Working With Chemical Reactions

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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

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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

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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?

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Which Presents a Problem

• 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?

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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.

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First Rethink Memorization

• Rote memorization has distinct disadvantages.
• It doesnt allow you to apply knowledge in a
  variety of situations.
• When thinking of ways to study, its essential
  that facts not remain isolated but that there is
  an understanding the same principles may apply in
  more that one case.

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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

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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.

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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

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REMEMBER!!!

• There are 3 things that drive reactions forward
• Formation of a Gaseous product
• Formation of water
• Formation of an insoluble precipitate

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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

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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.
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Sodium and Water
Source 1
The flame is Hydrogen Gas which has ignited due
to the heat produced by the reaction.
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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 ? CaCl2 H2
• Net Ionic Ca 2H ? Ca2 H2
• 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 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.

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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)
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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
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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.

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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
• wont form oxides.
• Since gold does not
• Oxidize these gold
• Artifacts from Egypt to
• Survive thousands of
• years.

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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.

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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.

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Example Question

• 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.

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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)

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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 ?2Ca2 2OH-
• The formation of hydroxide indicates this
  solution is basic.

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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.

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Example Questions

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

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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

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Example Questions

• Sulfur Dioxide gas is passed over Solid Calcium
  Oxide.
• Name the product compound.

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Example Questions

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

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• CaO P4O10 ? 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 P4O10 ? 2Ca3(PO4)2

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Reverse Reaction

• Carbonates will decompose back into metal oxides
  and carbon dioxide when roasted (heated).
• CaO CO2 ? CaCO3
• CaCO3 HEAT? CaO CO2

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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 ? Ca2 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

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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.

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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 H2O ? LiOH H2
• Net Ionic LiH H2O ? Li OH- H2
• Is the final solution acidic, basic or neutral?
• The resulting solution would be basic.

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Reaction Reversal

• Metal hydroxides may then decompose into metal
  oxides and water
• Zn(OH)2 ? ZnO H2O

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Other Patterns - Decomposition

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

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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.

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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.

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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

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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...

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Colors of Complex Ions
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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?vOQPwPGDQqzs
• So you could also say, heat is given off.

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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.

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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!

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Example Question

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

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• 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

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Example Question

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

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Example Question

• A solution of copper (II) chloride is added to a
  solution of sodium sulfide.
• Name the spectator ions in this reaction.

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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.

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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.

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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.

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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.
• CS2 O2 ? CO2 SO2

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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.

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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.

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Example Question

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

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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!!

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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/versuc
  he/english/v44-1-1.html

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