parenthesis subscript bold 2 open parentheses bold ac close parenthesis end subscript bold right arrow end style

The precipitate that forms is NiS nickel sulfide

style size 14px bold K subscript bold 2 bold type S subscript open parentheses bold ac close parenthesis end subscript bold more bold Neither open parentheses bold NO subscript bold 3 close parentheses subscript bold 2 open parentheses bold ac close parenthesis end subscript bold right arrow bold NiS subscript open bold parenthesis s close parentheses end subscript bold more bold 2 bold KNO subscript bold 3 open parentheses bold ac close parentheses end subscript end style

3. Write the balanced formula for the following acid-base reaction:

style size 14px bold HNO subscript bold 3 open parentheses bold ac close parenthesis end subscript bold more bold When opening parentheses bold OH close parentheses subscript bold 3 open bold parenthesis s close parentheses end subscript end style

Error converting from MathML to accessible text.

4. Write the balanced formula for the following acid-base reaction:

style size 14px bold HCl subscript open parentheses bold ac close parenthesis end subscript bold more bold Ca open parentheses bold OH close parentheses subscript bold 2 open parentheses bold ac close parentheses end subscript end style

style size 14px bold 2 bold HCl subscript open parentheses bold ac close parenthesis end subscript bold more bold Ca open bold parenthesis OH close parentheses subscript bold 2 open parentheses bold ac close parentheses end subscript bold right arrow bold CaCl subscript bold 2 subscript open bold parentheses ac close parenthesis end subscript bold more bold 2 bold H subscript bold 2 bold O end style

5. Write the balanced formula for the following acid-base reaction:

style size 14px bold HClO subscript bold 4 open parentheses bold ac close parentheses end subscript bold more bold Mg open parentheses bold OH close parenthesis subscript bold 2 open parentheses bold close parentheses end subscript end style

style size 14px bold 2 bold HCLO subscript bold 4 open parentheses bold ac close parentheses end subscript bold more bold Mg open parentheses bold OH close parentheses subscript bold 2 open parentheses bold close parenthesis end subscript bold right arrow bold type open bold parenthesis ClO subscript bold 4 close parentheses subscript bold 2 open parentheses bold ac close parentheses end subscript bold more bold 2 bold H subscript bold 2 bold O end style

6. Some of the substances most commonly used as antacids are magnesium oxide MgO, magnesium hydroxide Mg (OH) 2 and aluminum hydroxide AL (OH) 3 .

Write the balanced equation for the neutralization of hydrochloric acid for each of these substances.

Magnesium oxide

style size 14px bold 2 bold type MgO subscript open bold parenthesis s close parentheses end subscript bold more bold 2 bold HCL bold right arrow 2 bold MgCl subscript open parentheses bold ac close parentheses end subscript bold more bold subscript bold 2 subscript or subscript open parentheses bold l close parentheses end subscript end style

Magnesium hydroxide

style size 14px bold Mg open parentheses bold OH close parenthesis subscript bold 2 open parentheses bold type close parenthesis end subscript bold more bold 2 bold HCL right arrow bold MgCl subscript bold 2 open parentheses bold ac close parenthesis end subscript bold more bold subscript H 2 bold or subscript open bold parenthesis l close parentheses end subscript end style

Aluminum hydroxide

style size 14px bold When opening bold parenthesis OH close parenthesis subscript bold 3 open bold parenthesis s close parenthesis end subscript bold bold 3 bold HCl subscript open parentheses bold ac close parenthesis end subscript bold right arrow bold bold subscript subscript 3 open parentheses bold close parenthesis end subscript bold more bold 3 bold H subscript bold 2 bold O subscript open bold parenthesis l close parentheses end subscript end style

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