Learning Objective

In this lesson we will learn how to write chemical equations in a way that reflects the Law of Conservation of Mass.

Learning Outcomes

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

- Define the Law of Conservation of Mass.

- Explain how the Law of Conservation of Mass applies to chemical reactions.

- Write a balanced chemical equation.

## Conservation of Mass

- The Law of Conservation of Mass states that, during a physical or chemical change, matter is neither created nor destroyed.

- Therefore, for chemical reactions:

- The total numbers of each type of atom are the same before and after a reaction has occurred.

- The total mass of the products will be equal to the total mass of the reactants.

- In other words, although chemical reactions involve the formation of new substances, they don’t involve the formation of new atoms.

- The only difference between reactants and products is how atoms are arranged, as a result of the breaking and forming of chemical bonds.

## Chemical Equations

- So far we have learned two ways of writing chemical equations to represent the reactants and products of a chemical reaction:

- Word equations, which show the chemical names of all reactants and products.

- Formula equations, which show the chemical formulas of all reactants and products.

- Often a formula equation doesn’t show equal numbers of each type of atom on both sides of the equation.

- Consider the following equation for the reaction between nitrogen and hydrogen to form ammonia:

- The left side of the equation (the reactants) shows 2 nitrogen atoms whereas the right side of the equation (the product) shows 1 nitrogen atom.

- Similarly, the left side of the equation shows 2 hydrogen atoms whereas the right side of the equation shows 3 hydrogen atoms.

- When a formula equation shows unequal numbers of atoms on either side of the equation, the equation is said to be unbalanced.

- The equation is not demonstrating conservation of mass.

- To demonstrate conservation of mass by having equal numbers of atoms on either side of the equation, the equation needs to be balanced.

## Balancing Chemical Equations

- When first learning how to balance chemical equations, it can be helpful to draw diagrams of reactants and products.

- For example, the above reaction between nitrogen and hydrogen can be illustrated by the following diagram:

- We can also create a tally to show the total number of each type of atom for both sides of the equation:

Type of Atom |
Number of Atoms in Reactants |
Number of Atoms in Products |

N | 2 | 1 |

H | 2 | 3 |

- If a formula equation is unbalanced when it is first written, it means that the reactants and products do not exist in equal ratios.

- For example, in the above reaction, one nitrogen molecule does not combine with one hydrogen molecule to form one ammonia molecule.

- Therefore we need to determine the correct ratio for all reactants and products that will result in equal numbers of each type of atom on both sides of the equation.

- When balancing a chemical equation, deal with one type of atom at a time.

- Let’s balance the above equation by first balancing the number of nitrogen atoms.

- We will use diagrams and tallies to help us.

- The only way to make the number of atoms equal on both sides of an equation is to add more reactant or product molecules.

- For example, in the above equation we can’t simply add 1 nitrogen atom to the products to balance the number of nitrogen atoms.

- The only way we can increase the number of nitrogen atoms on the right side as to add another ammonia molecule:

- Now our diagram shows 2 nitrogen atoms on both sides of the equation.

- Let’s update the tally:

Type of Atom |
Number of Atoms in Reactants |
Number of Atoms in Products |

N | 2 | 2 |

H | 2 | 6 |

- Now we have balanced the number of nitrogen atoms, but the number of hydrogen atoms remains unbalanced. We have 2 hydrogen atoms on the left and 6 on the right.

- The only way to increase the number of hydrogen atoms on the left is to add hydrogen molecules. To have 6 hydrogen atoms in total, we need 3 hydrogen molecules, so we need to add 2 more:

- Now our diagram shows 6 hydrogen atoms on both sides of the equation.

- Let’s update the tally:

Type of Atom |
Number of Atoms in Reactants |
Number of Atoms in Products |

N | 2 | 2 |

H | 6 | 6 |

- Now we have balanced the number of nitrogen atoms and the number of hydrogen atoms.

- The last step is to rewrite the formula equation so that it corresponds with our diagram.

- We do this by adding numbers called coefficients in front of the chemical formulas of reactants and products that have more than one copy.

- So we need to add a ‘3’ in front of H
_{2}to represent 3 hydrogen molecules, and a ‘2’ in front of NH_{3}to represent 2 ammonia molecules.

- We don’t add a ‘1’ in front of N
_{2}to represent 1 nitrogen molecule (just as we don’t have a subscript 1 next to the N in ammonia to represent 1 nitrogen atom).

- Therefore our balance equation is:

## Tips for Balancing Chemical Equations

- When first learning to balance chemical equations, use diagrams and tallies.

- Only adjust one type of atom 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 H
_{2}O to H_{2}O_{2}, it must be written as 2 H_{2}O 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 NO
_{3}and SO_{4}, 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 H
_{2}+ 2 O_{2}→ 4 H_{2}O can be simplified to 2 H_{2}+ O_{2}→ 2 H_{2}O.

## Summary

- The Law of Conservation of Mass states that matter is neither created nor destroyed.

- To reflect the conservation of mass in a chemical reaction, chemical equations need to be balanced.

- Equations are balanced by adding coefficients (numbers) in front of the chemical formulas of reactants and products.

- A balanced equation shows equal numbers of each type of atom on both sides of the equation.

- It reflects the ratios of each reactant and product involved in the reaction.

- Steps for writing a balanced chemical equation:

- Write the formula equation showing all reactants and products.

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

- Place coefficients in front of reactants and products until there are equal numbers of each type of atom on both sides of the equation.