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CHEMISTRY – Chapter 8 Chemical Equations and Reactions.

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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. CH O 2 → CO H 2 O

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  MnO 2 → - 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  H 2 + Cl 2 → 2 HCl 1 mole of H 2 and 1 mole of Cl 2 Yields 2 moles HCl Relative masses of the reactants and products of a chemical rx can be determined from the coefficients  H 2 + Cl 2 → 2 HCl 1 mole H 2 = 2 g H 2 1 mole Cl 2 = 71 g Cl 2 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 (H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2 )  3. Balance the formula equation. Balance the different types of atoms one at a time 1 st 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) → ZnCl 2(aq) + H 2(g)  3. Zn (s) + 2HCl (aq) → ZnCl 2(aq) + H 2(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 → ZnCl 2 + H 2  Zn + 2 HCl → ZnCl 2 + H 2  Al 2 (SO 4 ) 3 + CaCl 2 → AlCl 3 + CaSO 4  Al 2 (SO 4 ) CaCl 2 → 2A lCl CaSO 4

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: 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.

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: 1.Explain the significance of an activity series. 2.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 ZnCl 2(ag) → 3 Zn (s) + 2AlCl 3(aq) Al will replace Zn This reaction will occur  Co (s) + 2 NaCl (s) → no reaction  Cr (s) + H 2 O (l) → no reaction (b/c water, not steam)

29 Examples  Zn (s) + H 2 O (l) 50°C → No reaction (b/c water is not steam)  Sn (s) + O 2(g) → yes, Sn reacts w/oxygen  Cd (s) + Pb(NO 3 ) 2(aq) → yes, Pb is below Cd  Cu (s) + HCl (aq) → no, Cu is below H 2

30 Review and Assignment 1.Explain the significance of an activity series. 2.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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