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Chemical Reactions: An Introduction

Chemical Reactions: An Introduction. Ch. 8. Chemical Equations. Chemical change involves a reorganization of the atoms in one or more substances. 4 Evidences a Chemical Reaction Has Occurred. 1. A color change is apparent. 2. A precipitate (solid) forms.

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Chemical Reactions: An Introduction

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  1. Chemical Reactions: An Introduction Ch. 8

  2. Chemical Equations Chemical change involves a reorganization of the atoms in one or more substances.

  3. 4 Evidences a Chemical Reaction Has Occurred • 1. A color change is apparent. • 2. A precipitate (solid) forms.

  4. 4 Evidences a Chemical Reaction Has Occurred • 3. A gas (bubbles) is formed. • 4. Evolution/absorption of energy • --heat, light, electrical energy.

  5. Reaction of Zinc and Iodine Reactants: Zn + I2 Product: Zn I2

  6. Chemical Equation • A representation of a chemical reaction: • word equation -- qualitative significance only • ethanol + oxygen yields carbon dioxide + water vapor • formula equation -- qualitative & quantitative C2H5OH(l) + 3O2(g)2CO2(g) + 3H2O(g) • reactants products

  7. Chemical Equations • Chemical equations give two important pieces of information: • 1. The identities of the reactants and products -- qualitative information. • 2. The relative number of each --quantitative information.

  8. Physical States • solid (s) • liquid (l) • gas (g) • aqueous (aq)

  9. Important Equation Symbols •  • heat -------> • light • light --------> • elect. • electricity ------> • yields ------> • cat. • catalyst -------> • H2SO4 • catalyst ------>

  10. Chemical EquationQuantitative Significance • C2H5OH (l)+3O2(g)2CO2(g)+3H2O(g) • The equation is balanced. • 1 mole of ethanol reacts with 3 moles of oxygen • to produce • 2 moles of carbon dioxide and 3 moles of water

  11. Chemical EquationsQuantitative Significance • 4 Al(s) + 3 O2(g) ---> 2 Al2O3(s) • This equation means • 4 Al atoms + 3 O2 molecules ---give---> • 2 molecules of Al2O3 • 4 moles of Al + 3 moles of O2 ---give---> • 2 moles of Al2O3

  12. Chemical Equations • Because of the principle of the conservation of matter, an equation must be balanced. • It must have the same number of atoms of the same kind on both sides. • The total mass for the reaction cannot change. Lavoisier, 1788

  13. Reactants are indicated by: reacts with….. combines with….. oxidizes…… burns in…… ….are required ….are needed ….decomposes Products are indicated by: ….is formed ….is produced ….is given off ….is precipitated yields…. ….is prepared …..is synthesized Key Words

  14. Coefficients, Subscripts, & Superscripts • Coefficient Superscript • Subscript

  15. Four Steps in Balancing Equations • 1. Get the facts down. • 2. Check for diatomic molecules (subscripts). • 3. Balance charges on compounds containing a metal, ammonium compounds, and acids (subscripts). • 4. Balance the number of atoms (coefficients). • a. Balance most complicated molecule first. • b. Balance other elements. • c. Balance hydrogen next to last. • d. Balance oxygen last.

  16. Balancing Equations Caution The identities (formulas) of the compounds must never be changed in balancing a chemical equation! Only coefficients can be used to balance the equation-subscripts will not change!

