Overview
This lecture explains how to balance chemical equations that include polyatomic ions, focusing on effective strategies and step-by-step procedures.
Introduction to Balancing Equations
- Balancing a chemical equation ensures the same number of each atom on both sides of the reaction.
- Law of Conservation of Mass states matter cannot be created or destroyed in a chemical reaction.
Identifying Polyatomic Ions
- Polyatomic ions are charged groups of covalently bonded atoms that behave as a unit in reactions.
- Common examples include sulfate (SO₄²⁻), nitrate (NO₃⁻), carbonate (CO₃²⁻), and phosphate (PO₄³⁻).
Strategy for Balancing Equations with Polyatomic Ions
- Treat polyatomic ions as single units if they appear unchanged on both sides of the equation.
- First, balance elements or ions that are not hydrogen or oxygen.
- Balance polyatomic ions next, followed by hydrogen and oxygen atoms.
- Use coefficients to adjust the number of molecules or units, not subscripts.
Step-by-Step Example
- Write down the unbalanced equation with all reactants and products.
- Identify and circle polyatomic ions that remain unchanged during the reaction.
- Count the number of each atom or polyatomic ion on both sides.
- Add coefficients to balance polyatomic ions as whole units first.
- Balance remaining individual elements last.
- Check that all atoms are balanced and the coefficients are in the simplest ratio.
Tips and Common Mistakes
- Do not alter chemical formulas or subscripts when balancing.
- Be careful with ions appearing only in one compound; balance them separately.
- Double-check your final equation for equal numbers of atoms or ions on both sides.
Key Terms & Definitions
- Chemical equation — a symbolic representation of a chemical reaction using formulas for reactants and products.
- Polyatomic ion — a group of atoms bonded covalently that has an overall charge and acts as a unit.
- Coefficient — a number placed in front of a compound or ion to balance the equation.
- Law of Conservation of Mass — principle that the total mass stays constant during a chemical reaction.
Action Items / Next Steps
- Practice by balancing five given chemical equations containing polyatomic ions.
- Read the textbook section on polyatomic ions and balancing equations.