Chemical Reactions and Equations

by: Prof. Liwayway Memije-Cruz

Chemical Reaction

Chemical Reactions

• 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.

• it 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.

Chemical Equations

• Provide 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.

• 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.

Steps in Writing a Balance Equation

• 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

Law of Conservation of Mass

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.Example:• 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”.

Types of Chemical Equations

Combination

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

Combination Reaction

Combination Reaction

Decomposition

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

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.

Activity series

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.

The activity series is used to predict the products of replacement

reaction.

Double Decomposition

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

Examples:AgNO3 + HCI → AgCl +HNO3

Ba(NO3)2 + 2NaOH → Ba (OH)2 + 2NaNO3

References:

• http://chemistry.about.com/od/chemicalreactions/a/reactiontypes.htm

• http://www.wisegeek.org/what-are-chemical-reactions.htm