1. Chapter 7 Chemical Reactions

2. 2 Experiencing Chemical Change chemical reactions are happening both around you and in you all the time some are very simple, others are complex in terms of the pieces – even the simple ones have a lot of interesting principles to learn from chemical reactions involve changes in the structures of the molecules, and many times we can experience the effects of those changes What are some examples of chemical reactions you experience?

3. 3 Chemical Reactions Reactions involve chemical changes in matter resulting in new substances Reactions involve rearrangement and exchange of atoms to produce new molecules Elements are not transmuted during a reaction Reactants  Products

4. 4 Evidence of Chemical Reactions look for evidence of a new substance visual clues (permanent) color change precipitate formation  solid that forms when liquid solutions are mixed gas bubbles large energy changes  container becomes very hot or cold  emission of light other clues new odor whooshing sound from a tube permanent new state

5. 5 Evidence of Chemical Change Color Change Formation of Solid Precipitate Formation of a Gas Emission of Light Release or Absorption of Heat

6. 6 Evidence of Chemical Change in order to be absolutely sure that a chemical reaction has taken place, you need to go down to the molecular level and analyze the structures of the molecules at the beginning and end Is boiling water a chemical change?

7. 7 Chemical Equations Shorthand 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 weights of reactants used and products that can be made

8. 8 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

9. 9 The Combustion of Methane methane gas burns to produce carbon dioxide gas and gaseous water whenever something burns it combines with O 2 (g)

10. 10 Combustion of Methane methane gas burns to produce carbon dioxide gas and gaseous water whenever something burns it combines with O 2 (g) CH 4 (g) + O 2 (g)  CO 2 (g) + H 2 O(g) H H C H H OO + O O C + O HH 1 C + 4 H + 2 O1 C + 2 O + 2 H + O 1 C + 2 H + 3 O

11. 11 Combustion of Methane Balanced to show the reaction obeys the Law of Conservation of Mass it must be balanced CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O(g) H H C H H + O O C + OO OO + O HH O HH + 1 C + 4 H + 4 O

12. 12 Chemical Equations CH 4 and O 2 are the reactants, and CO 2 and H 2 O are the products the (g) after the formulas tells us the state of the chemical the number in front of each substance tells us the numbers of those molecules in the reaction called the coefficients CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O(g)

13. 13 Chemical Equations this equation is balanced, meaning that there are equal numbers of atoms of each element on the reactant and product sides to obtain the number of atoms of an element, multiply the subscript by the coefficient 1  C  1 4  H  4 4  O  2 + 2 CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O(g)

14. 14 Symbols Used in Equations symbols used to indicate state after chemical (g) = gas; (l) = liquid; (s) = solid (aq) = aqueous = dissolved in water energy symbols used above the arrow for decomposition reactions  = heat h = light shock = mechanical elec = electrical

15. 15 Writing Balanced Chemical Equations 1.Write a skeletal equation by writing the formula of each reactant and product 2.Count the number of atoms of each element on each side of the equation polyatomic ions may often be counted as if they are one “element” 3.Pick an element to balance if an element is found in only one compound on both sides, balance it first leave elements that are free elements somewhere in the equation until last

16. 16 Writing Balanced Chemical Equations 4.Find the Least Common Multiple of the number of atoms on each side the LCM of 3 and 2 is 6 5.Multiply each count by a factor to make it equal to the LCM 6.Use this factor as a coefficient in the equation if there is already a coefficient there, multiply it by the factor it must go in front of entire molecules, not between atoms within a molecule 7.Recount and Repeat until Balanced

17. 17 Examples when magnesium metal burns in air it produces a white, powdery compound magnesium oxide Mg(s) + O 2 (g)  MgO(s)

18. 18 Examples Under appropriate conditions at 1000°C ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water NH 3 (g) + O 2 (g)  NO(g) + H 2 O(g)

19. 19 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

20. 20 Predicting Whether a Reaction Will Occur in Aqueous Solution “forces” that drive a reaction 1)formation of a solid 2)formation of water 3)formation of a gas 4)transfer of electrons when chemicals (dissolved in water) are mixed and one of these 4 things can occur, the reaction will generally happen

21. 21 Dissociation when ionic compounds dissolve in water, the anions and cations are separated from each other - this is called dissociationdissociation however not all ionic compounds are soluble in water! when compounds containing polyatomic ions dissociate, the polyatomic group stays together as one ion polyatomic ions dissociate

22. 22 Dissociation potassium iodide dissociates in water into potassium cations and iodide anions KI(aq)  K +1 (aq) + I -1 (aq) copper(II) sulfate dissociates in water into copper(II) cations and sulfate anions CuSO 4 (aq)  Cu +2 (aq) + SO 4 -2 (aq) K +1 I -1 K I Cu +2 SO 4 -2 Cu SO 4

