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Master the step-by-step process of balancing equations to ensure the number of atoms is equal on both sides.
Imagine you are a chef baking a cake, but halfway through, three eggs simply vanish into thin air. In the universe of chemistry, this is impossible! Every single atom you start with must be accounted for at the end.
In chemistry, we follow the Law of Conservation of Mass, which states that matter cannot be created or destroyed. In a chemical reaction, atoms are simply rearranged. This means the number of atoms for each element on the reactant side (left) must exactly match the number of atoms on the product side (right). Think of a chemical equation like an accounting ledger: if you start with 4 Hydrogens, you must end with 4 Hydrogens. If the sides don't match, the equation is 'unbalanced' and does not accurately represent reality.
Quick Check
If a reaction starts with 10 grams of reactants, how many grams of products must be formed in a closed system?
Answer
10 grams, because mass cannot be created or destroyed.
To balance an equation, you can only change the coefficient—the large number in front of a molecule. You must NEVER change the subscript—the small number below an atom. Changing a subscript changes the identity of the substance. For example, is life-sustaining water, but is toxic hydrogen peroxide! Adding a coefficient like means you have two separate water molecules, giving you a total of Hydrogen atoms and Oxygen atoms.
Balance the equation:
1. Inventory: Left has 2 H, 2 O. Right has 2 H, 1 O. 2. Identify Imbalance: Oxygen is missing one atom on the right. 3. Adjust Coefficient: Place a 2 in front of : . Now Oxygen is balanced (2 on each side), but Hydrogen is now 4 on the right. 4. Final Fix: Place a 2 in front of on the left: . 5. Check: Left (4H, 2O) = Right (4H, 2O). Balanced!
Quick Check
In the expression , how many total Oxygen atoms are present?
Answer
6 Oxygen atoms ().
The most reliable way to balance complex equations is the Inventory Method. Draw a vertical line under the arrow and list every element present. Count the atoms on both sides. If they aren't equal, pick an element (usually not Oxygen or Hydrogen first) and add a coefficient to increase its count. Update your inventory every time you add a coefficient. It is a game of 'trial and error'—if you get stuck, start over with a different element!
Balance the combustion of methane:
1. Inventory: Left: C=1, H=4, O=2 | Right: C=1, H=2, O=3. 2. Balance Hydrogen: We need 4 H on the right. Add coefficient 2 to : . 3. Update Inventory: Right side now has C=1, H=4, O=4 (2 from and 2 from ). 4. Balance Oxygen: We need 4 O on the left. Add coefficient 2 to : . 5. Final Check: C(1=1), H(4=4), O(4=4). Success!
Balance:
1. Inventory: Left: Al=1, Fe=2, O=3 | Right: Al=2, Fe=1, O=3. 2. Balance Iron (Fe): Add a 2 to on the right: . 3. Balance Aluminum (Al): Add a 2 to on the left: . 4. Check: Al(2=2), Fe(2=2), O(3=3). Even though it looked hard, the Oxygen was already balanced!
What does a coefficient represent in a chemical equation?
Which equation is correctly balanced?
Changing the subscript in a chemical formula is an acceptable way to balance an equation.
Review Tomorrow
Tomorrow morning, try to explain to a friend why you can't just change to to balance an equation.
Practice Activity
Find 5 unbalanced equations online and use the 'Inventory Method' to solve them without looking at the answers first.