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CHEMISTRY – Chapter 8

CHEMISTRY – Chapter 8. Chemical Equations and Reactions. Chapter 8 – Section 1. Objectives: 1. List three observations that suggest that a chemical reaction has taken place. 2. List three requirements for a correctly written chemical equation.

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CHEMISTRY – Chapter 8

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  1. CHEMISTRY – Chapter 8 Chemical Equations and Reactions

  2. Chapter 8 – Section 1 Objectives: 1. List three observations that suggest that a chemical reaction has taken place. 2. List three requirements for a correctly written chemical equation. 3. Write a word equation and a formula equation for a given chemical reaction.

  3. Chemical Reactions • A process by which one or more substances are changed into one or more different substances • Reactants – the original substances • Products – the resulting substances • Mass is always conserved

  4. Chemical Equation • Represents the identities and relative amounts of reactants and products in a chemical rx

  5. Chemical Reactions • A new substance must be produced • The following changes indicate a chemical reaction: • Heat and light – release of E • Production of gas – bubbles • Formation of a precipitate – a solid produced in a solution that settles • Color change

  6. Characteristics of Chemical Equations • Chemical Equations must do the following: • Represent known facts • Contain the correct formulas for the reactants and products • Diatomic molecules, etc. • Law of conservation of mass must be satisfied • Atoms are not created or destroyed, just rearranged

  7. Word Equations • Equations where products and reactants are represented by words • Qualitative • Does not give the whole story • No quantities are given • ex. methane + oxygen → carbon dioxide + water

  8. Formula equation • Represents reactants and products of a chemical rx with symbols or formulas • Use coefficients to balance • Small whole number that appears in front of a formula in an equation • ex. CH4 + 2 O2→ CO2 + 2 H2O

  9. Symbols in Chemical Equations • → - yields • ↔ - reversible reaction – chem rx in which the products reform the originals reactants • (s) – solid state • ↓ - precipitate • (l) – liquid • (aq) – aqueous solution (dissolved in water) • (g) – gas • heat→ - reactants are heated • 0°→ - temperature at which reaction is carried out • MnO2→ - catalyst – substance used to alter the rate of rx

  10. Review and Assignment 1. List three observations that suggest that a chemical reaction has taken place. 2. List three requirements for a correctly written chemical equation. 3. Write a word equation and a formula equation for a given chemical reaction. Assignment: HW 8-1 and HW 8-2

  11. Chapter 8 - Section 2 Objectives: 1.Balance a chemical formulas by inspection.

  12. Chemical Equations • Give us quantitative information • Coefficients indicate relative, not absolute amounts of reactants and products • H2 + Cl2→ 2 HCl • 1 mole of H2 and 1 mole of Cl2 • Yields 2 moles HCl • Relative masses of the reactants and products of a chemical rx can be determined from the coefficients • H2 + Cl2→ 2 HCl • 1 mole H2 = 2 g H2 • 1 mole Cl2 = 71 g Cl2 • Yields 2 moles HCl = 73 g HCl • Reverse reaction for a chemical equation has the same relative amounts of substances as the forward reaction

  13. Balancing Chemical Reactions • 1. Identify the names of the reactants and the products, and write a word equation. • 2. Write a formula equation by substituting correct formulas for the names of reactants and products. • Don’t forget diatomic molecules (H2, N2, O2, F2, Cl2, Br2, I2) • 3. Balance the formula equation. • Balance the different types of atoms one at a time • 1st balance atoms of elements that are combined and appear only once on each side • Balance polyatomic ions that appear on both sides • Balance H and O atoms • 4. Count atoms to be sure that the equation is balanced.

  14. Example • 1. zinc + hydrochloric acid → zinc chloride + hydrogen • 2. Zn(s) + HCl(aq)→ ZnCl2(aq) + H2(g) • 3. Zn(s) + 2HCl(aq)→ ZnCl2(aq) + H2(g) • 4. 1 Zn + 2H and 2 Cl → 1 Zn and 2 Cl + 2 H

  15. Demo. • Zinc + hydrochloric acid → zinc chloride + hydrogen gas • Aluminum sulfate + calcium chloride → aluminum chloride + calcium sulfate • Balance the equations from the previous demonstrations • Zn + HCl → ZnCl2 + H2 • Zn + 2 HCl → ZnCl2 + H2 • Al2(SO4)3 + CaCl2→ AlCl3 + CaSO4 • Al2(SO4)3 + 3 CaCl2→ 2AlCl3 + 3 CaSO4

  16. Review and Assignment 1. Balance a formula equation by inspection. Assignment: HW 8-3 and HW 8-4

  17. Chapter 8 - Section 2 Objectives: Define and give general equations for synthesis, decomposition, single-replacement, and double-replacement reactions. Classify a reaction as synthesis, decomposition, single-replacement, and double-replacement, or combustion.

  18. Types of Chemical Reactions 1. Synthesis reaction • A + X → AX 2. Decomposition • AX → A + X 3. Single-Replacement • A + BX → AX + B • Y + BX → BY + X 4. Double-Replacement • AX + BY → AY + BX

  19. Synthesis • 2 or more substances combine to produce a new compound • Also known as composition reaction • Forms oxides with oxygen and metals

  20. Decomposition • A single compound undergoes a reaction that produces two or more simpler substances • Most are endothermic when energy is supplied by electricity or heat • ex. electrolysis – decomposition of a substance by an electric current

  21. Single-Replacement • One element replaces a similar element in a compound • Also called displacement • Many are in aqueous solutions • Less E required • The more active element replaces the less active one • Most active metals (group 1) react w/water and produce metal hydroxides

  22. Double-Replacement • Ions of two compounds exchange places in an aqueous solution to form two new compounds • One compound formed is usually a precipitate, an insoluble gas, or a molecular compound (usually water) • Other compound remains dissolved

  23. Combustion Reaction • A substance combines with oxygen, releasing a large amount of E in the form of light and heat • Always have oxygen as a reactant

  24. Review and Assignment 1.Define and give general equations for synthesis, decomposition, single-replacement, and double-replacement reactions. 2. Classify a reaction as synthesis, decomposition, single-replacement, and double-replacement, or combustion. Assignment: WS, HW 8-5, HW 8-7

  25. Chapter 8 - Section 3 Objectives: Explain the significance of an activity series. Use the activity series to predict whether a given reaction will occur and what the products will be.

  26. Chemical Activity • The ability of an element to react • Metals lose e- easily • Non-metals gain e- easily

  27. Activity Series • A list of elements organized according to ease with which the elements undergo certain chemical reactions • Metals – greater activity = greater ease to lose e- • Non-metals – greater activity = greater ease to gain e- • Order is determined by single-replacement reactions • The metal at the top can replace anything beneath it • Predicts if a reaction will actually take place • d/o what they are reacting with (water vs. acid)

  28. Examples • 2 Al (s) + 3 ZnCl2(ag)→ 3 Zn(s) + 2AlCl3(aq) • Al will replace Zn • This reaction will occur • Co(s) + 2 NaCl(s)→ no reaction • Cr(s) + H2O(l) → no reaction (b/c water, not steam)

  29. Examples • Zn(s) + H2O(l)50°C→ • No reaction (b/c water is not steam) • Sn(s) + O2(g) → • yes, Sn reacts w/oxygen • Cd(s) + Pb(NO3)2(aq)→ • yes, Pb is below Cd • Cu(s) + HCl(aq) → • no, Cu is below H2

  30. Review and Assignment • Explain the significance of an activity series. • Use the activity series to predict whether a given reaction will occur and what the products will be. Assignment: HW 8-8

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