go to the AUS-e-TUTE homepage
home test Join AUS-e-TUTE game contact
 

 

Balancing Chemical Equations

Key Concept

The number of atoms of each element on the left hand side of the equation must be the same as the number of atoms of each element on the right hand side of the equation

Summary

  • Reactants are written on the left hand side of the chemical equation.

  • Products are written on right hand side of the chemical equation.

  • You can't change the formula of reactant or product molecules in the chemical equation.

  • You can only change the numbers of reactant or product molecules in the chemical equation.

Animated Tutorial

Example

When zinc metal reacts with hydrochloric acid, hydrogen gas and zinc chloride are produced.
Write a balanced chemical equation for this reaction.

  1. Identify the reactants and products:
    reactants :zinc and hydrochloric acid
    products :hydrogen and zinc chloride

  2. Write the word equation:
    general form of equation: reactants products
    word equation for this reaction:     zinc + hydrochloric acid hydrogen + zinc chloride

  3. Write the formula for all reactants and products:

    zinc : Zn
    hydrochloric acid : HCl
    hydrogen gas : H2
    zinc chloride : ZnCl2

    Once you have written the formula for each reactant and product you cannot change them during the process of balancing the equation.

  4. Write the unbalanced chemical equation by replacing the words in the word equation with the formulae above:
    word equation: zinc + hydrochlodic acid hydrogen + zinc chloride
    unbalanced chemical equation:       Zn + HCl H2 + ZnCl2

  5. Balancing the equation:

    • Balance the Zn atoms:
      Count the number of Zn atoms on the left hand side of the equation = 1
      (ie 1 Zn atom in Zn)

      Count the number of Zn atoms on the right hand side of the equation = 1
      (ie, 1 Zn atom in ZnCl2)

      number of Zn atoms on the left hand side = number of Zn atoms on the right hand side
      Zn atoms are balanced.

    • Balance the H atoms in the chemical equation:
      Zn + HCl → H2 + ZnCl2

      Count the number of H atoms on the left hand side of the equation = 1
      (ie, 1 H atom in HCl)

      Count the number of H atoms on the right hand side of the equation = 2
      (ie, 2 H atoms in H2)

      number of H atoms on the left hand side ≠ number of H atoms on right hand side
      H atoms are NOT balanced.

      In order to balance the number of H atoms, we need twice as many H atoms on the left hand side of the equation, but, we cannot change the formula of hydrochloric acid so we must multiply the number of molecules of hydrochloric acid by 2:

      Zn + 2HCl → H2 + ZnCl2

      Check that the number of H atoms on each side of the equation is now the same:
      Count the number of H atoms on the left hand side of the equation = 2
      (ie, 2 H atoms in 2HCl)

      Count the number of H atoms on the right hand side of the equation = 2
      (ie, 2 H atoms in H2)

      number of H atoms on the left hand side = number of H atoms on right hand side
      H atoms are balanced.

    • Balance the Cl atoms using the new chemical equation :
      Zn + 2HCl → H2 + ZnCl2

      Count the number of Cl atoms on the left hand side of the equation = 2
      (ie, 2 Cl atoms in 2HCl)

      Count the number of Cl atoms on the right hand side of the equation = 2
      (ie, 2 Cl atoms in ZnCl2)

      number of Cl atoms on the left hand side = number of Cl atoms on right hand side
      Cl atoms are balanced.

    • The chemical equation is balanced when the numbers of atoms of each element on the left hand side of the equation is equal to the number of atoms of each element on the right hand side of the equation:
      Zn + 2HCl H2 + ZnCl2
      number Zn atoms 1     =     1 Zn atoms are balanced
      number H atoms       2 = 2     H atoms are balanced
      number Cl atoms         2 =         2 Cl atoms are balanced
      The number of atoms of each element on the left hand side of the equation is equal to the number of atoms of each element on the right hand side of the equation so the chemical equation is balanced.


What would you like to do now?
advertise on the AUS-e-TUTE website and newsletters
 
 

Search this Site

You can search this site using a key term or a concept to find tutorials, tests, exams and learning activities (games).
 

Become an AUS-e-TUTE Member

 

AUS-e-TUTE's Blog

 

Subscribe to our Free Newsletter

Email email us to
subscribe to AUS-e-TUTE's free quarterly newsletter, AUS-e-NEWS.

AUS-e-NEWS quarterly newsletter

AUS-e-NEWS is emailed out in
December, March, June, and September.

 

Ask Chris, the Chemist, a Question

The quickest way to find the definition of a term is to ask Chris, the AUS-e-TUTE Chemist.

Chris can also send you to the relevant
AUS-e-TUTE tutorial topic page.

 
 
 

Share this Page

Bookmark and Share
 
 

© AUS-e-TUTE