Chapter 7 Chemical Reactions

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1 Chapter 7 Chemical Reactions

2 Evidence of Chemical Change Release or Absorption of Heat Color Change Emission of Light Formation of a Gas Formation of Solid Precipitate Tro's "Introductory 2

3 How Do We Represent A Chemical Reaction? Reactions are represented by Chemical Equations Chemical Equations Short-hand way of describing a reaction. Provides information about the reaction. Formulas of reactants and products. States of reactants and products. Relative numbers of reactant and product molecules that are required. Can be used to determine masses of reactants used and products that can be made. 3

4 Symbols Used in Equations Symbols used to indicate state after chemical. (g) = gas; (l) = liquid; (s) = solid. (aq) = aqueous = dissolved in water. C(s) + O 2 (g) CO 2 (g) reactants products Energy symbols used are: D = heat. hn = light. 4

5 Conservation of Mass Matter cannot be created or destroyed. Therefore, the total mass cannot change. And the total mass of the reactants will be the same as the total mass of the products. In a chemical reaction, all the atoms present at the beginning are still present at the end. If all the atoms are still there, then the mass will not change. 5

6 Balancing Equations activity Rules for balancing equations: -Balance only one element at a time -Always balance free elements last 6

7 Aqueous Solutions Many times, the chemicals we are reacting together are dissolved in water. Mixtures of a chemical dissolved in water are called aqueous solutions. Dissolving the chemicals in water helps them to react together faster. The water separates the chemicals into individual molecules or ions. The separate, free-floating particles come in contact more frequently so the reaction speeds up. 7

8 Dissociation When ionic compounds dissolve in water, the anions and cations are separated from each other. This is called dissociation. However, not all ionic compounds are soluble in water! When compounds containing polyatomic ions dissociate, the polyatomic group stays together as one ion. 8

9 Dissociation, Continued Potassium iodide dissociates in water into potassium cations and iodide anions. KI(aq) K +1 (aq) + I -1 (aq) K I K +1 I -1 Copper(II) sulfate dissociates in water into copper(ii) cations and sulfate anions. CuSO 4 (aq) Cu +2 (aq) + SO 4-2 (aq) Cu SO 4 Cu +2 SO 4-2 9

10 Dissociation, Continued Potassium sulfate dissociates in water into potassium cations and sulfate anions. K 2 SO 4 (aq) 2 K +1 (aq) + SO 4-2 (aq) K SO 4 K K +1 SO 4-2 K +1 10

11 Electrolytes Electrolytes--- substances whose water solution is a conductor of electricity. All electrolytes have ions dissolved in water. Tro's "Introductory 11

12 Electrolytes, Continued In strong electrolytes, all the electrolyte molecules or formula units are separated into ions. In weak electrolytes, a small percentage of the molecules are separated into ions In nonelectrolytes, none of the molecules are separated into ions. Tro's "Introductory 12

13 Types of Electrolytes Salts = Water soluble ionic compounds. Mostly strong electrolytes. Acids = Form H + ions and anions in water solution. Sour taste. React and dissolve many metals. Bases = Water-soluble metal hydroxides. Form OH - ions in water solution. Bitter taste, slippery (soapy) feeling solutions. Tro's "Introductory 13

14 When Will a Salt Dissolve? A compound is soluble in a liquid if it dissolves in that liquid. NaCl is soluble in water, but AgCl is not. A compound is insoluble if a significant amount does not dissolve in that liquid. AgCl is insoluble in water. Though there is a very small amount dissolved, but not enough to be significant. 14

15 Solubility Rules: Compounds that Are Generally Soluble in Water Compounds containing the following ions are generally soluble Li +, Na +, K +, NH 4 + NO 3, C 2 H 3 O 2 Exceptions (when combined with ions on the left the compound is insoluble) none none Cl, Br, I Ag +, Hg 2 2+, Pb 2+ SO 4 2 Ca 2+, Sr 2+, Ba 2+, Pb 2+ 15

16 Solubility Rules: Compounds that Are Generally Insoluble Compounds containing the following ions are generally insoluble Exceptions (when combined with ions on the left the compound is soluble or slightly soluble) OH Li +, Na +, K +, NH 4+, H + Ca 2+, Sr 2+, Ba 2+ S 2 Li +, Na +, K +, NH 4+, H + Ca 2+, Sr 2+, Ba 2+ CO 2 3, PO 3 4 Li +, Na +, K +, NH 4+, H + 16

