Unit #1Unit #2Unit #3Unit #4Unit #5Unit #6Unit #7Unit #8Unit #9Unit #10
spacerUNIT # 8  
spacerspacerIntroduction
spacerspacerObjectives
spacerspacerReading


spacer
8.1 Conventions
spacerand Symbols
spacer8.2 Balancing
spacerChemical
spacerEquations

spacer8.3 Methods of
spacerBalancing
spacerEquations

spacerBalance by Inspection
spacerBalance algebraically
spacer8.4 Types of
spacerChemical
spacerReactions
spacer8.5 Calculations
spacerBased on
spacerChemical
spacerEquations
spacerMole-mole calculations
spacerMass-mass calculations
spacerMass-mole calculations
spacerLimiting Reagent calculations



spacerProblems
spacer1 | 2
| 3 | 4

Unit #8CHEMICAL REACTIONS

8.1 - Conventions and Symbols

The reaction of hydrogen gas and oxygen gas produces water. This reaction requires an input of energy and can be written as:

  • a word equation (shown below in blue) or
  • a chemical equation (shown below in black using chemical formulas).



As we learned in unit 2, section 2.5.2, this is an exothermic reaction because energy is released. Contrast to the above exothermic reaction is the reverse reaction, the decomposition reaction of water.

The decomposition reaction of water, as given below, is an endothermic reaction. To decompose water into its elements, energy is required. A process to decompose water is by electrolysis.

We will cover the different types of chemical reactions, including decomposition reactions in section 8.4.

For now, let's focus on the symbols used to write a chemical reaction. The following table summarizes the symbols and terms used in chemical equations:

Symbols/Terms Meaning
Reactants
and Products
reactants are what is written on the left-hand-side of the arrow.
products are what is written on the right-hand-side of the arrow.
separates the reactants from the products
Physical states indicates the physical states of each species in the chemical reaction. For example,
  • (s) a solid substance,
  • (l) a liquid substance,
  • (g) a gaseous substance,
  • (aq) an aqueous solution (substance dissolved in water)
Coefficients are numbers written in front of each substance. If nothing is written in front, the default value is 1.

eg. the 2 written in front of H2 (g) is a coefficient.
the 2 written in front of H2O (l) is a coefficient.

In the decomposition reaction of water, it means that:
  • there are two molecules H2 (g) and 1 molecule of O2 (g) formed from the decomposition of two molecules of H2O (l). OR, scaled up,
  • there are two moles H2 (g) and 1 mole of O2 (g) formed from the decomposition of two moles of H2O (l).

 

Let's count the number of each type of atoms on both sides of the decomposition reaction of water:

  • The number of hydrogen atoms on the left-hand-side of the chemical equation:
    • Each water molecule has 2 hydrogen atoms.The coefficient of '2' tells us that there are two water molecules at the start of the reaction. Therefore, there are 2 H atoms x 2 =
      4 hydrogen atoms on the left-hand-side
      .
  • The number of oxygen atoms on the left-hand-side of the chemical equation:
    • each water molecule has 1 oxygen atom. The coefficient of '2' tells us that there are two water molecules at the start of the reaction. Therefore, there are 1 O atom x 2 =
      2 oxygen atoms on the left-hand-side
      .

  • The number of hydrogen atoms on the right-hand-side of the chemical equation:
    • two H2 (g) molecules are formed. Each H2 (g) molecules has 2 hydrogen atoms. The coefficient of '2' tells us that there are two H2 (g) produced. Therefore, there are 2 H atoms x 2 =
      4 hydrogen atoms on the right-hand-side
      .
  • The number of oxygen atoms on the right-hand-side of the chemical equation:
    • one O2 (g) molecules is formed. Each O2 (g) molecules has 2 oxygen atoms. There is no coefficient written in from of theO2 (g). By default, the coefficient is '1'. Therefore, there are 2 O atoms x 1 =
      2 oxygen atoms on the right-hand-side

Note: The number of each type of atoms on both sides of the chemical equation is the SAME.

No of atoms:
LHS
RHS
Hydrogen atoms
4
4
Oxygen atoms
2
2

This is not a coincident. The above reaction is called a 'balanced' chemical reaction. All chemical reactions must be 'balanced' because they must obey the Law of Conservation of Mass. Read on ....

Section 9.1

Definition of a Chemical Equation ..p216
Section 9.2
Terms, Symbols, and Their Meanings ..p217

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