Working With Chemical Reactions

1. Working With Chemical Reactions • When we discuss chemical reactions we break them down into categories: - Combination (Synthesis, Addition) - Single Replacement - Metathesis (Double Replacement) - Combustion - Decomposition - Organic - Redox - Acid Base Reactions

2. Additional Categories • Lewis Acid Base Reactions – Adduct formation • Reactions of Anhydrides – Substances that do not contain water, but react with water to produce acids and bases, usually in the form of a gas bubbled through water or a solid. May also have acidic and basic reactants. • Non-trivial redox – slightly unpredictable

3. Identification Problems It is difficult to identify a reaction using only the reactants. For example… Solid Calcium Oxide is exposed to a stream of Carbon Dioxide Gas. You are asked to write the equation, in net ionic form, if possible and to answer the following question: What type of reaction has occurred?

4. Presented with the complete reaction Solid Calcium Oxide is exposed to a stream of Carbon Dioxide Gas. CaO + CO2 → CaCO3 It would be easy to determine that this is a combination reaction... but how can the behavior of CaO be predicted? Which Presents a Problem

5. Strategies to Help • Focus on gathering information about the reactants so we can determine the kinds of reactions they will undergo and better predict products. Learn strategies that can help to incorporate large amounts of information into our existing knowledge base.

6. First… Rethink Memorization • Rote memorization has distinct disadvantages. • It doesn’t allow you to apply knowledge in a variety of situations. • When thinking of ways to study, it’s essential that facts not remain isolated but that there is an understanding the same principles may apply in more that one case.

7. The Purpose of Memorization, is to allow you to acquire a working knowledge base that can be readily recalled so that you can use your mental energy to focus on higher level problem solving. In order to readily recall and remember information IT MUST BE ORGANIZED IN A WAY THAT IS Associated with your existing knowledge Grouped so that it can be readily used Memorization – Acquiring a Knowledge Base

8. To Help You to Learn How to Do This • Reactants can be organized into broad categories, and breaking them down according to element/compound recognition (when looking at the reactants). • Key tools you will need mainly: solubility guidelines, acid/base lists, the reactivity series, must also be memorized.

9. Reactions By Reactant • Starting from the reactions that are most numerous, and working towards narrower categories. 1) Simple Reactions Involving Metals 2) Non-metal Reactions – not organic 3) Acid Base Reactions 4) Complex Ions 5) Non-trivial Redox

10. There are 3 things that drive reactions forward: Formation of a Gaseous product Formation of water Formation of an insoluble precipitate REMEMBER!!!

11. Reactions of Elemental Metals • Active metals react with cold water, steam, and non-oxidizing metals to varying degrees. • These are single replacement reactions The table on page 153 in your text clearly shows this trend

13. Reactions of Elemental Metals • The most active metals (K, Na, Li, & Ca), will react with cold water to produce a metal oxide and hydrogen gas. Small chunks of Sodium are added to cold water. 2Na + 2H2O → 2NaOH + H2 Net Ionic 2Na + 2H2O → 2Na+ + 2OH- + H2 Describe a test to confirm the gaseous product in your reaction: Hold a match to a flask containing a small sample and listen for a pop.

14. Sodium and Water Source 1 The flame is Hydrogen Gas which has ignited due to the heat produced by the reaction.

15. Reactions of Elemental Metals • The active metals from Al to Iron in the reactivity series will react with steam to produce the metal oxide and hydrogen gas. Steam is passed over solid magnesium. Mg + H2O  MgO + H2

16. Reactions of Elemental Metals • Active to less active metals, (Li to Pb on reactivity series), will react with non-oxidizing acids to produce the salt of the metal and Hydrogen gas. Calcium Metal is added to a dilute solution of Hydrochloric Acid. Ca + HCl → CaCl2 + H2 Net Ionic: Ca + 2H+ → Ca+2 + H2 Give at least one visual observation for this reaction: The release of hydrogen gas would cause bubbling.

17. Reactions of Active Metals with O2 • SLPMg ='s 4 active metals: Sodium, Lithium, Potassium and Magnesium (only magnesium is stable in air and has to be ignited) Lithium Metal is burned in air. Besides Combustion, what type of reaction could this be classified as? 4Li + O2 → 2Li2O This is a synthesis, or combination reaction.