17 Using the Solubility Rules to Predict an Ionic Compound s Solubility in Water First check the cation: If it is Li +, Na +, K +, or NH 4+, then the compound will be soluble in water. Regardless of the anion. If the cation is not Li +, Na +, K +, or NH 4+, then follow the rule for the anion. If a rule says the compounds are mostly soluble, then the exceptions are insoluble. If a rule says the compounds are mostly insoluble, then the exceptions are soluble. Note: slightly soluble insoluble. 17

18 Examples of Soluble and Insoluble Compounds KOH Soluble, because the cation is K +. AgBr CaCl 2 Insoluble, even though most compounds with Br are soluble, this is an exception. Soluble, most compounds with Cl are soluble. Pb(NO 3 ) 2 Soluble, because the anion is NO 3. PbSO 4 Insoluble, even though most compounds with SO 4 2 are soluble, this is an exception. 18

19 Types of Reactions (Part A) Precipitation Reactions Acid-Base Reactions Gas Evolution Reactions 19

20 Reaction Type 1 A: Precipitation Reactions A reaction that takes place from the exchange of cations and anions This results in the formation of a solid known as a precipitate Note: The reactants can be in any state other than the solid state. 20

21 Precipitation Reactions, Continued 2 KI(aq) + Pb(NO 3 ) 2 (aq) 2 KNO 3 (aq) + PbI 2 (s) 21

22 No Precipitate Formation = No Reaction KI(aq) + NaCl(aq) KCl(aq) + NaI(aq) All ions still present, no reaction. 22

23 Process for Predicting the Products of a Precipitation Reaction 1. Write the formula for the reactants and Determine what ions each aqueous reactant has. 2. Exchange ions. (+) ion from one reactant with (-) ion from the other. 3. Balance charges of combined ions to get formula of each product. 4. Balance the equation. Count atoms. 5. Determine solubility of each product in water. Use the solubility rules. If product is insoluble or slightly soluble, it will precipitate. If neither product will precipitate, no reaction. 23

24 Example 7.7 Write a balanced chemical equation for the reaction between an Aqueous Solution of Sodium Carbonate and an Aqueous Solution of Copper(II) Chloride 1. Write the form the reactants and Determine the ions present when each reactant dissociates. 2. Exchange the ions. Na 2 CO 3 (aq) + CuCl 2 (aq) (Na + + CO 3 2- ) + (Cu +2 + Cl - ) (Na + + CO 3 2- ) + (Cu +2 + Cl - ) (Na + + Cl - ) + (Cu +2 + CO 3 2- ) 24

25 Example 7.7 Write a balanced chemical equation for the reaction between an Aqueous Solution of Sodium Carbonate and an Aqueous Solution of Copper(II) Chloride 3. Write the formulas of the products. Cross charges and reduce. Na 2 CO 3 (aq) + CuCl 2 (aq) NaCl + CuCO 3 4. Balance the equation. Na 2 CO 3 (aq) + CuCl 2 (aq) 2 NaCl + CuCO 3 25

26 Example 7.7 Write a balanced chemical equation for the reaction between an Aqueous Solution of Sodium Carbonate and an Aqueous Solution of Copper(II) Chloride 5. Determine the solubility of each product. Write an (s) after the insoluble products and a (aq) after the soluble products NaCl is soluble. CuCO 3 is insoluble. Na 2 CO 3 (aq) + CuCl 2 (aq) 2 NaCl(aq) + CuCO 3 (s) 26

27 Practice Predict the Products and Balance KCl(aq) + AgNO 3 (aq) the Equation Na 2 S(aq) + CaCl 2 (aq) 27

28 Reaction Type 2 A: Acid Base Reactions Also called neutralization reactions Acid contains H + and base contains OH- In all acid-base reactions, the H + and the OH - combine to make H 2 O. acid + base salt + water 2 HNO 3 (aq) + Ca(OH) 2 (aq) Ca(NO 3 ) 2 (aq) + 2 H 2 O(l) 28

29 Predicting the products of an acid-base reactions: Example 7.11 Write the Molecular Equation for the Reaction of Aqueous Nitric Acid with Aqueous Calcium Hydroxide. 1. Write the formulas of the reactants. HNO 3 (aq) + Ca(OH) 2 (aq) 2. Since this is an acid-base reaction, one of the products must be water! H 2 O(l). HNO 3 (aq) + Ca(OH) 2 (aq) H 2 O(l) + other product 29

