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Intro to Reactions

Intro to Reactions. Ch. 7 – Chemical Reactions. A.Signs of a Chemical Reaction. Change in heat and light Formation of a gas Formation of a precipitate Color change Odor. B.Law of Conservation of Mass. In a chemical reaction, matter is not created or destroyed. Atoms can only rearrange.

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Intro to Reactions

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  1. Intro to Reactions Ch. 7 – Chemical Reactions

  2. A.Signs of a Chemical Reaction • Change in heat and light • Formation of a gas • Formation of a precipitate • Color change • Odor

  3. B.Law of Conservation of Mass • In a chemical reaction, matter is not created or destroyed. • Atoms can only rearrange. • Discovered by Lavoisier. 4H 2O 4H 2O

  4. C. Chemical Reaction • A change in which one or more substances are converted to different substances. A+BC+D REACTANTS PRODUCTS

  5. C. Chemical Equations • Reactants = Starting substances • Products = Substances Formed

  6. C. Chemical Equations p. 635

  7. C. Chemical Equations • A chemical equation is a way to describe a chemical reaction using chemical formulas and other symbols. • Chemical equations written with symbols makes it much easier to tell what is happening in the reaction.

  8. C. Chemical Equations Aqueous lead(II) nitrate plus two units of aqueous potassium iodide produces solid lead(II) iodide and two units of aqueous potassium nitrate.  KI KNO3 Pb(NO3)2 2 2 + PbI2 + Coefficient - # of units of each substance

  9. Describing Coefficients: individual atom = “atom” 2Mg  2 atoms of magnesium covalent substance = “molecule” 3CO2 3 molecules of carbon dioxide ionic substance = “unit” 4MgO  4 units of magnesium oxide C. Chemical Equations

  10. II. Balancing Equations Ch.7– Chemical Reactions C. Johannesson

  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 SO4” instead of “1 S” and “4 O”

  13. C. Balancing Example Al + CuCl2 Cu + AlCl3 Aluminum and copper(II) chloride react to form copper and aluminum chloride. 2 3 2 3

  14. D. Writing Balanced Equations • Determine the reactants and products • Write the reactants on the left side of the equation and the products on the right • Show an equal number of atoms for each element on both sides of the equation.

  15. Example When propane gas burns in the air, the reactants are propane (C3H8)and oxygen (O2). The products formed are carbon dioxide (CO2)and water (H2O). Step 1: Find the reactants and products.

  16. Example Step 2: Write a chemical equation C3H8 H2O + O2 CO2 +

  17. Example Step 3: Balance the chemical equation *In balancing a chemical equation, change only the coefficients. Never change the subscripts. C3H8 + O2 CO2 + H2O 4 5 3

  18. Homework Assignment • Page 235: 33-36 • Show inventory and balance the reactions

  19. D. Writing Reactions • Identify the reactants and products and write the equation for each of the following chemical reactions. • The key to getting this right is to balance each of the pieces correctly (Write the balanced formula. Remember from chapter 4?) • After writing the reactions balance them.

  20. D. Writing Reactions • There are a few elements that need to be treated in a special way because of how they bond with each other. • Example: Elemental oxygen, is never found by itself. It is always found as O2. Oxygen is one of the diatomic elements. This means they are always paired up. • There are 7 diatomic elements: H, I, Br, O, N, Cl, F. (Remember • HI BrONClF!) These elements are all gases!

  21. D. Writing Reactions • The Seven Diatomic Elements Br2 I2 N2 Cl2 H2 O2 F2 HIBrONClF

  22. Writing Reactions • When writing the balanced formula for elemental metals they are always written by themselves. • Example: Elemental Copper is just Cu. Elemental Aluminum is just Al. • This will be the case for all elemental metals.

  23. Writing Reactions Example 1: Solid magnesium metal reacts with liquid water to form solid magnesium hydroxide and hydrogen gas. Step 1: Find the reactants and products

  24. Writing Reactions The reactants are Mg metal and Water, the products are Mg hydroxide and H gas. Step 2: Write the balanced formula for each piece. Mg (s) = Magnesium Metal H2O (l) = Water Mg(OH)2 (s) = Magnesium Hydroxide H2 (g)= Hydrogen gas

  25. Writing Reactions The reactants are Mg metal and Water, the products are Mg hydroxide and H gas. Step 2: Write the balanced formula for each piece. Mg (s) = Magnesium Metal H2O (l) = Water Mg(OH)2 (s) = Magnesium Hydroxide H2 (g)= Hydrogen gas

  26. Writing Reactions Step 3: Put the reactants and products into a chemical reaction. Mg(s) + H2O(l)  Mg(OH)2 (s) + H2(g) Step 4: Balance the reaction

  27. D. Writing Reactions Write the following reactions and then balance them: 1. Solid Ammonium dichromate decomposes to form solid chromium (III) oxide, gaseous nitrogen, and gaseous water. 2. Gaseous ammonia (NH3) reacts with gaseous oxygen to form gaseous nitrogen monoxide and gaseous water.

