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Chemical Reactions

Chemical Reactions. General Reactions. Combination Rxns : 2 or more substances react to form a single product 2 H 2 + O 2  2 H 2 O. Decomposition Rxns : single substance decomposes into 2 or more products opposite of combination rxns 2 H 2 O  2 H 2 + O 2.

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Chemical Reactions

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  1. Chemical Reactions

  2. General Reactions • Combination Rxns: 2 or more substances react to form a single product • 2 H2 + O2 2 H2O

  3. Decomposition Rxns: single substance decomposes into 2 or more products • opposite of combination rxns • 2 H2O  2 H2 + O2

  4. Single Replacement/Displacement: one element reacts with a compound to form a new compound and release a new element • 2 Na + 2 H2O  2 NaOH + H2

  5. Double Replacement/Displacement: an interchange of partners between two compounds • Also called “Exchange Reactions” • Pb(NO3)2(aq) + KI (aq)  PbI2 (s) + 2 KNO3(aq)

  6. Combustion Rxns: the burning of a compound, usually a hydrocarbon, in oxygen to form carbon dioxide and water

  7. Redox Reaction • A reaction in which there is a transfer of electrons from one reactant to another

  8. +

  9. Rules for Assigning Oxidation States Free elements have an oxidation state = 0 Na = 0 and Cl2 = 0 in 2 Na(s) + Cl2(g) Monatomic ions have an oxidation state equal to their charge Na = +1 and Cl = -1 in NaCl Fluorine is always –1 in compounds with other elements Cl, Br, and I are always –1 in compounds except when combined with O or F. Cl = -1 in NaCl Cl = +1 in ClO-

  10. Rules for Assigning Oxidation States The oxidation # of H is +1 in most compounds except when forming a binary compound with a metal H = +1 in HCl H = -1 in NaH The oxidation # of O is –2 in most compounds except when forming a peroxide O = -2 in H2O O = -1 in H2O2

  11. The sum of the oxidation states of all the atoms in a compound is 0 Na = +1 and Cl = -1 in NaCl, (+1) + (-1) = 0 The sum of the oxidation states of all the atoms in a polyatomic ion equals the charge on the ion N = +5 and O = -2 in NO3–, (+5) + 3(-2) = -1

  12. Practice – Assign an Oxidation State to Each Element in the following Br2 MgBr2 KBr LiF CO2 CO SO42- Na2O2

  13. Oxidation and ReductionOIL RIG oxidation occurs when an atom’s oxidation state increases during a reaction reduction occurs when an atom’s oxidation state decreases during a reaction oxidation reduction CH4 + 2 O2 → CO2 + 2 H2O -4 +1 0 +4 –2 +1 -2

  14. Oxidation–Reduction oxidation and reduction must occur simultaneously if an atom loses electrons another atom must take them the reactant that reduces an element in another reactant is called the reducing agent the reducing agent contains the element that is oxidized the reactant that oxidizes an element in another reactant is called the oxidizing agent the oxidizing agent contains the element that is reduced 2 Na(s) + Cl2(g) → 2 NaCl(s)

  15. Identify the Oxidizing and Reducing Agents in Each of the Following Fe2O3(s) + 2 Al(s)  2 Fe(s) + Al2O3(s) MnO2 + 4 HBrMnBr2 + Br2 + 2 H2O 3 H2S + 2 NO3– + 2 H+3S + 2 NO + 4 H2O MnO4-(aq) + Fe2+(aq)  Mn2+ + Fe3+(aq)

  16. Common Oxidizing Agents 20

  17. Common Reducing Agents 21

  18. Collision Theory For a reaction to occur, there must be: • Collision • Orientation • Energy

  19. Reaction Rates Rate: change in the quantity of something over a given period of time Rate of rxn = Rate of rxn =

  20. Questions What is your speed, in mph, if you travel 12 km in 10 minutes? Which reaction, in the above diagram, is faster? at t = 0 [A] = 8 [B] = 8 [C] = 0 at t = 0 [X] = 8 [Y] = 8 [Z] = 0 at t = 16 [A] = 4 [B] = 4 [C] = 4 at t = 16 [X] = 7 [Y] = 7 [Z] = 1

