Learning Objective

In this lesson we will revise how to balance a chemical equation to reflect the conservation of mass during a chemical reaction.

Learning Outcomes

By the end of this lesson you will be able to:

  • Write word equations and formula equations to represent chemical reactions.

  • Include states of matter in formula equations.

  • Balance formula equations to reflect conservation of mass.

 

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Describing Chemical Reactions With Equations

  • A chemical reaction involves one or more substances, known as reactants, interacting to form one or more new substances, known as products.
  • A chemical equation is a short-hand way of representing a chemical reaction.
  • The general format of a chemical equation is:

 
chemical reaction general formula reactants products

 

  • A chemical equation can be written as a word equation or a formula equation.
  • A word equation contains the chemical names of reactants and products.
    Example
    calcium hydroxide  +  carbon dioxide  →  calcium carbonate  +  water
  • A formula equation contains the chemical formulas of reactants and products.
    Example
    Ca(OH)2  +  CO2  →  CaCO3  +  H2O
  • Formula equations may also include the states of matter of reactants and products.
    These are written in brackets after each chemical formula:
    • (s) represents solids.
    • (l) represents liquids.
    • (g) represents gases.
    • (aq) represents aqueous solutions, which are substances dissolved in water, such as acids and soluble salts.
    Example
    Ca(OH)2 (aq)  +  CO2 (g)  →  CaCO3 (s)  +  H2O (l)

 

Conservation of Mass

  • The Law of Conservation of Mass states that matter is neither created nor destroyed.
    This means that even though chemical reactions result in the formation of new substances, the total mass of reactants will always equal the total mass of products.
    In other words, the total numbers of each type of atom are the same before and after a chemical reaction has occurred – they are just arranged differently.
  • In the above example:
    • There is 1 calcium atom on each side of the equation.
    • There are 4 oxygen atoms on each side of the equation.
    • There are 2 hydrogen atoms on each side of the equation.
    Therefore, this equation shows the conservation of mass.
  • However, it is usually not the case that a formula equation will show the conservation of mass.
    For example, in the following equation:
    FeCl3  +  K2S  →  Fe2S3  +  KCl
    • There is 1 iron atom on the left side of the equation and 2 iron atoms on the right side of the equation.
    • There are 3 chlorine atoms on the left side of the equation and 1 chlorine atom on the right side of the equation.
    • There are 2 potassium atoms on the left side of the equation and 1 potassium atom on the right side of the equation.
    • There is 1 sulfur atom on the left side of the equation and 3 sulfur atoms on the right side of the equation.
    Therefore, this equation does not show the conservation of mass.
    This doesn’t mean that the reaction can’t take place, it simply means that the reactants and products do not occur in a 1:1:1:1 ratio as they do in the previous example.
    The equation is said to be unbalanced.
  • For the equation to be balanced, the ratio of substances in the equation needs to be adjusted so there are equal numbers of each type of atom on both sides of the equation.
    This is done by placing numbers (coefficients) in front of the chemical formulas of substances.

 

Steps for Balancing a Chemical Equation

  • Balancing equations successfully requires a systematic approach.
    We will illustrate this using the above reaction between iron (III) chloride and potassium sulfide as an example.

Step 1

  • Write the formula equation showing all reactants and products.

    • FeCl3  +  K2S  →  Fe2S3  +  KCl

Step 2

  • Tally up the total numbers of each type of atom for both sides of the equation.

    • .

Atom Reactants Products
Fe 1 2
Cl 3 1
K 2 1
S 1 3

Step 3

  • Place coefficients in front of substances until there are equal numbers of each type of atom on both sides of the equation.
    Each time a coefficient is added, update the tally.
    Placing a “2” in front of FeCl3 will balance the number of Fe atoms:
    2 FeCl3  +  K2S  →  Fe2S3  +  KCl
    It will also change the number of Cl atoms on the left side.
    The updated tally is:

Atom Reactants Products
Fe 2 2
Cl 6 1
K 2 1
S 1 3

    The iron atoms are now balanced, but the other atoms are still unbalanced.
    Placing a “6” in front of KCl will balance the number of Cl atoms:
    2 FeCl3  +  K2S  →  Fe2S3  +  6 KCl
    It will also change the number of K atoms on the right side.
    The updated tally is:

Atom Reactants Products
Fe 2 2
Cl 6 6
K 2 6
S 1 3

    The iron and potassium atoms are now balanced, but the other atoms are still unbalanced.
    Placing a “3” in front of K2S will balance the number of K atoms as well as the number of S atoms:
    2 FeCl3  +  3 K2S  →  Fe2S3  +  6 KCl
    The updated tally is:

Atom Reactants Products
Fe 2 2
Cl 6 6
K 6 6
S 3 3

    The equation is now balanced.
    It shows that, for this reaction, the ratio of substances is 2:3:1:6.
    (There is no need to place a “1” in front of substances in a balanced equation.)

 

Tips for Balancing Chemical Equations

  • Only adjust one coefficient at a time.
  • Remember that coefficients change the tallies for all atoms in a substance, not just the atom you are trying to balance.
  • If the equation contains elements, leave them until last when adjusting coefficients. This is because they can be adjusted without affecting the tallies of other atoms.
  • You can only balance an equation by placing coefficients in front of substances. Never adjust the subscript numbers that are part of a chemical formula.
    For example, you can’t change H2O to H2O2, it must be written as 2 H2O if you wish to double the number of oxygen atoms.
  • Only place whole numbers (2, 3, 4 etc) in front of chemical formulas. If you find that a fraction is required to balance an equation, multiply all coefficients by the smallest number required to convert the fraction to a whole number.
  • Polyatomic groups, such as NO3 and SO4, often stay together in reactions. If this is the case, consider them as one group in the tally, rather than as individual atoms – this will simplify the process.
  • Check that the coefficients are the smallest possible numbers.

    • For example,  4 H2 + 2 O2 → 4 H2O  can be simplified to  2 H2 + O2 → 2 H2O.

 

Summary

  • A word equation shows the chemical names of all reactants and products involved in a chemical reaction.
  • A formula equation shows the chemical formulas of reactants and products.
  • Formula equations may also include the states of matter of reactants and products, in brackets after each chemical formula.
    (s) represents solids.
    (l) represents liquids.
    (g) represents gases.
    (aq) represents aqueous solutions.
  • A balanced equation shows the ratios of reactants and products.
  • A balanced equation reflects the Law of Conservation of Mass and shows that atoms are neither created nor destroyed during a chemical reaction, but only rearranged.
  • Steps for balancing a chemical equation:
    1. Write the formula equation showing all reactants and products.
    2. Tally up the total numbers of each type of atom for both sides of the equation.
    3. Place coefficients in front of reactants and products until there are equal numbers of each type of atom on both sides of the equation.

 
balanced equation particle diagram

 
 

(Header image: JSquish, Wikimedia Commons)

 

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