Balancing Chemical Equations with Polyatomic Ions

Main Ideas and Lessons

The video presents two strategies for making equations with polyatomic ions easier to balance:

1. Keep an unchanged polyatomic ion together. If the same polyatomic ion appears on both sides of the equation, count and balance it as a unit rather than separating it into its atoms.
2. Rewrite water as HOH when hydroxide (OH) is on the opposite side. This can make it easier to track hydrogen and hydroxide while balancing.

Balancing Method

• List the individual elements and polyatomic ions on each side in the same order.
• Count each item, paying attention to subscripts and parentheses:
  • A subscript outside parentheses applies to the whole group. For example, ((SO_4)_3) means three sulfate ions.
  • A group without parentheses represents one copy of that group.
• Adjust coefficients in front of whole compounds to balance the counts. A coefficient multiplies every element and group in that compound.
• Recheck the counts after each change.
• When possible, leave individual hydrogen and oxygen atoms until the end.

Examples

Iron Sulfate and Potassium Hydroxide

Keep the sulfate ion together because it appears on both sides. Balance iron first, then sulfate, followed by potassium and hydroxide:

[ Fe_2(SO_4)_3 + 6KOH \rightarrow 2Fe(OH)_3 + 3K_2SO_4 ]

Aluminum Hydroxide and Sulfuric Acid

Rewrite water as HOH to help track hydrogen and hydroxide. Balance aluminum first, followed by hydroxide and sulfate, then check hydrogen at the end:

[ 2Al(OH)_3 + 3H_2SO_4 \rightarrow Al_2(SO_4)_3 + 6H_2O ]

Speaker

Melissa Maribel — chemistry tutor and presenter.

The instructor closes by encouraging viewers to keep practicing and stay determined.