Write & balance rxns

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  • 1. I. Intro to Reactions Chp 11: Chemical Reactions
  • 2. A.Signs of a Chemical Reaction
    • Evolution of heat and light
    • Formation of a gas
    • Formation of a precipitate
    • Color change
  • 3. B.Law of Conservation of Mass
    • mass is neither created nor destroyed in a chemical reaction
    4 H 2 O 4 H 2 O 4 g 32 g 36 g
    • total mass stays the same
    • atoms can only rearrange
  • 4. C. Chemical Equations
    • A+B  C+D
    REACTANTS PRODUCTS
  • 5. C. Chemical Equations
  • 6. D. Writing Equations
    • Identify the substances involved.
    • Use symbols to show:
    2H 2 ( g ) + O 2 ( g )  2H 2 O( g )
      • How many? - coefficient
      • Of what? - chemical formula
      • In what state? - physical state
    • Remember the diatomic elements.
  • 7. D. Writing Equations
    • Two atoms of aluminum react with three units of aqueous copper(II) chloride to produce three atoms of copper and two units of aqueous aluminum chloride.
    • How many?
    • Of what?
    • In what state?
    Al 2 ( s ) + 3 CuCl 2 ( aq )  3 Cu ( s ) + 2 AlCl 3 ( aq )
  • 8. E. Describing Equations
    • Describing Coefficients :
      • individual atom = “atom”
      • covalent substance = “molecule”
      • ionic substance = “unit”
    3 molecules of carbon dioxide 2 atoms of magnesium 4 units of magnesium oxide 3CO 2  2Mg  4MgO 
  • 9. E. Describing Equations
    • to produce
    • How many?
    • Of what?
    • In what state?
    Zn( s ) + 2HCl( aq )  ZnCl 2 ( aq ) + H 2 ( g ) One atom of solid zinc reacts with two molecules of aqueous hydrochloric acid one unit of aqueous zinc chloride and one molecule of hydrogen gas.
  • 10. II. Balancing Equations Chp 11: – Chemical Reactions
  • 11. A. Balancing Steps
    • 1. Write the unbalanced equation.
    • 2. Count atoms on each side.
    • 3. Add coefficients to make #s equal.
    • Coefficient  subscript = # of atoms
    • 4. Reduce coefficients to lowest possible ratio, if necessary.
    • 5. Double check atom balance!!!
  • 12. B. Helpful Tips
    • Balance one element at a time.
    • Update ALL atom counts after adding a coefficient.
    • If an element appears more than once per side, balance it last.
    • Balance polyatomic ions as single units.
      • “1 SO 4 ” instead of “1 S” and “4 O”
  • 13. C. Balancing Example
    • Aluminum and copper(II) chloride react to form copper and aluminum chloride.
    1 1 1 1 2 3 2  3  6   3 3 3 2 2  2  6 Al + CuCl 2  Cu + AlCl 3 Al Cu Cl