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Chemical Reactions

Chemical Reactions. What IS a Chemical Reaction?. A process whereby… (reactants) (products). Starting Materials. New Substances. Get converted into. A Chemical Equation is What We Write to Represent A Chemical Reaction. Reactants

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Chemical Reactions

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  1. Chemical Reactions

  2. What IS a Chemical Reaction? A process whereby… (reactants) (products) Starting Materials New Substances Get converted into

  3. A Chemical Equation is What We Write to Represent A Chemical Reaction Reactants always go on the LEFT side Products always go on the RIGHT side

  4. What Happens During a Chemical Reaction? Let’s look at the reaction of the combustion of methane H  H CH  H OO O=O +

  5. What Happens During a Chemical Reaction? First, the bonds of the reactants must be broken. This requires an input of energy. H  H CH  H OO O=O +

  6. What Happens During a Chemical Reaction? First, the bonds of the reactants must be broken. This requires an input of energy. H H CH H O O O O +

  7. What Happens During a Chemical Reaction? Second, atoms must rearrange themselves H O O + H C H O O H

  8. What Happens During a Chemical Reaction? Third, new bonds are formed thus creating “new substances”. Energy is released during this part. O H H + O = C = O O H H

  9. How Do You Know a Reaction Happened? The new products will have new and unique properties from the reactants O H H + O = C = O O H H

  10. How Do You Know a Reaction Happened? The new products will have new and unique properties from the reactants CH4 + 2O2 CO2 + 2H2O have completely different properties from these products. These reactants…

  11. Evidence That a Reaction Happened The following observations might indicate that a chemical reaction has occurred: 1. A color change 2. Formation of a gas (bubbles) 3. Formation of a solid (precipitate) 4. A change in energy (heat or light)

  12. 0 Writing Chemical Equations

  13. 0 What IS a Chemical Equation? The written description of a chemical reaction using compound formulasand symbols.

  14. 0 What IS a Chemical Equation? We perform the reaction in the lab, but what we write down to represent it is called the equation

  15. 0 An Example of a Chemical Equation C3H8 + 5O2 4H2O + 3CO2 You may notice several numbers in the equation

  16. 0 An Example of a Chemical Equation C3H8 + 5O2 4H2O + 3CO2 The small numbers to the lower right corner of an element are called subscripts

  17. 0 An Example of a Chemical Equation C3H8 + 5O2 4H2O + 3CO2 Subscripts tell you how many atoms are present in the compound

  18. 0 An Example of a Chemical Equation C3H8 + 5O2 4H2O + 3CO2 For example, in the first compound there are: 3 carbon atoms and 8 hydrogen atoms

  19. 0 An Example of a Chemical Equation C3H8 + 5O2 4H2O + 3CO2 The big numbers out in front of compounds are called coefficients

  20. 0 An Example of a Chemical Equation C3H8 + 5O2 4H2O + 3CO2 Coefficients are “multipliers” of everything after them

  21. 0 An Example of a Chemical Equation C3H8 + 5O2 4H2O + 3CO2 For example, the product “H2O” has a coefficient of “4”

  22. 0 An Example of a Chemical Equation C3H8 + 5O2 4H2O + 3CO2 This means there are 2 x 4 = 8 hydrogen atoms and 1 x 4 = 4 oxygen atoms

  23. 0 An Example of a Chemical Equation C3H8 + 5O2 4H2O + 3CO2 If no subscript or coefficient is written, we assume it is a “1”

  24. Counting Atoms To determine the number of atoms present, MULTIPLY the subscriptby the coefficient

  25. Example The coefficient for this compound is 3 3MgCl2 There are: 1 x 3 = 3 Mg2+ ions 2 x 3 = 6 Cl- ions

  26. “Double” Subscripts Sometimes polyatomic ions have a “double” subscript 4Al(NO2)3 One subscript is INSIDE the parenthesis and one subscript is OUTSIDE the parenthesis

  27. “Double” Subscripts To count the number of atoms in a compound with “double” subscripts, you must MULTIPLY the subscripts 4Al2(CO2)3 There are: 2 x 4 = 8 Al atoms 3 x 1 x 4 = 12 C atoms 3 x 2 x 4 = 24 O atoms This “2” applies only to the O This “3” applies to both C and O

  28. Example The coefficient for this compound is 5 5Mg(OH)2 There are: 1 x 5 = 5 Mg2+ ions 2 x 1 x 5 = 10 O atoms 2 x 1 x 5 = 10 Hatoms

  29. More Examples • How many of each atom in Ba3(PO4)2? • How many of each atom in 4Ba3(PO4)2? • How many of each atom in Al2(Cr2O7)3? • How many of each atom in 2Al2(Cr2O7)3?

  30. Answers • 3-Ba, 2-P, 8-0 • 12-Ba, 8-P, 32-O • 2-Al, 6-Cr, 21-O • 4-Al, 12-Cr, 42-O

  31. Now We’re Ready to BalanceSome Equations!!!

  32. 0 Chemical Equations MustAlways Be Balanced That means they must conform to the “Law of Conservation of Matter”

  33. 0 Chemical Equations MustAlways Be Balanced The Law of Conservation of Matter states: “Matter can not be created nor destroyed”

  34. 0 Chemical Equations MustAlways Be Balanced In other words, your equation can’t show atoms magically appearing or disappearing

  35. Watch me pull atoms right out of my…….. magic hat.

  36. Balancing Equations You must have the same amount of each type of atom on the left and right side of the equation Ex. If you have 3 Carbon atoms as reactants, you MUST have 3 Carbon atoms as products

  37. Let’s Check to Make Sure It’s Balanced 4 Al atom reactants 4Al + 3O2 2Al2O3 2 x 2 = 4 Al atom products Put “4” in, get “4” out!

  38. Let’s Check to Make Sure It’s Balanced 2 x 3 = 6 Oxygen atom reactants 4Al + 3O2 2Al2O3 3 x 2 = 6 Oxygen atom products Put “6” in, get “6” out!

  39. Some Good Balancing Tips Use subscripts to balance formulas first!!! • Make sure to balance charges within a formula MgCl2, Al2(SO4)3 • Don’t forget to put subscripts on diatomic elements when they are by themselves (H2, O2, N2, Cl2, Br2, I2, F2)

  40. Some Good Balancing Tips Use coefficients to balance the equation!!! • Balance METALS 1st • Balance POLYATOMIC IONS 2nd • Balance NONMETALS 3rd • Balance HYDROGENS • Balance OXYGENS last

  41. Balance This Equation • BaClO3 BaCl + O • Ba(ClO3)2 BaCl2 + O2 • Use subscripts to balance formulas first • Don’t forget about subscripts on diatomic atoms

  42. Example • Ba(ClO3)2 BaCl2 + O2 • Check coefficients now that subscripts are correct!!! • Ba(ClO3)2 BaCl2 + 3O2 • Count each atom to verify its balanced!!!

  43. Writing Equations from Word Problems • Determine what are the reactantsand what are the products. • Use nomenclature rules to write the compound formulas. • Use subscriptsto balance charges within formulas. • Use coefficientsto balance equation.

  44. Example • The white solid material, silver chloride, can be producedby mixing aqueous silver nitrate and sodium chloride in a test tube. A by product of this reaction is sodium nitrate. • AgNO3 + NaCl  AgCl + NaNO3

  45. Now you just need tons of practice problems!

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