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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

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

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?

Which Presents a Problem

Presented with the complete reaction Solid Calcium Oxide is exposed to a stream of Carbon Dioxide Gas. CaO + CO 2 → CaCO 3 It would be easy to determine that this is a combination reaction... but how can the behavior of CaO be predicted?

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.

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.

Memorization – Acquiring a Knowledge Base

• 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

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.

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

REMEMBER!!!

There are 3 things that drive reactions forward: 1) 2) 3) Formation of a Gaseous product Formation of water Formation of an insoluble precipitate

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

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 + 2H 2 O → 2NaOH + H 2 Net Ionic 2Na + 2H 2 O → 2Na + + 2OH + H 2 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.

Sodium and Water

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

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

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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 → CaCl 2 + H 2 Net Ionic: Ca + 2H + → Ca +2 + H 2 Give at least one visual observation for this reaction: The release of hydrogen gas would cause bubbling.

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Reactions of Active Metals with O

2 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 + O 2 → 2Li 2 O This is a synthesis, or combination reaction.

Reactions of Active Metals with O

2   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)

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Less Reactive Metals + O

2 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.

Less Reactive Metals + O

2 Iron is heated and allowed to react in the presence of excess oxygen. 3Fe + 2O 2  Fe 3 O 2 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.

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.

Reactions of Group 1 + 2 Metals With 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.

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Reactions of Group 1 + 2 Metals With Nitrogen

Group 1 - only Lithium reacts with Nitrogen at room temperature to yield Li 3 N.

6Li + N 2  2Li 3 N  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.

Example Question

 Magnesium Metal is added to nitrogen gas.

3Mg + N 2 → Mg 3 N 2  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.

Decomposition of the Products

  6Li + N 2 → 2Li 3 N (salt like solid) Decomposes to:   Li 3 N + 3H 2 O → 3Li + 3M + N 2 → M 3 N 2 + OH + NH 3 (s) – where M= Mg,Ca, Ba, Sr Decomposes when heated to:  M 3 N 2 (s) → 3M (s) + N 2 (g) Or the Metal Nitride can react with water to produce ammonia and the metal hydroxide M 3 N 2 (s) + 6H 2 O (l) → 3M(OH) 2 (aq) + 2NH 3 (g)

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 + O 2 → 2CaO If the product of this solution is added to water is the resulting mixture acidic or basic?

2CaO + H 2 O → 2Ca +2 + 2OH The formation of hydroxide indicates this solution is basic.

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?

Li 2 O + H 2 O  2LiOH Net ionic: Li 2 O + H 2 O  2Li + + OH The resulting solution is basic.

Example Questions

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

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 + CO 2  CaCO 3 What type of reaction has occurred?

Synthesis/Combination

** notice that this forms an insoluble salt

Example Questions

Sulfur Dioxide gas is passed over Solid Calcium Oxide.

Name the product compound.

Example Questions

A mixture of solid Calcium Oxide, And solid Tetraphosphorus Decaoxide is heated.

Is the product compound soluble in water?

CaO + P 4 O 10 → Ca + 4P + 11O There is the potential to make phosphate and therefore get an insoluble product •

6 calcium oxides and 1 tetraphosphorous decaoxide make exactly 2 calcium phosphates.

6CaO + P 4 O 10 → 2Ca 3 (PO 4 ) 2

Reverse Reaction

 Carbonates will decompose back into metal oxides and carbon dioxide when roasted (heated).

CaO + CO 2  CaCO 3 CaCO 3 + HEAT  CaO + CO 2

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  CaCl 2 + H 2 O  Net Ionic: CaO + 2HCl  Ca +2 + Cl + H 2 O 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

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.

Reactions of Metal Hydrides

 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 + H 2 O → LiOH + H 2 Net Ionic: LiH + H 2 O → Li + + OH + H 2 Is the final solution acidic, basic or neutral?

The resulting solution would be basic.

Reaction Reversal

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Metal hydroxides may then decompose into metal oxides and water Zn(OH)

2 →

ZnO + H

2

O

Other Patterns - Decomposition

 Chlorates decompose into metal chlorides and oxygen gas  Alkali and alkaline nitrates decompose into nitrites and oxygen gas

Example Question

 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.

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.

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 + CuCl 2 → AlCl 3 Balanced: 2Al + 3CuCl 2 → + Cu 2AlCl 3 Net Ionic: 2Al + 3Cu 2+ → 2Al 3+ + 3Cu + 3Cu

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...

Colors of Complex Ions

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.

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.

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!

Example Question

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

• Write your reaction and balance • 2AgNO 3 + Na 2 CrO 4 → Ag 2 CrO 4 + 2NaNO 3 • Now you must write is it in net ionic form: • • 2Ag + + CrO 4 -2 → Ag 2 CrO 4 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

Example Question

 Solutions of Manganese (II) Sulfate and Ammonium Sulfide are mixed.

 List any precipitate that forms during this reaction.

Example Question

 A solution of copper (II) chloride is added to a solution of sodium sulfide.

 Name the spectator ions in this reaction.

Reactions of Non-Metals

   Organic reactions are specific and have their own section.

Non-metals and oxygen react to form non-metal oxides, limited oxygen might produce a monoxide (like Carbon monoxide), excess produces the dioxide.

2 non-metals can undergo Combination/synthesis and form molecular compounds.

Reactions of Non-metals

 Non-metal oxides and water form ternary acids.

ie. Dinitrogen pentoxide and water form?

Carbon Dioxide and water form?

Reverse Reaction: Ternary acids decompose into non-metal oxides and water.

Answers

 Nitric Acid  Carbonic Acid  These are synthesis/combination reactions so all the reactants are in the product, acid should be written in ionic form because they are soluble.

Individual Reactions

 Unfortunately some characteristic reactions do need to be recognized and memorized.

 Carbon disulfide has a characteristic reaction with oxygen to produce Carbon Dioxide and Sulfur Dioxide. CS 2 + O 2 → CO 2 + SO 2

Other Patterns - Decomposition

  Some ammonium salts decompose to lose ammonia.

Ammonium salts containing nitrate or dichromate decompose to produce an oxide, water and nitrogen gas. The N or Cr undergoes reduction.

Ex. Solid ammonium carbonate is heated.

Classify the reaction type.

Yields 2 ammonia, Carbon dioxide, and water. Heat is your clue that this is decomposition. Remove ammonia, and also carbonates decompose to carbon dioxide, the remaining elements form water.

Other Patterns - Decomposition

 Hydrogen peroxide decomposes in the presence of a catalyst to water and oxygen Ex. A solution of Hydrogen Peroxide is catalytically decomposed. Name the element being reduced and the element being oxidized. This needs to be balanced, two hydrogen peroxide yields two water and one oxygen molecule.

Example Question

 A solution of Hydrogen Peroxide is catalytically decomposed.  Name the element being reduced and the element being oxidized.

Recommended Practice

 Create your own way to study the reactivity series, either as a visual or a memory trick. Be prepared to hand in a copy.

 Create your own way of representing the solubility rules, you can use the table example given in class, but make sure it includes all the precipitates you are responsible for.  Reaction Schematic – Organize all the reactions covered so far into a graphic format, like a table or flow chart. Make connections!!

Homework

 Complete the reaction worksheet

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Sources and Links

1) University of Siegen, Germany – cool chemistry photo page http://www2.uni-siegen.de/~pci/versuche/english/v44-1-1.html