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Chemical Reactions: An Introduction

Chemical Reactions: An Introduction. Chapter 6. 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 Chapter 6

  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. Balancing Equation Prerequisites • Student must have memorized: • 44 chemical symbols • Table 5.2 on page 133 in text • Table 5.3 on page 137 • Table 5.4 on page 142 • Table 5.5 on page 146 • Table 5.6 on page 146 • Type I, II, III, and acid nomenclature • Count HOFBrINCl

  14. 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

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

  16. 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.

  17. 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!

  18. 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)

  19. 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)

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

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

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

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

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

  25. 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

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

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

  28. 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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