23. 23 Dissociation 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 +1 SO 4 -2 K +1 KK SO 4

24. 24 Electrolytes electrolytes are substances whose water solution is a conductor of electricity all electrolyte have ions dissolved in water

25. 25 Electrolytes 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

26. 26 Types of Electrolytes salts = water soluble ionic compounds all strong electrolytes acids = form H +1 ions in water solution sour taste react and dissolve many metals strong acid = strong electrolyte, weak acid = weak electrolyte bases = water soluble metal hydroxides bitter taste, slippery (soapy) feeling solutions increases the OH -1 concentration

27. 27 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

28. 28 When will a Salt Dissolve? Predicting whether a compound will dissolve in water is not easy The best way to do it is to do some experiments to test whether a compound will dissolve in water, then develop some rules based on those experimental results we call this method the empirical method

29. 29 Compounds Containing the following Ions are Generally Soluble Exceptions (when combined with ions on the left the compound is insoluble) Li +, Na +, K +, NH 4 + none NO 3 –, C 2 H 3 O 2 – none Cl –, Br –, I – Ag +, Hg 2 2+, Pb 2+ SO 4 2– Ca 2+, Sr 2+, Ba 2+, Pb 2+ Solubility Rules Compounds that are Generally Soluble in Water

30. 30 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 +, Ca 2+, Sr 2+, Ba 2+ S 2– Li +, Na +, K +, NH 4 +, Ca 2+, Sr 2+, Ba 2+ CO 3 2–, PO 4 3– Li +, Na +, K +, NH 4 + Solubility Rules Compounds that are Generally Insoluble

31. 31 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 but if a rule says the compounds are mostly insoluble, then the exceptions are soluble note: slightly soluble  insoluble

32. 32 Determine if Each of the Following is Soluble in Water KOH AgBr CaCl 2 Pb(NO 3 ) 2 PbSO 4

33. 33 Determine if Each of the Following is Soluble in Water KOHSoluble, because the cation is K + AgBrInsoluble, even though most compounds with Br - are soluble, this is an exception CaCl 2 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

34. 34 Precipitation Reactions many reactions are done by mixing aqueous solutions of electrolytes together when this is done, often a reaction will take place from the cations and anions in the two solutions exchanging if the ion exchange results in forming a compound that is insoluble in water, it will come out of solution as a precipitate

35. 35 Precipitation Reactions Pb(NO 3 ) 2 (aq) + 2 KI(aq)  2 KNO 3 (aq) + PbI 2 (s)

36. 36 Precipitation Reactions Pb(NO 3 ) 2 (aq) + 2 KI(aq)  2 KNO 3 (aq) + PbI 2 (s)

37. 37 No Precipitate Formation = No Reaction KI(aq) + NaCl(aq)  KCl(aq) + NaI(aq) all ions still present,  no reaction

38. 38 Process for Predicting the Products of a Precipitation Reaction 1.Determine what ions each aqueous reactant has 2.Exchange Ions (+) ion from one reactant with (-) ion from 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

39. 39 Example 7.7 - When an aqueous solution of sodium carbonate is added to an aqueous solution of copper(II) chloride, a white solid forms 1.Write the formulas of the reactants Na 2 CO 3 (aq) + CuCl 2 (aq)  2.Determine the ions present when each reactant dissociates (Na + + CO 3 2- ) + (Cu +2 + Cl - )  3.Exchange the Ions (Na + + CO 3 2- ) + (Cu +2 + Cl - )  (Na + + Cl - ) + (Cu +2 + CO 3 2- )

40. 40 4.Write the formulas of the products cross charges and reduce Na 2 CO 3 (aq) + CuCl 2 (aq)  NaCl + CuCO 3 5.Balance the Equation Na 2 CO 3 (aq) + CuCl 2 (aq)  NaCl + CuCO 3 Example 7.7 - When an aqueous solution of sodium carbonate is added to an aqueous solution of copper(II) chloride, a white solid forms

41. 41 6.Determine the solubility of each product NaCl is soluble CuCO 3 is insoluble 7.Write an (s) after the insoluble products and a (aq) after the soluble products Na 2 CO 3 (aq) + CuCl 2 (aq)  NaCl(aq) + CuCO 3 (s) Example 7.7 - When an aqueous solution of sodium carbonate is added to an aqueous solution of copper(II) chloride, a white solid forms

42. 42 Ionic Equations equations which 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 which describe the actual dissolved species are called 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)  K +1 (aq) + 2NO 3 -1 (aq) + Mg(OH) 2(s)