30 Example 7.11 Write the Molecular Equation for the Reaction of Aqueous Nitric Acid with Aqueous Calcium Hydroxide, Continued. 3. The other product will be formed by combining the ions other than H + and OH -. i.e. NO 3 and Ca 2+ HNO 3 (aq) + Ca(OH) 2 (aq) H 2 O(l) + Ca(NO 3 ) 2 4. Balance the equation. 2 HNO 3 (aq) + Ca(OH) 2 (aq) Ca(NO 3 ) H 2 O(l) 30

31 Example 7.11 Write the Molecular, Ionic, and Net- Ionic Equation for the Reaction of Aqueous Nitric Acid with Aqueous Calcium Hydroxide, Continued. 5. Determine the solubility of the salt. Ca(NO 3 ) 2 is soluble. a. Write an (s) after an insoluble salt and an (aq) after a soluble salt. 2 HNO 3 (aq) + Ca(OH) 2 (aq) Ca(NO 3 ) 2 (aq) + 2 H 2 O(l) 31

32 Practice Complete and Balance These NH 4 OH(aq) + H 2 SO 4 (aq) Acid Base Reactions. Al(OH) 3 (aq) + H 2 SO 3 (aq) Ba(OH) 2 (aq) + HCl(aq) 32

33 Reaction type 3A: Gas Evolution Reactions Reactions that produce a gas. Note: the reactants can be in any state other than a gas. To identify a gas evolution reaction one of the reactant is usually an acid or a base but not both. K 2 S(aq) + H 2 SO 4 (aq) K 2 SO 4 (aq) + H 2 S(g) Note that if an acid and a base are present as reactants then the reaction will be an acid-base reaction. Other reactions form a gas by the decomposition of one of 33 their products into a gas and water.

34 Predicting the Products of Gas Evolving Reactions (Table 7.4) Reactant type Reacting with Ion exchange product Decompose? Gas formed Example Metal n S, metal HS Acid H 2 S No H 2 S K 2 S(aq) + 2HCl(aq) 2KCl(aq) + H 2 S(g) Metal n CO 3, Acid H 2 CO 3 Yes CO 2 K 2 CO 3 (aq) + 2HCl(aq) metal HCO 3 2KCl(aq) + CO 2 (g) + H 2 O(l) Metal n SO 3 Acid H 2 SO 3 Yes SO 2 K 2 SO 3 (aq) + 2HCl(aq) metal HSO 3 2KCl(aq) + SO 2 (g) + H 2 O(l) (NH 4 ) n anion Base NH 4 OH Yes NH 3 KOH(aq) + NH 4 Cl(aq) KCl(aq) + NH 3 (g) + H 2 O(l) Decomposition products H 2 CO 3 (aq) CO 2 (g) + H 2 O(l) H 2 SO 3 (aq) SO 2 (g) + H 2 O(l) NH 4 OH(aq) NH 3 (g) + H 2 O(l) 34

35 Process for Predicting the Products of a Gas-Evolving Reaction 1. Write the formula of the reactants 2. Use table 7.4 to predict the ion exchange product i.e.: H 2 S, H 2 CO 3, H 2 SO 3, or NH 4 OH 3. Form the second product from the remaining cation and anion 4. Check to see if either product decomposes. If so, rewrite as H 2 O(l) and a gas. See Table Balance the equation. 6. Determine solubility of other product in water. 35

36 Example Write a molecular equation for the reaction between an Aqueous Solution of Sodium Sulfite and an Aqueous Solution of Nitric Acid. 1. Write the formulas of the reactants. Na 2 SO 3 (aq) + HNO 3 (aq) 2. Use table 7.4 to predict the ion exchange product i.e.: H 2 S, H 2 CO 3, H 2 SO 3, or NH 4 OH Na 2 SO 3 (aq) + HNO 3 (aq) H 2 SO 3 (aq) + other product 36

37 Example Write a molecular equation for the reaction between an Aqueous Solution of Sodium Sulfite and an Aqueous Solution of Nitric Acid. 3. Form the second product from the remaining cation and anion Na 2 SO 3 (aq) + HNO 3 (aq) NaNO 3 + H 2 SO 3 4. Check to see if either product decomposes. If so, rewrite as H 2 O(l) and a gas. See Table 7.4 Na 2 SO 3 (aq) + HNO 3 (aq) NaNO 3 + SO 2 (g) + H 2 O(l) 37

