Chemical Reactions and Chemical Equations



A chemical reaction is all about chemical change. The ripening of fruits, photosynthesis, tarnishing of iron, burning of woods, digestion of foods, and even cooking foods are few examples of chemical changes and chemical reactions happening around us and even inside our bodies. A chemical reaction involves the transformation of one or more substances into another substance or substances. it involves a change in composition and is represented by a chemical equation.

A chemical equation provides a concise picture of a chemical change. It is used to convey the pertinent information about the chemical reaction which includes the substances involved and their quantitative ratio.

Chemical equations are representations of chemical reactions in terms of symbols of elements and formulas of compounds involved in the reactions. The substances that enter into a chemical reaction are called reactants and the substances formed are the products.

An example of a chemical equation
An example of a chemical equation

Amazing Chemical Reactions

Writing and Balancing Chemical Equations

Steps in Writing a Balance Equation

  1. Write the symbols and formulas of the reactant/s on the left side of the arrow and symbol/s and formula/s of the products on the right. Monoatomic elements are represented by their symbols without subscript. Examples: Ca, Mg and Zn. Diatomic elements are represented by their symbols with subscript 2. Examples: H2, O2, N2, F2, CI2, Br2 and I2
  2. Chemical changes occur in accordance with the Law of Conservation of Mass. It is therefore necessary to balance the number of atoms of each element in the reactants with the number of atoms of the same element in the product. Balancing chemical equations by inspection simply requires placing the coefficient before any of the symbol/s and formula/s until there are exactly the same numbers of each kind of atom on both sides of the equation.
  • Pointers to consider in using coefficient:
  1. There is no need to write a coefficient, which is 1.
  2. Use simplest whole numbers as coefficients.


Write the balance chemical equation for the reaction of hydrogen with oxygen to produce water.

2 H2 + O2 2H2O

“The reaction of 2 moles of hydrogen and 1 mole of oxygen yields 2 moles of water”.

Symbols used in Writing Chemical Equations

Symbols used in writing chemical equations
Symbols used in writing chemical equations

Law of Conservation of Mass and Balancing of Chemical Equations

Types of Chemical Reactions

1. Combination reaction is a type of reaction in which two or more substances (either elements or compounds) react to form one product.

Example: 2 Na + Cl2 → 2NaCl

2. Decomposition reaction is a type of reaction in which one compound decomposes to form two or more new substances. Usually heat is to cause these reactions to take place.

Types of Decomposition reaction:

a. Hydrates - when heated, decompose to yield water and anhydrous salt. A hydrate is a salt that contains one or more molecules of water for each formula unit of salt, built right into the crystal structure.

Example: BaCl2 2H2O → BaCl2 + 2H2O

b. Chlorates – when heated, decompose to form chlorides and oxygen gas.

Example: 2KCIO3 → 2KCI + 3O2

c. A few metal oxides decompose when heated to form the free metal and oxygen gas.

Example: 2HgO → 2Hg + 02

d. Most carbonates when heated, decompose to form oxides and CO2.

Example: CaCO3 → CaO + CO2

The group IA carbonates do not decompose readily.

e. Most hydrogen carbonates (bicarbonates), when heated decompose to form an oxide plus water and CO2.

Example: Ca(HCO3) → CaO = 2CO2 + H2O

When hydrogen carbonates of the Group IA metals are heated they form a carbonate plus water and CO2.

Example: 2NaHCO3 → Na2CO3 + CO2 + H2O

3. Substitution or Replacement reaction is a type of reaction in which a metal replaces another metal ion from a solution or a nonmetal replaces a less active nonmetal in a compound.

The activity series is used to predict the products of replacement reaction. In using this series, any free metal that is higher on the list will displace from a solution another metal that is lower. Hydrogen is included in the series although it is not a metal. Any metal above hydrogen in the series will displace hydrogen gas from an acid.

Activity series of metals

The activity series is used to predict the products of replacement reaction.
The activity series is used to predict the products of replacement reaction.

4. Double Decomposition reaction is a type of reaction wherein two compounds react to form two new compounds. This involves exchange of ion pairs.


