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Balancing Chemical Equations

Balancing Chemical Equations. Unit 2 – Lecture 8. Chemical Equations. A shorthand way to describe a chemical reaction using chemical symbols and formulae Three Parts to a Chemical Equation Reactants substances present before the reaction Yield Arrow indicates the direction of a reaction

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Balancing Chemical Equations

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  1. Balancing Chemical Equations Unit 2 – Lecture 8

  2. Chemical Equations • A shorthand way to describe a chemical reaction using chemical symbols and formulae • Three Parts to a Chemical Equation • Reactants • substances present before the reaction • Yield Arrow • indicates the direction of a reaction • some rxns are reversible [double-sided arrow] • Products • substances present after the reaction

  3. Chem. Equations – cont’d Chemical symbols give a “before-and-after” picture of a chemical reaction Reactants Products MgO + C CO + Mg magnesium oxide to form carbon monoxide reacts with carbon and magnesium or… magnesium oxide reacting with carbon yields carbon monoxide and magnesium

  4. Chemical Equations – cont’d • Coefficient – (large) # in front of an element • applies to all elements in the group to the right of it • ex: 4C • 4 carbon atoms • ex: 5KI • 5 potassium atoms & 5 iodine atoms • ex: 4SO4 • 4 Sulfur atoms & 16 Oxygen atoms (4x4)

  5. Chemical Equations – cont’d • Subscript – (small) # to the lower right of an element • applies only to the element it is attached to • if outside of parentheses [(XY)3] applies to everything inside of parentheses • ex: H2 • 2 hydrogen atoms • ex: CO2 • 1 carbon atom, 2 oxygen atoms • ex: (SO4)3 • 3 Sulfur atoms (1x3), 12 Oxygen atoms (4x3)

  6. Chemical Equations - cont’d Try this: • Al2(SO4)3 • Aluminum? • 2 atoms • Sulfur? • 3 atoms • Oxygen? • 12 atoms (4x3)

  7. Balancing Chem Equations • Law of Conservation of Matter • matter is neither created nor destroyed in a chemical reaction, it is conserved • biology: energy is lost, matter is RECYCLED • (changed from one thing to another) • ie, if you begin a reaction with 5 atoms of Hydrogen, you must end up with 5 atoms of Hydrogen

  8. Balancing – cont’d • Balanced = same # of each element on each side Al + S Al2S3 2Al + 3S Al2S3 Unbalanced Balanced

  9. Balanced or Unbalanced? • C3H8 + 5O2 4H2O + 3CO2 • Carbon? • = 3 vs 3 • Hydrogen? • = 8 vs 8 • Oxygen • = 10 vs 10 (4 + 6) • Is it balanced? • Yes.

  10. Balanced or Unbalanced? • Fe3O4 + H2 Fe +H2O • Iron • = 3 vs 1 • Oxygen • = 4 vs 1 • Hydrogen • = 2 vs 2 • Is it balanced? • No. • So, now what?

  11. Balancing a Chemical Reaction • Set up two columns: labeled “reactants” &“products.” • Identify & record reacting elements on the left. • Record elements in the SAME ORDER on the right. • Count & Record # of each reacting element. • Count & Record # of each element produced. • Unequal? Balance Coefficients. [can’t change subscripts] • you can treat Polyatomic Ions as one if they appear on both sides of the equation like (SO4) in parentheses • change your greatest inequality first • leave solitary elements for last

  12. Practice • Fe3O4 + H2 Fe +H2O

  13. Practice • On your worksheet, complete the two problems on the front of your sheet. ____H2 + ____O2 ____H2O ____Cl2 + ____NaBr  ____Br2 + ____NaCl 2 2 1 1 2 1 2

  14. Homework • Page 6 – • Balancing Chemical Equations w/s • 1-9 on-level • all for honors

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