  28. Assignment • Page 234 #’s 9-11, 13, 18, 21, 25-28 • I want you to write the question, circle the reactants, underline the products. Then write out the reaction in symbol form and balance it.

  29. III. Types of Chemical Reactions Ch. 8 – Chemical Reactions

  30. A. Combustion • the burning of any substance in O2 to produce heat A + O2 B CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)

  31. A. Combustion • Products: • contain oxygen • hydrocarbons form CO2 + H2O Na(s)+ O2(g)  Na2O(s) 4 2 C3H8(g)+ O2(g)  CO2(g)+ H2O(g) 5 3 4

  32. B. Synthesis • the combination of 2 or more substances to form a compound • only one product A + B  AB

  33. B. Synthesis H2(g) + Cl2(g)  2 HCl(g) C. Johannesson

  34. B. Synthesis • Products: • ionic - cancel charges • covalent - hard to tell Al(s)+ Cl2(g)  AlCl3(s) 2 3 2

  35. C. Decomposition • a compound breaks down into 2 or more simpler substances • only one reactant AB  A + B

  36. C. Decomposition 2 H2O(l)  2 H2(g) + O2(g) C. Johannesson

  37. C. Decomposition • Products: • binary - break into elements • others - hard to tell KBr(l)  K(s) + Br2(l) 2 2

  38. D. Single Replacement • one element replaces another in a compound • metal replaces metal (+) • nonmetal replaces nonmetal (-) A + BC  B + AC C. Johannesson

  39. D. Single Replacement Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s) C. Johannesson

  40. D. Single Replacement • Products: • metal  metal (+) • nonmetal  nonmetal (-) • free element must be more active(check activity series) Fe(s)+ CuSO4(aq)  Cu(s)+ FeSO4(aq) Br2(l)+ NaCl(aq)  N.R. C. Johannesson

  41. E. Double Replacement • ions in two compounds “change partners” • cation of one compound combines with anion of the other AB + CD  AD + CB C. Johannesson

  42. E. Double Replacement Pb(NO3)2(aq) + K2CrO4(aq)  PbCrO4(s) + 2KNO3(aq) C. Johannesson

  43. E. Double Replacement • Products: • switch negative ions Pb(NO3)2(aq)+ KI(aq)  2 2 PbI2(s)+ KNO3(aq) NaNO3(aq)+ KI(aq)  N.R.

  44. Decide the RXN Type • WO3 + 3H2 W + 3H2O • RbCl + 2O2  RbClO4 • RbBr + AgCl  AgBr + RbCl • 2KNO3  2KNO2 + O2 • 3Mg + N2  Mg3N2 • 2C2H6 + 7O2  4CO2 + 6H2O

  45. Chemical Changes • Chemical reactions can be classified into 5 different types. • These 5 types are Single Displacement, Double Displacement, Decomposition, Synthesis, and Combustion. • You will be given the general form for each of the 5 reactions.

  46. Single Replacement • In a single displacement reaction one element displaces or changes places with another element in a compound. • General Form: • Element + Compound Element + Compound • Ex. Cl2 + 2KBr  2KCl + Br2 • Chlorine changes places with Bromine

  47. Single Replacement • Look at the reactants in the following single displacement reaction and decide which elements will change places: Al + Fe2O3 • Be sure to find the charges of all the elements. • Which ones will switch?

  48. Double Replacement • In a double displacement reaction the positive and negative parts of two compounds switch places. • General form: • Compound + Compound  Compound + Compound • Ex: PbCl2 + Li2SO4 PbSO4 + 2LiCl

  49. Double Replacement • Look at the reactants for the following double displacement reaction and predict what will happen. • ZnBr2 + AgNO3

  50. Decomposition • In a decomposition reaction a compound break up or decompose down into simpler substances. • General form: Compound  two or more elements or compounds • Ex: 2Ag2O  4Ag + O2

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