  17. O H H O + + C O O C H H H H O 1 C + 4 H + 2 O 1 C + 2 O + 2 H + O 1 C + 2 H + 3 O Combustion of Methane • methane gas burns to produce carbon dioxide gas and liquid water • whenever something burns it combines with O2(g) • CH4(g) + O2(g)  CO2(g) + H2O(l)

  18. O O O O H H H H + + + C C + H H O O O O H H 1 C + 4 H + 4 O 1 C + 4 H + 4 O Combustion of MethaneBalanced • The balanced reaction must obey the Law of Conservation of Mass: • CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(l)

  19. Balancing Equations • ___ Al(s) + ___ Br2(l) ---> ___ Al2Br6(s)

  20. Chemical Reactions • What evidence did you observe that a chemical reaction occurred? • 1. Evolution of heat/light • 2. Color change

  21. Balancing Equations • 2 Al(s) + 3 Br2(l) ---> Al2Br6(s)

  22. Balancing Equations • ___C3H8(g) + ___ O2(g) ----> ____CO2(g) + ____ H2O(g) • ___B4H10(g) + ___ O2(g) ----> ____ B2O3(g) + ____ H2O(g)

  23. Balancing Equations C3H8(g) + 5 O2(g) ----> 3CO2(g) + 4 H2O(g) 2 B4H10(g) + 11 O2(g) ----> 4 B2O3(g) + 10 H2O(g)

  24. Final Equation Balancing Check • SiO2(s) + 4 HF(aq) -----> SiF4(g) + 2 H2O(l) • Make a grid to do a final check to be sure the Law of Conservation of Matter has been obeyed: • Reactants Products • 1 Si 1 Si • 2 O 2 O • 4 H 4 H • 4 F 4 F

  25. Figure 6.5(a): The reaction of potassium with water

  26. Figure 6.5(b)&(c): The reaction of potassium with water What evidence is there for a chemical reaction?

  27. Oxidation-Reduction Reactions(Redox Reactions) • Redox reaction -- involves the transfer of electrons • loss and gain of electrons must be exactly equal. • loss and gain of electrons must be simultaneous.

  28. Oxidation • loss of electrons • metal atoms -- Na, Ca, & K • Nao ---> Na1+ + 1e- • Cao ---> Ca2+ + 2e- • nonmetal ions -- Cl-, S2-, & O2- • Cl1- ---> Clo +1e- • S2- ---> So + 2e-

  29. Reduction • gain of electrons. • nonmetal atoms. • Oo + 2e- ---> O2- • Fo + 1e- ---> F1- • metal ions. • K1+ + 1e- ---> Ko • Ba2+ + 2e- ---> Bao

  30. Figure 7.7: When powdered aluminum and iodine (shown in the foreground) are mixed (and a little water added), they react vigorously

  31. Oxidation & Reduction Half-Reactions • Always add electrons (negative) to the more positive side of the equation. • The charge on both sides of an equation must be equal. • Oxidation -- Nao ----> Na1+ + 1 e- • Reduction -- Cl2 + 2 e- ----> 2 Cl-

  32. OIL RIG • Oxidation Is Loss. • Reduction Is Gain.

  33. Redox Reactions • Metal-nonmetal reactions are always redox reactions. • Any reaction that has a free element (such as O2) as a reactant or product are redox. • All single replacement reactions are redox. • All combustion reactions are redox.

  34. Synthesis I (Composition) • A + X ----> AX ALWAYS REDOX • Element + Element -----> Binary Compound • Fe(s) + S(s) ----> FeS(s) • 4 Al(s) + 3 O2(g) ----> 2 Al2O3(s)

  35. Synthesis II (Composition) • A + X ----> AX NOT REDOX • Compound + Compound ----> Compound (3 or more elements) • Ammonia + Acid ----> Ammonium Salt • NH3(g) + HCl(g) ----> NH4Cl(s) • 2 NH3(aq) + H2SO4(aq) ----> (NH4)2SO4(aq)

  36. Synthesis II (Composition)Continued • Water + An Oxide • Rule # 1 --Water + Metal Oxide ----> Metal Hydroxide (Base) • HOH(l) + CaO(s) ----> Ca(OH)2(s) • HOH(l) + Na2O(s) ----> 2 NaOH(aq) • Rule # 2 --Water +Nonmetal Oxide ----> Acid • HOH(l) + SO3(g) ----> H2SO4(aq) • HOH(l) + N2O5(g) ----> 2 HNO3(aq)