18. Reactions of Active Metals with O2 • SLPMg will burn in Oxygen to produce metal oxides that will all become strong bases when dissolved in water. • These are combustion reactions and Synthesis reactions (combination), and redox reactions Mg being ignited Mg Burning (Source 1)

19. Less Reactive Metals + O2 • Less reactive metals will also react with Oxygen to form oxides, however they do so without combustion. • Six commonly react with unheated oxygen, each more slowly from Al, Zn, Fe, Sn, Pb, Cu. Rust - Iron Oxide Oxidized Copper

20. Less Reactive Metals + O2 Iron is heated and allowed to react in the presence of excess oxygen. 3Fe + 2O2  Fe3O2 The word excess in the question indicates that you should give Iron the higher oxidation state. What is the oxidation state of the Iron in the resulting compound. The Iron has adopted a 3+ charge.

21. Least Active Metals • Hg, Ag, Pd, Pt and Au are the least reactive metals. • If they occur as oxides, they will readily decompose if heated. • Gold and Platinum are the least reactive, and won’t form oxides. Since gold does not Oxidize these gold Artifacts from Egypt to Survive thousands of years.

22. Reactions of Group 1 + 2 MetalsWith Nitrogen • You may remember that Nitrogen as generally nonreactive due to its triple bond. • Nitrogen does react with certain main group elements, but may require intense heat to provide the energy needed to drive the reaction.

23. Reactions of Group 1 + 2 MetalsWith Nitrogen • Group 1 - only Lithium reacts with Nitrogen at room temperature to yield Li3N. 6Li + N2 2Li3N • Group 2 - Mg, Ca, Ba and Sr will all react with nitrogen to produce compounds, but must be strongly heated. In all cases, the Nitrogen will adopt a -3 oxidation state.

24. Magnesium Metal is added to nitrogen gas. 3Mg + N2→ Mg3N2 What is the oxidation number of Magnesium before and after the reaction? No Net Ionic equation is necessary. In this case since the magnesium is elemental its Oxidation state is initially 0 and goes to a +2. Example Question

25. Decomposition of the Products • 6Li + N2 → 2Li3N (salt like solid) • Decomposes to: Li3N + 3H2O → 3Li+ + OH- + NH3 • 3M + N2 → M3N2 (s) – where M= Mg,Ca, Ba, Sr • Decomposes when heated to: M3N2 (s) → 3M (s) + N2(g) • Or the Metal Nitride can react with water to produce ammonia and the metal hydroxide M3N2 (s) + 6H2O (l) → 3M(OH)2 (aq) + 2NH3 (g)

26. Reactions of Metal Oxides • Metal Oxides that are soluble, will react with water to form hydroxides. Calcium metal is heated strongly in the presence of oxygen. 2Ca + O2 → 2CaO If the product of this solution is added to water is the resulting mixture acidic or basic? 2CaO + H2O →2Ca+2 + 2OH- The formation of hydroxide indicates this solution is basic.

27. Reactions of Metal Oxides • Metal Oxides that are soluble, will react with water to form hydroxides. Solid Lithium Oxide is added to excess water. Is the final solution acidic, basic or neutral? Li2O + H2O  2LiOH Net ionic: Li2O + H2O  2Li+ + OH- The resulting solution is basic.

28. Solid Barium Oxide is added to distilled water. Is the resulting solution acidic basic or neutral? Example Questions

29. Reactions of Metal Oxides • Metal oxides will react with non-metal oxides to form salts. Solid Calcium Oxide is exposed to a stream of Carbon Dioxide gas. CaO + CO2 CaCO3 What type of reaction has occurred? Synthesis/Combination ** notice that this forms an insoluble salt

30. Sulfur Dioxide gas is passed over Solid Calcium Oxide. Name the product compound. Example Questions

31. A mixture of solid Calcium Oxide, And solid Tetraphosphorus Decaoxide is heated. Is the product compound soluble in water? Example Questions

32. CaO + P4O10 → Ca + 4P + 11O There is the potential to make phosphate and therefore get an insoluble product 6 calcium oxides and 1 tetraphosphorousdecaoxide make exactly 2 calcium phosphates. 6CaO + P4O10 → 2Ca3(PO4)2