43. 43 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)  K +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)

44. 44 Acid-Base Reactions also called neutralization reactions because the acid and base neutralize each others properties in the reaction of an acid with a base, the H +1 from the acid combines with the OH -1 from the base to make water the cation from the base combines with the anion from the acid to make the salt acid + base  salt + water 2 HNO 3 (aq) + Ca(OH) 2 (aq)  Ca(NO 3 ) 2 (aq) + 2 H 2 O(l) the net ionic equation for an Acid-Base reaction is H +1 (aq) + OH -1 (aq)  H 2 O(l) as long as the salt that forms is soluble in water

45. 45 Example 7.11 - Write the molecular, ionic and net-ionic 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.Determine the ions present when each reactant dissociates (H + + NO 3 - ) + (Ca +2 + OH - )  3.Exchange the ions, H +1 combines with OH -1 to make H 2 O(l) (H + + NO 3 - ) + (Ca +2 + OH - )  (Ca +2 + NO 3 - ) + H 2 O(l)

46. 46 4.Write the formulas of the products cross charges and reduce HNO 3 (aq) + Ca(OH) 2 (aq)  Ca(NO 3 ) 2 + H 2 O(l) 5.Balance the Equation may be quickly balanced by matching the numbers of H and OH to make H 2 O coefficient of the salt is always 1 2 HNO 3 (aq) + Ca(OH) 2 (aq)  Ca(NO 3 ) 2 + 2 H 2 O(l) Example 7.11 - Write the molecular, ionic and net-ionic equation for the reaction of aqueous nitric acid with aqueous calcium hydroxide

47. 47 6.Determine the solubility of the salt Ca(NO 3 ) 2 is soluble 7.Write an (s) after the insoluble products and a (aq) after the soluble products 2 HNO 3 (aq) + Ca(OH) 2 (aq)  Ca(NO 3 ) 2 (aq) + 2 H 2 O(l) Example 7.11 - Write the molecular, ionic and net-ionic equation for the reaction of aqueous nitric acid with aqueous calcium hydroxide

48. 48 Example 7.11 - Write the molecular, ionic and net-ionic equation for the reaction of aqueous nitric acid with aqueous calcium hydroxide 8.Dissociate all aqueous materials to get complete ionic equation not H 2 O 2 H + (aq) + 2 NO 3 - (aq) + Ca +2 (aq) + 2 OH - (aq)  Ca +2 (aq) + 2 NO 3 - (aq) + H 2 O(l) 9.Eliminate spectator ions to get net-ionic equation 2 H +1 (aq) + 2 OH -1 (aq)  H 2 O(l) H +1 (aq) + OH -1 (aq)  H 2 O(l)

49. 49 Gas Evolving Reactions Some reactions form a gas directly from the ion exchange K 2 S(aq) + H 2 SO 4 (aq)  K 2 SO 4 (aq) + H 2 S(g) Other reactions form a gas by the decomposition of one of the ion exchange products into a gas and water K 2 SO 3 (aq) + H 2 SO 4 (aq)  K 2 SO 4 (aq) + H 2 SO 3 (aq) H 2 SO 3  H 2 O(l) + SO 2 (g)

50. 50 Compounds that Undergo Gas Evolving Reactions Reactant Type Reacting With Ion Exchange Product Decom pose? Gas Formed Example metal n S, metal HS acidH2SH2SnoH2SH2S K 2 S(aq) + 2HCl(aq)  2KCl(aq) + H 2 S(g) metal n CO 3, metal HCO 3 acidH 2 CO 3 yesCO 2 K 2 CO 3 (aq) + 2HCl(aq)  2KCl(aq) + CO 2 (g) + H 2 O(l) metal n SO 3 metal HSO 3 acidH 2 SO 3 yesSO 2 K 2 SO 3 (aq) + 2HCl(aq)  2KCl(aq) + SO 2 (g) + H 2 O(l) (NH 4 ) n anionbaseNH 4 OHyesNH 3 KOH(aq) + NH 4 Cl(aq)  KCl(aq) + NH 3 (g) + H 2 O(l)

51. 51 Process for Predicting the Products of a Gas Evolving Reaction 1.Determine what ions each aqueous reactant has 2.Exchange Ions (+) ion from one reactant with (-) ion from other 3.Balance Charges of combined ions to get formula of each product 4.Check to see if either product is H 2 S 5.Check to see if either product decomposes, if so rewrite as H 2 O(l) and a gas see table 6.Balance the Equation 7.Determine Solubility of Other Product in Water