  37. Single Replacement I • A + BX ----> AX + B ALWAYS REDOX • Element + Compound ----> Different Element + Different Compound • Metal Reactivities: • K > Na > Ca > Mg > Al > Zn > Cr > Fe > Cd > Co > Ni > Sn > Pb > H > Sb > Cu > Hg > Ag > Pt > Au

  38. Single Replacement I(Continued) • A is a metal. • A is more reactive than B • A + BX ----> AX + B • Cu(s) + 2 AgNO3(aq) ---->Cu(NO3)2(aq) + 2 Ag(s) • Zn(s) + 2 HCl(aq) ----> ZnCl2(aq) + H2(g) • Rule # 3 -- Active Metal + Water ----> Metal Hydroxide + Hydrogen • Ca(s) + 2 HOH(l) ----> Ca(OH)2(s) +H2(g)

  39. Single Replacement I(Continued) • A is a metal. • A is less reactive than B • A + BX ----> No Reaction (NR) • Cu(s) + HCl(aq) ----> NR

  40. Single Replacement II • Y + BX ----> BY + X ALWAYS REDOX • Element + Compound ----> Different Element + Different Compound • Nonmetal Reactivities: • F > O > Cl > Br > I

  41. Single Replacement II(Continued) • Y is a nonmetal. • Y is more reactive than X • Y + BX ----> BY + X • Cl2(g) + 2 KBr(aq) ----> 2 KCl(aq) + Br2(aq) • Y is less reactive than X • Y + BX ----> No Reaction • Cl2(g) + KF(aq) ----> NR

  42. Figure 7.6: The thermite reaction gives off so much heat that the iron formed is molten

  43. Decomposition I • AX ----> A + X ALWAYS REDOX • AX is a binary compound. • Binary Compound ----> Element + Element • elect • 2 NaCl(l) ----> 2 Na(l) + Cl2(g) • elect • 2 HOH(l) ----> 2 H2(g) + O2(g)

  44. Decomposition II • AX -----> A + X MAY OR MAY NOT BE REDOX • AX is a ternary compound. • Rule # 4 -- Metal Chlorates ----> Metal Chlorides + Oxygen •  • 2 KClO3(s) ----> 2 KCl (s) + 3 O2(g) REDOX

  45. Decomposition II(Continued) • Rule # 5 -- Metal Carbonates ----> Metal Oxides + Carbon Dioxide •  • CuCO3(s) ----> CuO (s) + CO2(g) NOTREDOX

  46. Decomposition II(Continued) • Rule # 6 -- Metal Hydroxides ----> Metal Oxides + Water •  • Ca(OH)2(s) ----> CaO (s) + HOH(g) NOTREDOX

  47. Decomposition II(Continued) • Rule # 7 -- Acid ----> Nonmetal Oxide + Water •  • H2SO4(aq) ----> SO3(g) + HOH(g) NOTREDOX

  48. Combustion • AX + O2 ALWAYS REDOX • AX is a hydrocarbon. •  • CxHy + O2(g) ----> CO2(g) + HOH(g) •  • CH4(g) + 2 O2(g) ----> CO2(g) + 2 HOH(g) •  • 2 C6H6(l) + 15 O2(g) ----> 12 CO2(g) + 6 HOH(g)

  49. Decomposition I • AX ----> A + X ALWAYS REDOX • AX is a binary compound. • Binary Compound ----> Element + Element • elect • 2 NaCl(l) ----> 2 Na(l) + Cl2(g) • elect • 2 HOH(l) ----> 2 H2(g) + O2(g)

  50. Reaction of Potassium and Water Write and balance the word and formula equation for potassium and water. potassium(s) + water(l) potassium hydroxide(aq) + hydrogen(g) 2K(s) + 2HOH(l) 2KOH(aq) + H2(g)

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