Click images to preview the worksheet for this lesson and the Year 9 Chemistry Workbook (PDF and print versions)

 

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.

 

For any chemical reaction, the mass of reactants is equal to the mass of products.

 

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 H₂ to represent 3 hydrogen molecules, and a ‘2’ in front of NH₃ to represent 2 ammonia molecules.

• We don’t add a ‘1’ in front of N₂ 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 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

• 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.

 

 

---

 

     

Click images to preview the worksheet for this lesson and the Year 9 Chemistry Workbook (PDF and print versions)