33. Reverse Reaction • Carbonates will decompose back into metal oxides and carbon dioxide when roasted (heated). CaO + CO2 CaCO3 CaCO3 +HEATCaO + CO2

34. Reactions of Metal Oxides • Metal Oxides will react with acids to produce water and a salt of the metal...this is like a version of a neutralization reaction. CaO + 2HCl  CaCl2 + H2O Net Ionic: CaO + 2HCl  Ca+2 + Cl- + H2O • From your packet: “A metal oxide is, in a real sense, a basic anhydride, i.e. a base with the water removed.” So an acid plus a basic anhydride = neutralization

35. Reactions of Metal Hydrides • Typical metal hydrides are those formed from the reaction of a active group one or two metal, and hydrogen gas. • In a metal hydride, hydrogen will carry a -1 charge, instead of +1. You will notice that in hydrides H is not in front, but follows the metal cation, since here it is an anion.

36. Metal hydrides react with water to produce the metal hydroxide, which you will recognize as a strong base, and hydrogen gas will be evolved. Solid Lithium Hydride is added to distilled water. LiH + H2O → LiOH + H2 Net Ionic: LiH + H2O → Li+ + OH- + H2 Is the final solution acidic, basic or neutral? The resulting solution would be basic. Reactions of Metal Hydrides

37. Reaction Reversal • Metal hydroxides may then decompose into metal oxides and water Zn(OH)2 → ZnO + H2O

38. Other Patterns - Decomposition • Chlorates decompose into metal chlorides and oxygen gas • Alkali and alkaline nitrates decompose into nitrites and oxygen gas

39. Solid Potassium Chlorate is heated in the presence of manganese dioxide as a catalyst. How many moles of reaction products can be produced from one mole of potassium chlorate. Example Question

40. Difficulties • One of the more difficult types of questions are those that demand recall of a particular reaction in terms of physical observations. • For example: Aluminum Metal is added to a solution of Copper (II) Chloride. List at least 2 observations that indicate a chemical reaction is occurring.

41. Working through it • Single replacement, double replacement, redox?? Well sort of all of the above. • We can get credit for at least half this equation because we know our reactivity series and we know Aluminum is more active than copper so: Al + CuCl2 → AlCl3 + Cu Balanced: 2Al + 3CuCl2 → 2AlCl3 + 3Cu Net Ionic: 2Al + 3Cu2+ → 2Al3+ + 3Cu

42. Now What? • What if you've never done this reaction? • Some information that can help: If you would like you can memorize the colors of some transition metal ions. In water they form complex ions, which give them colors...

43. Colors of Complex Ions

44. That's one clue... • If you know Copper is being reduced to Copper metal, its complex ion will come out of solution, so the blue/green color of the solution will fade. • Now think harder, where is the copper going? Well it's being converted back to copper metal, which is a reddish color, so we could take a stab, and say there will be a red precipitate. • Third... if you would like to see this for yourself, this reaction is most definitely exothermic go to: http://www.youtube.com/watch?v=OQPwPGDQqzs • So you could also say, heat is given off.

46. Basic Comparison of Active Metals • If the two metals are in the same group, the one in the lower position will be more reactive. • If the two metals are in the same period, the one further left will be more reactive. • Transition metals can be memorized as a list, often people use a mnemonic device, using the first letters.

47. Good Old Precipitates • Knowing your solubility guidelines can really give you a lot of insight into reactions. • A question that says both compounds are in solution can be written out in ionic form, where a pair of suspected precipitators can be identified. • Additionally, a lot of complex ion reactions, involve the dissolving of an otherwise insoluble compound, using a strong base or acid, another clue!

48. Solutions of Silver Nitrate and Sodium Chromate are mixed. What is the oxidation number of the chromium in the chromate ion? Example Question

49. Write your reaction and balance 2AgNO3 + Na2CrO4 → Ag2CrO4 + 2NaNO3 Now you must write is it in net ionic form: 2Ag+ + CrO4-2 → Ag2CrO4 The Oxidation number of chromium in chromate is always the same, so even if you struggled with reaction, you should be able to say that this chromium is a +6

50. Solutions of Manganese (II) Sulfate and Ammonium Sulfide are mixed. List any precipitate that forms during this reaction. Example Question