AgNO3 + HCI → AgCl +HNO3

Ba(NO3)2 + 2NaOH → Ba (OH)2 + 2NaNO3

Oxidation Numbers

Oxidation numbers are arbitrary numbers based upon the following rules:

1. The oxidation number of uncombined elements is zero.

2. The common oxidation state of hydrogen in compound is +1, -1 for hydrites. For oxygen, it is -2.

3. The common oxidation state for Group VIIA elements in binary compounds is -1. It varies in tertiary compounds.

4. The common oxidation state for Group IA ions is +1; for Group IIA is +2, and for Group IIIA is +3.

5. The oxidation state for an ion is calculated if the oxidation states of all other ion in the compound are known, since the sum of all oxidation states in a compound is zero.


1. Calculate the oxidation state of Mn in KmnO4.

Assign the oxidation number of the other ions and let x be the oxidation number of Mn.

+1 x -2

K Mn O4

Applying rule no. 5

(+1) + (X) + (-2)4 = 0

1 + X -8 = 0

X = +7

Therefore the oxidation state of Mn in KMnO4 is +7

2. Calculate the oxidation number of Cl in Mg(ClO3)2.

+2 X -2

Mg (Cl 03)2

(+2) 1 + (X) + (-2)6 = 0

X = +5

Therefore the oxidation state of Cl in Mg(ClO3)2 is +5

Oxidation-Reduction Reactions

Oxidation is a chemical change in which electrons are lost by an atom or group of atoms, and reduction is a chemical change in which electrons are gained by an atom or group of atoms. A transformation that converts a neutral atom to a positive ion must be accompanied by the loss of electrons and must, therefore, be an oxidation.

Example: Fe = Fe+2 + 2e

Electrons (e) are written explicitly on the right side and provide equality to total charge on the two sides of the equation. Similarly, the transformation of neutral element to an anion must be accompanied by electron gain and is classified as a reduction.


Cl2 + 2e = 2Cl­­­-

S + O2 → SO2

Since the oxidation number of the sulfur atom increase from 0 to +4, sulfur is said to be oxidized. The oxidation number of the oxygen atom decreases from 0 to -2, then the oxygen is said to be reduced.

In an oxidation-reduction reaction, the substance that is oxidized is called the reducing agent since it is causes something else to be reduced. On the other hand, the substance that is reduced causes something else to be oxidized and it is called oxidizing agent.

Oxidation Number Method of Balancing Oxidation-Reduction Equations

Given the following expression:

HNO3 + H2S → NO + S + H2O

Balance the redox reaction using the oxidation number method:

1. Determine the oxidation numbers of the atoms/ions in the equation in order to identify atoms/ions undergoing oxidation or reduction.

HNO3 + H2S → NO + S H2O

Nitrogen is reduced from +5 to +2, a decrease of 3, and sulfur is oxidized from -2 to 0, an increase of 2.

2. Balance the total increase and the total decrease of oxidation number by placing the necessary coefficients, using the smallest number possible.

We have a decrease of 3 and an increase of 2 as indicated in the unbalanced expression. The lowest common multiple of 3 and 2 is 6. Since:

(-1) (2) = -6

(+2) (3) = +6


2HNO3 + 3H2S → 2NO + 3S + H2O

3. Complete the balancing by inspection.

Balance those substances that changed in oxidation number and then any atoms other than hydrogen and oxygen. Then finally balance hydrogen and oxygen.

2HNO3 + 3H2S → 2NO + 3S + 4H2O

HNO is the oxidizing agent and H2S is the reducing agent.

Oxidation-reduction reaction
Oxidation-reduction reaction

Factors Affecting The Rates of Chemical Reactions

For a chemical reaction to take place, the molecules/ions of the reacting substances must collide. However, not all collisions may result to chemical change. For a collision to be effective, the colliding particles must be in the right orientation and must possess the necessary energy to reach the activation energy.

Activation energy is the added energy that reacting substances must have in order to take part in a chemical reaction. Any factor that affects the frequency and effectiveness of collisions of reacting substances also affects the rate of chemical reaction, which is the rate of formation of products or the rate of disappearance of reactants. These rates maybe affected by the following factors:

1. Nature of Reactants

The nature of the reactants determines the nature of the activation energy or the height of the energy barrier that must be overcome in order for the reaction to take place. Reactions with low activation energy occur rapidly while those of higher activation energy occur slowly. Ionic reactions occur rapidly since the ions have an attraction for each other and therefore do not need additional energy. In covalent molecules, the collisions may not be enough to break the bonds, hence have higher activation energy.