  21. Calculate the rates of rxn from t=16 to t=32 and from t=32 to t=48 Using the rates of rxn from problems 2 and 3, what can you conclude about the rate of a rxn as the rxn proceeds? Why does this happen? at t = 16 [A] = 4 [B] = 4 [C] = 4 at t = 32 [A] = 2 [B] = 2 [C] = 6 at t = 48 [A] = 0 [B] = 0 [C] = 8

  22. Factors Influencing the Speed of Reactions • Nature of reactants • Temperature • Concentration • Catalysts

  23. Nature of Reactants: what kind of reactant molecules and what physical condition they are in small molecules tend to react faster than large molecules gases tend to react faster than liquids which react faster than solids powdered solids are more reactive than “blocks” ions tend react faster than molecules

  24. Temperature Increase temp = Increase rxn rate Decrease temp = Decrease rxn rate

  25. Reactant Concentration • Increase [reactants] = Increase rxn rate • Decrease [reactants] = Decrease rxn rate

  26. Catalysts: substances that affect the speed of a reaction without being consumed themselves • Provide an alternative pathway that has a lower activation energy (Ea)

  27. Ozone Depletion over the Antarctic mechanism without catalyst O3(g) + O(g)  2 O2(g) V. Slow mechanism with catalyst Cl(g) + O3(g)  O2(g) + ClO(g) Fast ClO(g) + O(g)  O2(g) + Cl(g) Slow

  28. Polar stratospheric clouds contain ice crystals that catalyze reactions that release Cl from atmospheric chemicals Energy Profile of Catalyzed Reaction

  29. Enzymatic Hydrolysis of Sucrose

  30. Chemical Equilibrium • H2 (g) + I2(g)  2 HI (g) • H2 (g) + I2(g) 2 HI (g) • H2 (g) + I2(g) 2 HI (g) • Equilibrium: a condition in which the rate of the forward reaction is equal to the rate of the reverse reaction • Concentrations of reactants and products are constant, but not necessarily equal

  31. The Equilibrium Constant Expression • K = equilibrium constant • For aA + bB cC + dD @ equilib.

  32. Questions • Write the equilibrium constant expression for the following reactions: • H2 (g) + I2(g) 2 HI (g) • CH3OH (g) CO(g) + 2 H2(g) • N2(g) + H2(g) NH3(g)

  33. The Meaning of K • When K >> 1 • Rxn is product favored • [P]’s higher than [R]’s @ equilibrium K = 1.9 x 1019 @ 25 °C

  34. When K << 1 • Rxn is reactant favored • [R]’s is higher than [P]’s @ equilibrium K = 4.1 x 10-31 @ 25 °C

  35. When K ≈ 1 • Neither direction is favored • Rxn proceeds about ½ way • Calculations must be done to figure out whether the [R]’s or the [P]’s is higher

  36. Questions • Is the following reaction reactant favored or product favored? • Cd(NH3)42+(aq) Cd2+(aq) + 4 NH3(aq) • K = 1.0 x 10-7 • A rxn mixture that originally contains 11 mol A and 0 mol B in a 1.0 L container is allowed to reach equilibrium. What are the [A] and [B] @ equilibrium? • A(g) B(g) K = 10 • Phosgene, COCl2, is a toxic substance that is produced by the reaction of carbon monoxide and chlorine. The Kc for the reaction is 5.0. If the equilibrium concentrations for the reactio are [Cl2] = 0.25 M and [COCl2] = 0.80 M, what is the equilibrium concentration of CO(g)? • CO(g) + Cl2(g) COCl2(g)

  37. When Country A’s citizens feel overcrowded, some will emigrate to Country B. Le Châtelier’s Principle

  38. However, as time passes, emigration will occur in both directions at the same rate, leading to populations in Country A and Country B that are constant, though not necessarily equal

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