52. 52 Example - When an aqueous solution of sodium sulfite is added to an aqueous solution of nitric acid, a gas evolves 1.Write the formulas of the reactants Na 2 SO 3 (aq) + HNO 3 (aq)  2.Determine the ions present when each reactant dissociates (Na +1 + SO 3 -2 ) + (H +1 + NO 3 -1 )  3.Exchange the Ions (Na +1 + SO 3 -2 ) + (H +1 + NO 3 -1 )  (Na +1 + NO 3 -1 ) + (H +1 + SO 3 -2 )

53. 53 4.Write the formulas of the products cross charges and reduce Na 2 SO 3 (aq) + HNO 3 (aq)  NaNO 3 + H 2 SO 3 5.Check to see either product H 2 S - No 6.Check to see of either product decomposes – Yes H 2 SO 3 decomposes into SO 2 (g) + H 2 O(l) Na 2 SO 3 (aq) + HNO 3 (aq)  NaNO 3 + SO 2 (g) + H 2 O(l) Example - When an aqueous solution of sodium sulfite is added to an aqueous solution of nitric acid, a gas evolves

54. 54 7.Balance the Equation Na 2 SO 3 (aq) + 2 HNO 3 (aq)  NaNO 3 + SO 2 (g) + H 2 O(l) 8.Determine the solubility of other product NaNO 3 is soluble 9.Write an (s) after the insoluble products and a (aq) after the soluble products Na 2 SO 3 (aq) + 2 HNO 3 (aq)  NaNO 3 (aq) + SO 2 (g) + H 2 O(l) Example - When an aqueous solution of sodium sulfite is added to an aqueous solution of nitric acid, a gas evolves

55. 55 Other Patterns in Reactions the precipitation, acid-base, and gas evolving reactions all involved exchanging the ions in the solution other kinds of reactions involve transferring electrons from one atom to another – these are called oxidation-reduction reactions also known as redox reactions unlike the others, many of these reactions are not done by dissolving the reactants in water

56. 56 Oxidation-Reduction Reactions We say that the element that loses electrons in the reaction is oxidized and the substance that gains electrons in the reaction is reduced you cannot have one without the other

57. 57 Reactions of Metals with Nonmetals Reactions of Metals with Nonmetals (Oxidation-Reduction) metals react with nonmetals to form ionic compounds ionic compounds are solids at room temperature the metal loses electrons and becomes a cation the metal undergoes oxidation the nonmetal gains electrons and becomes an anion the nonmetal undergoes reduction In the reaction, electrons are transferred from the metal to the nonmetal 2 Na(s) + Cl 2 (g)  NaCl(s)

58. 58 Oxidation-Reduction Reactions any reaction that has an element that is uncombined on one side and combined on the other is a redox reaction uncombined = free element 2 CO + O 2  2 CO 2 2 N 2 O 5  4 NO 2 + O 2 3 C + Fe 2 O 3  3 CO + 2 Fe Mg + Cl 2  MgCl 2 any reaction where a cation changes charge is redox CuCl + FeCl 3  FeCl 2 + CuCl 2 SnCl 2 + F 2  SnCl 2 F 2

59. 59 Combustion Reactions Reactions in which O 2 (g) is a reactant are called Combustion Reactions Combustion reactions release lots of energy 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)

60. 60 Combustion Products to predict the products of a combustion reaction, combine each element in the other reactant with oxygen ReactantCombustion Product contains CCO 2 (g) contains HH 2 O(g) contains SSO 2 (g) contains NNO(g) or NO 2 (g) contains metalM 2 O n (s)

61. 61 Classifying Reactions one way is based on the process that happens precipitation, neutralization, formation of a gas, or transfer of electrons

62. 62 Classifying Reactions another scheme classifies reactions by what the atoms do Type of ReactionGeneral Equation Synthesis A + B  AB Decomposition AB  A + B Displacement A + BC  AC + B Double Displacement AB + CD  AD + CB

63. 63 Synthesis Reactions also known as Composition or Combination 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 2 + 2 KI  K 2 HgI 4

64. 64 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, make 2 or more products

65. 65 Decomposition of Water

66. 66 Displacement Reactions also known as single-displacement reactions Reactions that involve one anion being transferred from one cation to another X  Y  + A   X + A  Y  Zn(s) + 2 HCl(aq)  ZnCl 2 (aq) + H 2 (g) 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)

67. 67 Displacement of Copper by Zinc

68. 68 Double Displacement Reactions two ionic compounds exchange ions may be followed by decomposition of one of the products to make a gas X  Y  (aq) + A  B  (aq)  XB + AY precipitation, acid-base and gas-evolving reactions are also double displacement reactions