2. Concentration of Reactants

Concentration of a substance Is a measure of the number of molecules in a given volume. The rate of reaction of reaction increases as the molecules become more concentrated and become more crowded, hence, there is an increase in the frequency of collisions. Concentration can be expressed as moles per liter for reactions carried out in liquid solutions. For reactions involving gases, the concentration is expressed in terms of the pressure of the individual gases.

3. Temperature

An increase in temperature will cause the molecules to move rapidly resulting in more collisions. Because they are moving rapidly, they have sufficient energy and they collide with greater impact.

4. Catalyst

A catalyst is a substance that alters the speed of reaction without itself undergoing a permanent chemical change. Catalyst are usually used to increase the speed of chemical reaction, but there are also catalysts called inhibitors or negative catalysts, which slow down a chemical reaction.


2SO2 + O2 → 2SO2 (SLOW)

2NO + O2 → 2NO2 (FASTER)

The catalyst forms an intermediate compound with one of the reactants.

NO2 + SO2 → SO3 + NO

The catalyst is regenerated

Catalysts are important in industrial processes because aside from increasing the production, their use cut off production cost. Enzymes, which are the biological catalysts, metabolize reactions in our body.


Invertase cleaves sucrose (table sugar) into simple sugars, glucose and fructose, glucose in the form absorbed in the bloodstream

Factors Affecting the Rate of Chemical Reactions
Factors Affecting the Rate of Chemical Reactions

Questions for Study and Review

I. Write a balanced equation that describes each of the following chemical reactions:

  1. When heated, pure aluminum reacts with air to give Al2O3.
  2. CaSO4 • 2H2O, decomposes when heated, giving calcium sulfate, CaSO4, and water.
  3. During photosynthesis in plants, carbon dioxide and water are converted into glucose, C6H12O6, and oxygen, O2.
  4. Water vapor reacts with sodium metal to produce gaseous hydrogen, H2, and solid sodium hydroxide, NaOH.
  5. Acetylene gas, C2H2, burns in air forming gaseous carbon dioxide, CO2 and water.

II. Balance the following equations and indicate the type of reaction:

  1. K + CI → KCI
  2. AI + H2SO4 → AI2(SO4)3 + H2
  3. CuCO3 + HCI → H2O + CO2
  4. MnO2 + KOH → H2O + K2MnO4
  5. AgNO3 + NaOH → Ag2O + NaNO3
  6. C6H6 + O2 → CO2 + H2O
  7. N2 + H2 → NH3
  8. Na2CO3 + HCI → NaCI + CO2 + H2O
  9. MgCI2 + Na3PO4 → Mg3(PO4)2 + NaCI
  10. P2O5 + H2O → H3PO4

III. Balance the following redox equations using the oxidation number method. Be able to identify the oxidizing and reducing agent.

  1. HNO3 + H2S → NO + S + H2O
  2. K2Cr2O7 + HCl → KCl + Cr + Cl2 + H2O + Cl

IV. Choose the condition, which will have a higher reaction rate and identify the factor affecting the rate of reaction.

1. a. 3 moles of A reacting with 1 mole of B

b. 2 moles of A reacting with 2 moles of B

2. a. A2 + B2 ----- 2AB at 200 C

b. A2 + B2 ----- 2AB at 500 C

3. a. A + B ----- AB

b. A + C ----- AC

AC + B ----- C

4. a. Iron exposed in moist air

b. Silver exposed in moist air

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Comments 2 comments

Angelica Salinas profile image

Angelica Salinas 12 months ago

Angelica G. Salinas

BSBA 1-1


A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. The substance (or substances) initially involved in a chemical reaction are called reactants or reagents. Chemical reactions are usually characterized by a chemical change, and they yield one or more products, which usually have properties different from the reactants. Chemical reactions are described with chemical equations, which symbolically present the starting materials, end products, and sometimes intermediate products and reaction conditions. Chemical equations are used to graphically illustrate chemical reactions. They consist of chemical or structural formulas of the reactants on the left and those of the products on the right. They are separated by an arrow (→) which indicates the direction and type of the reaction; the arrow is read as the word "yields".

ismaellozano123 profile image

ismaellozano123 11 months ago from Baliuag Bulacan Philippines

Lozano, John Ismael E. (BSA 1-2)

According to what I have read, cell physiology is the physical process and activities of cell in our body (even in other organisms).it is the reason why all living organisms maintain their lives on their life span. It is the most important of all about cells because it includes the important process of cells (Membrane Transport, Photosynthesis, Protein Synthesis, Cell Division, and cellular Respiration) that essential to us to be learned.

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