38 Example When an Aqueous Solution of Sodium Sulfite Is Added to an Aqueous Solution of Nitric Acid, a Gas Evolves, Continued. 6. Balance the equation. Na 2 SO 3 (aq) + 2 HNO 3 (aq) 2 NaNO 3 + SO 2 (g) + H 2 O(l) 7. Determine the solubility of other product. NaNO 3 is soluble. a. Write an (s) after the insoluble products and an (aq) after the soluble products. Na 2 SO 3 (aq) + 2 HNO 3 (aq) 2 NaNO 3 (aq) + SO 2 (g) + H 2 O(l) 38

39 Practice Complete the Following Reactions. PbS(s) + H 2 SO 4 (aq) HNO 3 (aq) + NaHCO 3 (aq) 39

40 Ionic Equations Equations that describe the chemicals put into the water and the product molecules are called molecular equations. 2 KOH(aq) + Mg(NO 3 ) 2 (aq) 2 KNO 3 (aq) + Mg(OH) 2 (s) Equations that describe the chemicals in terms of ions are called complete ionic equations. Aqueous electrolytes are written as ions. Soluble salts, strong acids, strong bases. Insoluble substances and nonelectrolytes written in molecule form. Solids, liquids, and gases are not dissolved, therefore, molecule form. 2K +1 (aq) + 2OH -1 (aq) + Mg +2 (aq) + 2NO 3-1 (aq) 2K +1 (aq) + 2NO 3-1 (aq) + Mg(OH) 2(s) 40

41 Ionic Equations Precipitation, acid-base, and gas evolution reactions can all be written as ionic equations. When Writing Ionic Equations Only substances in an aqueous state can be written as ions 41

42 Writing Ionic Equations Only aqueous substances can be written as ions 2 KOH(aq) + Mg(NO 3 ) 2 (aq) 2 KNO 3 (aq) + Mg(OH) 2 (s) 2K +1 (aq) + 2OH -1 (aq) + Mg +2 (aq) + 2NO 3-1 (aq) 2K +1 (aq) + 2NO 3-1 (aq) + Mg(OH) 2(s) 42

43 Net Ionic Equations Ions that are both reactants and products are called spectator ions. 2K +1 (aq) + 2OH -1 (aq) + Mg +2 (aq) + 2NO 3-1 (aq) 2K +1 (aq) + 2NO 3-1 (aq) + Mg(OH) 2(s) An ionic equation in which the spectator ions are removed is called a net ionic equation. 2OH -1 (aq) + Mg +2 (aq) Mg(OH) 2(s) 43

44 **Example** Write the Ionic and Net Ionic Equation. K 2 SO 4 (aq) + Ba(NO 3 ) 2 (aq) 2 KNO 3 (aq) + BaSO 4 (s) 44

45 Other Reaction Types Oxidation-reduction reactions Combustion reactions Decomposition Synthesis or combination reactions Single Displacement reactions Double Displacement reactions 45

46 Reduction-Oxidation Reactions Also known as RedOx Involves transferring electrons from one atom to another Can be easily identified as Reactions in which either one or more of the reactants or propducts are free elements. Reactions that involve a change in oxidation state from reactant to product 46

47 Oxidation Reduction Reactions Oxidation: An element is oxidized when: it loses electrons OR when there is an increase in the oxidation state Reduction: An element is reduced when: it gains electrons OR when there is an decrease in the oxidation state Whenever you think of Oxidation- Reduction reactions, think of OIL-RIG 47

48 What is OIL-RIG? Oxidation Is Loss Reduction Is Gain

49 Electron Loss/Gain as Redox In the following reaction: Mg(s) + S(s) MgS(s) The magnesium atoms are oxidized. Mg 0 Mg e The oxygen atoms are reduced. S e S 2 49

50 Practice Decide Whether Each of the Following Reactions Is a Redox Reaction, Continued. 2 Al(s) + 3 Br 2 (l) 2 AlBr 3 (s) Yes, metal + nonmetal. CaSO 3 (s) + 2 HCl(aq) CaCl 2 (aq) + SO 2 (g) + H 2 O(l) No, this is a gas evolving reaction. Fe 2 O 3 (s) + C(s) 2 Fe(s) + 3 CO(g) Yes SO 2 (g) + O 2 (g) + H 2 O(l) H 2 SO 4 (aq) Yes, O 2 reactant. 50

