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CHAPTER 8

CHAPTER 8. Chemical Equations and Reactions Joshua Jo Bessy chen. Chemical equation. Chemical equation represents, with symbols and formulas, the identities and relative amounts of the reactants and products in a chemical reaction. ex. (NH 4 ) 2 Cr 2 O 7(g) N 2(9) +Cr 2 O 3(s) +4H 2 O (g)

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CHAPTER 8

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  1. CHAPTER 8 Chemical Equations and Reactions Joshua Jo Bessy chen

  2. Chemical equation • Chemical equation represents, with symbols and formulas, the identities and relative amounts of the reactants and products in a chemical reaction. ex. (NH4)2Cr2O7(g)N2(9)+Cr2O3(s)+4H2O(g) • Indications • Characteristics • Significance • balancing

  3. Indications • Evolutions of heat and light Ex. Ammonium dichromate release energy to produce heat & light • Production of a gas Ex. The reaction of vinegar & baking soda • Formation of a precipitate Ex.combine of Water solutions of ammonium sulfide & cadmium nitrate (yellow precipitate)

  4. characteristics Word: methane+oxygencarbon dioxide+water Formula: CH4(g)+2O2(g)CO2(g)+2H2O(g)

  5. Additional symbols used in chemical equations

  6. Significance • 1. The coefficients of a chemical reaction indicate relative, not absolute, amounts of reactants and products. Ex. H2 + Cl2  2HCl 1molH2 : 1mol Cl2 : 2mol HCl • 2. The relative masses of the reactants and products of a chemical reaction can be determined from the reaction's coefficients

  7. Significance • 3. The reverse reaction for a chemical equation has the same relative amounts of substances as the forward reaction.

  8. Balancing • 1. Identify the names of the reactants and the products, and write a word equation. waterhydrogen+oxygen • 2. Write a formula equation by substituting correct formulas for the names of the reactants and the products H2O(l)H2(g)+O 2(g)

  9. Balancing • 3. Count atoms to be sure that the equation is balanced 2H2O(l)  2H2(g)+O2(g) (4H+2O) = (4H) + (2O) • 4. Balance the formula equation according to the law of conservation of mass 2H2O(l)H2(g)+O2(g) • Balance the different types of atoms one at a time. • First balance the atoms of elements that are combined and that appear only once on each side of the equation. • Balance polyatomic ions that appear on both sides of the equation as single units. • Blance H and O after atoms of all other elements have been balanced.

  10. Types of Chemical Reactions • Synthesis Reaction - Two or more substances combine to form a new compound A + B → AB A+B+C → ABC

  11. a. Reactions with Oxygen & Sulfur - Element + Oxygen → Oxide ex)2Mg(s) + O2(g) → 2MgO(s) - Element + Sulfur → Sulfide ex)8Ba(s) + S8(s) → 8BaS(s) b. Reactions with Halogens - Metal + Halogen → ionic compound ex)2Na(s) + Cl2(g) → 2NaCl(s) Mg(s) + F2(g) → MgF2(s) c. Reactions with Oxides -Active metal oxide + water → Metal hydroxide ex) CaO(s) + H2O(l) → Ca(OH)2(s) - Nonmetal oxide + water → Oxyacid ex) SO2(g) + H2O(l) → H2SO3(aq)

  12. Decomposition Reactions - A single compound breaks down to form two or more smaller compounds or elements - Most decomposition reactions take place only when energy in the form of electricity (electrolysis) or heat is added AB → A + B ABC → A+B+C

  13. a. Decomposition of a Binary Compound into its element ex) 2H2O(l) → 2H2(g) + O2(g) b. Metal Carbonates - Metal Carbonate → Metal oxide + Carbon Dioxide gas ex) CaCO3(s) → CaO(s) + CO2(g) c. Metal Hydroxides (except those containing Group 1 metals) - Metal Hydroxide → Metal oxide + water ex) Ca(OH)2(s) → CaO(s) + H2O(g) d. Metal Chlorates - Metal chlorate → Metal chloride + Oxygen ex) 2KClO3(s) → 2KCl(s) + 3O2(g)

  14. Single-Replacement Reactions - One element replaces a similar element in a compound - Many Reactions take place in aqueous solution. - More active metal replaces another. - Activity Series A + BX → AX + B Ex) 2Al(s) + 3Pb(NO3)2(aq) → 3Pb(s) + 2Al(NO3)3(aq)

  15. Double-Replacement Reactions - The ions of two compounds exchange places in an aqueous solution to form two new compounds. - One of the compounds formed is usually a precipitate, an insoluble gas, or water. - The other compound remains dissolved in solution. AX + BY → AY + BX

  16. a. Formation of a Precipitate - The cations of one reactant combine with the anions of another reactant to form an insoluble compound. ex) 2KI(aq) + Pb(NO3)2(aq) → PbI2(s) + 2KNO3(aq) b. Formation of a Gas ex) FeS(s) + 2HCl(aq) → H2S(g) + FeCl2(aq) c. Formation of Water ex) HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

  17. Combustion Reactions - A substance combines with oxygen, releasing a large amount of energy in the form of light and heat. X + O2(g) → XO2 + heat Ex) C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g)

  18. Activity Series • A list of elements organized according to the ease with which the elements undergo certain chemical reactions • The order in which the elements are listed is usually determined by single-replacement reactions. • An element can replace any element below it but not any above it. Ex. 1) 2Al(s) + 3ZnCl2(aq) → 3Zn(s) + 2AlCl3(aq) Ex. 2) Co(s) + 2NaCl(aq) → no reaction

  19. Animations • http://www.sci.brooklyn.cuny.edu/~marciano/labs.htm

  20. Experiment • Purpose: To observe the indications of a chemical reaction 1. Production of a gas Baking soda + Vinegar  Production of CO2 gas (NaHCO3) + (H+) → Na+ + H2O + CO2 2. Evolution of heat and light Decomposition of ammonium dichromate  Releasing energy (NH4)2Cr2O7(s) → Cr2O3(s) + N2(g) + 4H2O(g)

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