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How to Balance a Chemical Equation

Chemistry Core with Orlando Broomfield

Learning Goal

Chemical Equations and Chemical Reactions

  • Chemical equations are simple summaries of chemical reactions.
  • Types of Reactions:
    • Synthesis
      • A + B AB
    • Decomposition
      • AB A + B
    • Single displacement / Substitution
      • A + BC AC + B
    • Ionic precipitation / Double displacement
      • AB + CD AD + CB
    • Neutralization
      • Acid + base salt + water
    • Combustion
      • A + O₂ CO₂ + H₂O
  • In a chemical equation, the reactants are found on the left-hand side, and the products are found on the right-hand side.

                 left-hand side                            right-hand side
                    Reactants                                 Products
                        A + B                                        C + D
             ‘+ reacts with      ‘produces           ‘+and

  • Subscript shows how many atoms of an element are in a molecule/compound. 
    • e.g., H₂O [two atoms of hydrogen (H)‏; one atom of oxygen (O)‏]
  • Coefficient shows how many molecules there are of a particular chemical.
    • e.g., 3 H₂O means there are 3 water

Word Chemical Equations

Word equations use the name of the substances involved in the chemical reaction to describe the chemical equation. A word equation is the easiest way to describe a chemical reaction.

  • For e.g., when sodium reacts with chlorine, sodium chloride is the product formed. 
  • How do we write the word equation for this reaction? 

           sodium + chlorine sodium chloride

However, chemists usually summarize reactions using symbol equations. Symbol equations tell us the number and type of elements on each side of the equation. The elements can exist on their own, or in compounds. Therefore, we need to develop the skill of naming compounds.

Naming & Writing Compounds

These are general rules to follow when writing the symbol for elements.

  1. Metallic elements in main groups I. II, III of the Periodic Table, and the transition metals are represented by:
    • the symbol for the element
      • For e.g., calcium has the symbol Ca
  2. Non-metallic elements, such as the elements in the main groups V, VI, VII of the Periodic Table are represented as:
    • diatomic molecules (they form molecules in nature).
      • For e.g., oxygen has the symbol O₂, and that of chlorine is Cl₂.
  3. Ionic compounds, formed when metals and non-metals combine, are represented with:
    • The metal first, followed by the non-metal
      • For e.g., fluorine and magnesium will react to form magnesium fluoride.
    • ‘–ide’ if there are only two elements
      • For e.g., beryllium and oxygen, forming beryllium oxide.
    • ‘–ite’ or ‘–ate’ if there is a third element, which is usually oxygen
      • For e.g., magnesium, sulfur and oxygen, forming magnesium sulfate
  4. Covalent compounds, formed by a combination of non-metals, are represented with prefixes:
    • mono’ – meaning one (1) 
      • For e.g., one carbon atom combining with one oxygen atom, forming carbon monoxide [CO]
    • di’ – meaning two (2) 
      • For e.g., one carbon atom combining with two oxygen atoms, forming carbon dioxide [CO₂]
    • tri’ – meaning 3, such as SO₃ sulfur trioxide
    • tetra’ – meaning 4, such as CCl₄ tetrachloromethane
    • penta’ – meaning 5, such as P₂O₅ diphosphorus pentoxide
  5. For transition metals with roman numerals, the number represents 
    • the charge on the metal
      • e.g., copper(I)    –    Cu²⁺
      • e.g., iron(II)       –    Fe²⁺
      • e.g., iron(III)     –    Fe³⁺

How to balance a Chemical Equation

A balanced chemical equation shows equal numbers of each element on both sides of the equation. If an equation obeys the Law of Conservation of Mass, then it is balanced.

Step 1

Make sure that all formulae are accurately written for each reactant and product.

Step 2

Be aware that subscripts cannot be added, removed, or changed, as they are part of the identity/make-up of the chemical substances.

Step 3

If the equation is not already balanced, balance the chemical equation by placing coefficients in front of chemical formulae, never in the middle of a formula.

Step 4

Add state symbols to the right of each chemical substance to indicate their physical states during the reaction.
solid – (s) liquid – (l) gas (g) aqueous (aq)

More tips.

Through trial and improvement, place numbers in front of the chemical formulae to balance the equation. Place reaction conditions, such as temperature, catalysts, and pressure, above and below the arrow.

Solubility Rules

  Soluble compounds include:

o   Group 1 metal cations (Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) and the ammonium ion (NH₄⁺)

o   The acetate (C₂H₃O₂⁻), bicarbonate (HCO₃⁻), nitrate (NO₃⁻) ions

o   The halide ions (Cl⁻, Br⁻, I⁻)

  • Exceptions: Ag⁺, Hg²⁺, and Pb²⁺

o   The sulfate ion (SO₄²⁻)

  • Exceptions: Ag⁺, Pb²⁺, Ba²⁺

  Insoluble compounds include:

o   Carbonate (CO₃²⁻), hydrogen carbonate (HCO₃⁻), phosphate (PO₄³⁻), and hydroxide (OH⁻) ions

  • Exceptions: group 1 metals (Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) and the ammonium ion (NH₄⁺)

Demonstration on Balancing Chemical Equations

  • Question: 

Write a balanced Chemical equation with state symbols for the reaction that occurs when lithium burns in steam to form lithium hydroxide and hydrogen gas.

Balancing Method

Step 1 – Write down the word and symbol equations.

  • Word equation:
    • Lithium + steam lithium hydroxide + hydrogen gas
  • Symbol equation (use the correct formula for the compounds):
    •    Li + H₂O           LiOH      +         H₂

Is the equation balanced?

No

Step 2 – List the elements that are on the left-hand side, and on the right-hand side of the equation. 

       Left-Hand Side              Right-Hand Side
             Li1                                    Li1
             H2                                    H1
             O1                                     O3

Step 3 – Count how many of each element you have on both sides, and identify which elements have different amounts on each side of the equation.

       Left-Hand Side              Right-Hand Side
             Li1                                    Li1
             H2                                    H1
             O1                                    O3

Step 4 – Through trial and improvement, place numbers in front of the chemical formulae to balance the equation. Then include the state symbols.

  2 Li (s)   +   2 H₂O (g)  2 LiOH (s)   +   H₂ (g)

Practice Balancing Chemical Equations

Instructions: Balance the following chemical equations with state symbols.
You can Find more practice questions on balancing chemical equations here.

  1. Na   +   O₂       N₂O₄
  2. H₂ +   Br₂   HBr
  3. Zn  + HNO₃ Zn(NO₃)₂  + H₂
  4. C + Cl₂ CCl₄
  5. K +   I₂ →  KI
  6. H₂  +  O₂  H₂O
  7. Be +   O₂ →BeO
  8. N₂ + H₂  NH₃

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