51 Combustion Reactions Reactions in which O 2 (g) is a reactant are called combustion reactions. Combustion reactions release lots of energy. They are exothermic. Combustion reactions are a subclass of oxidation reduction reactions. 2 C 8 H 18 (g) + 25 O 2 (g) 16 CO 2 (g) + 18 H 2 O(g) 51

52 Products of Combustion To predict the products of a combustion reaction, combine each element in the other reactant with oxygen. Reactant Contains C Contains H Contains S Contains N Contains metal Combustion product CO 2 (g) H 2 O(g) SO 2 (g) NO(g) or NO 2 (g) M 2 O n (s) 52

53 Example Write the Equation for Each Reaction Combustion of the anesthetic cyclopropane, C 3 H 6 (g). 2 C 3 H 6 (g) + 9 O 2 (g) 6 CO 2 (g) + 6 H 2 O(g) Combustion of the non-toxic antifreeze propylene glycol, C 3 H 8 O 2 (l). 2 C 3 H 6 O 2 (l) + 7 O 2 (g) 6 CO 2 (g) + 6 H 2 O(g) 53

54 Classifying Reactions, Continued Another scheme classifies reactions by what the atoms do. Type of reaction Synthesis Decomposition Single Displacement Double displacement General equation A + B AB AB A + B A + BC AC + B AB + CD AD + CB 54

55 Combination Reactions Also known as composition or synthesis reactions. Two (or more) reactants combine together to make one product. Simpler substances combining together. 2 CO + O 2 2 CO 2 2 Mg + O 2 2 MgO HgI KI K 2 HgI 4 55

56 Decomposition Reactions A large molecule is broken apart into smaller molecules or its elements. Caused by addition of energy into the molecule. Have only one reactant, makes 2 or more products. 2 FeCl 3 elec ( s) 2 FeCl 2 ( l) Cl 2 ( g) 2 HgO (s) D 2 Hg(l) O 2 ( g) 2 O 3 h 3 O 2 56

57 Single Displacement Reactions Reactions that involve one atom displacing another and replacing it in a compound. In the reaction Zn(s) + 2 HCl(aq) ZnCl 2 (aq) + H 2 (g), the atom Zn displaces H from the compound. Other examples of displacement reactions are: Fe 2 O 3 (s) + Al(s) Fe(s) + Al 2 O 3 (s) 2 Na(s) + 2 H 2 O(aq) 2 NaOH(aq) + H 2 (g) Tro's "Introductory 57

58 Double Displacement Reactions Two ionic compounds exchange ions. May be followed by decomposition of one of the products to make a gas. X Y q (aq) + A B q (aq) XB + AY Precipitation, acid base, and gas evolving reactions are also double displacement reactions. 58

59 Examples of Double Displacement CaCl 2 (aq) + Na 2 CO 3 (aq) CaCO 3 (s) + 2 NaCl(aq) Ba(OH) 2 (s) + 2 HNO 3 (aq) Ba(NO 3 ) 2 (aq) + 2 H 2 O(l) Li 2 CO 3 (aq) + 2 HCl(aq) 2 LiCl(aq) + CO 2 (g) + H 2 O(l) 59

60 Example Classify the Following Reactions as Synthesis, Decomposition, Single Displacement, or Double Displacement, Continued. 3 Mg(s) + 2 FeCl 3 (aq) 3 MgCl 2 (aq) + 2 Fe(s) Single displacement. CO 2 (g) + H 2 O(l) H 2 CO 3 (aq) Synthesis. 3 KOH(aq) + H 3 PO 4 (aq) K 3 PO 4 (aq) + 3 H 2 O(l) Double displacement. heat 3 g Decomposition. CaCO ( s) CaO( s) CO2( Tro's "Introductory 60 )

61 Recommended Study Problems Chapter 7 NB: Study problems are used to check the student s understanding of the lecture material. Students are EXPECTED TO BE ABLE TO SOLVE ALL THE SUGGESTED STUDY PROBLEMS. If you encounter any problems, please talk to your professor or seek help at the HACC-Gettysburg learning center. Questions from text book Chapter 7, p 232 5, 7, 29, 37, 53, 55, 61, 65, 67, 75, 83, 85, 87, 89, 93, 115 ANSWERS -The answers to the odd-numbered study problems are found at the back of your textbook 61

Chapter 7. Chapter opener with space shuttle launch. Chemical Reactions

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