Types of Chemical Reactions

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Types of Chemical Reactions

• 6.2 Assigning oxidation numbers for individual
  atoms of monatomic and polyatomic ions
• 6.3 Identifying the nomenclature of ionic
  compounds, binary compounds, and acids
• 6.4 Classifying chemical reactions as
  composition, decomposition, single replacement,
  or double replacement
• AHSGE Reading 4.2 Demonstrate the ability to
  preview and predict.
• Other AOD C.6.1 Define stoichiometry,
  reactants, and products.

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What are some types of chemical reactions?

• Synthesis
• Decompostion
• Single Replacement/Single Displacement
• Double Replacement/Double Displacement
• Combustion
• Complete combustion yields ________.
• Incomplete combustion yields __________.

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Classes of Reactions

• Precipitation reactions
• Acid-base reactions
• Oxidation-reduction reactions
• Almost all reaction types can be put into one of
  these three classes.

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Precipitation Reactions

• What do you think this reaction would involve?
• AKA, double replacement/double displacement
• Example K2CrO4(aq) Ba(NO3)2(aq) ? ???
• What does (aq) mean?
• What are some other designations?
• What is the name of K2CrO4? Ba(NO3)2?
• What would the products be in a double
  displacement reaction?
• Would either product be a solid (precipitate)?

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Three Ways to Write the Reaction Equation

• Molecular equation shows the overall balanced
  reaction (reactants and products)
• Complete ionic equation shows a balanced
  equation of the actual FORMS of the reactants and
  products in the reaction
• Net ionic equation balanced equation showing
  ONLY the ions involved in the reaction.
• Ions that remain in solution (as ions) are not
  shown. (spectator ions)

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K2CrO4(aq) Ba(NO3)2(aq) ? ???

• Molecular equation
  K2CrO4(aq) Ba(NO3)2(aq) ? 2 KNO3 (aq)
  BaCrO4(s)
• Complete ionic equation
  2K(aq) CrO42-(aq) Ba2(aq) 2NO3-(aq) ?
  BaCrO4(s) 2K(aq) 2NO3-(aq)
• Net ionic equation
• What is the only CHANGE in this rxn?
• CrO42-(aq) Ba2(aq) ? BaCrO4(s)

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Examples

• See Sample Exercise 4.9, p.155.
• P. 182 (29a)
• BaCl2(aq) Na2SO4(aq) ? BaSO4(s) 2NaCl(aq)
• Net ionic equation
  Ba2(aq) SO42-(aq) ? BaSO4(s)
• What happened to the Cl2 and the Na2 part of
  the molecular equation?
• What would the complete ionic equation look like?

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Assignment

• P.182 30, 34, 36
• Special note on 34 You are MAKING UP a chemical
  reaction that will produce the specified solid.
• It should be a precipitation (double
  displacement) reaction.

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Acid-Base Reactions

• Acid-Base definitions
• Arrhenius
• Acid a substance that produces H when
  dissolved in water
• Base - a substance that produces OH- when
  dissolved in water
• Bronsted-Lowry
• Acid a proton donor
• Base a proton acceptor

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Predicting Acid-Base Reactions

• What element do acid chemical formulas start
  with?
• Most bases contain hydroxide ions.
• Using Arrhenius definitions, H OH- ? ??
• So Acid-Base reactions yield _________.

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Example NaOH HCl

• What is the acid? Base?
• Molecular equation NaOH HCl ? NaCl H2O
  (Whats missing from this equation???)
• Complete ionic equation Na(aq) OH-(aq)
  H(aq) Cl-(aq) ? Na(aq) Cl-(aq) H2O(??)
• Net ionic equation ????
• Assignment P.183 (46, 48)

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Oxidation-Reduction (Redox) Reactions

• Def reactions in which one or more electrons are
  transferred
• Example 2Na(s) Cl2(g) ? 2NaCl(s)
• What is the charge on Na(s)?
• On Cl in Cl2?
• On Na in Nacl?
• On Cl in NaCl?

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Redox Reactions, continued

• Includes most reactions involving energy
  production (i.e., photosynthesis, combustion).
• Example CH4(g) 2O2(g) ? CO2(g) 2H2O(g)
  energy
• How do we calculate the charges on elements in
  covalent molecules to determine a transfer of
  electrons?

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Oxidation States (aka, Oxidation Numbers)

• Def the imaginary charges the atoms in a
  covalent molecule would have if the shared
  electrons were assigned to the atom with the most
  electronegativity
• Rules for Assigning Oxidation States (Table 4.2,
  p.167)

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Rules for Assigning Oxidation States

• Give the oxidation states for all the atoms in
  the following molecules
• O2
• CH4
• CO2
• H2O
• SF6
• NO3-
• Fe3O4

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CH4(g) 2O2(g) ?
CO2(g) 2H2O(g) energy

• For which atoms did the oxidation state change?
• C went from -4 to 4.
• This change shows that each C atom LOST 8
  electrons.
• O went from 0 to -2.
• This shows that each O atom ______ ____ electrons.

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Oxidation and Reduction

• Oxidation an INCREASE in oxidation state (LOSES
  e-s)
• Reduction a DECREASE in oxidation state (GAINS
  e-s)
• If an atom is oxidized, it is called the reducing
  agent (electron donor).
• If an atom is reduced, it is called the _______
  agent (electron acceptor).
• OIL RIG Oxidation Involves Loss Reduction
  Involves Gain

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CH4(g) 2O2(g) ?
CO2(g) 2H2O(g) energy

• Which element is oxidized in this equation?
• Which element is reduced?
• Which compound is the oxidizing agent?
• Which compound is the reducing agent?
• Example 2Al(s) 3I2(s) ? 2AlI3(s)
• Assignment Pp.183-184 (58, 60, 62)

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Balancing Redox Reactions

• If it does not balance easily using the normal
  rules, use the half-reaction method.
• Uses two separate reaction equations
• Oxidation half-reaction balance the substance
  being oxidized
• Reduction half-reaction balance the substance
  being reduced
• Example Ce4(aq) Sn2(aq) ? Ce3(aq)
  Sn4(aq)
• Sn2(aq) ? Sn4(aq) ---- __________ half-reaction
• Ce4(aq) ? Ce3(aq) --- __________ half-reaction
• But how would you balance this????

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Actual Balancing Steps

• Varies by whether the reaction occurs in acidic
  or basic solutions!
• Acidic solutions (solns) follow steps on p.172
• Basic solns follow acidic steps, THEN add steps
  2-4 on p.177.

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Steps in Acidic Solutions

1. Write the two half-reactions.
2. For each half-reaction!
3. Balance all elements EXCEPT H and O.
4. Balance O by adding H2O.
5. Balance H by adding H.
6. Balance any charge by adding e-s.
7. Multiple either of the half-reactions by an
   integer to have equal number of e-s in both
   half-reactions, if needed. (You want the e-s to
   cancel in the next step.)
8. Add the half-reactions, cancelling any identical
   species.
9. Check that both elements AND charges are balanced
   in final equation.

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Example
Zn(s) NO3-(aq) ? Zn2 NO2(g)

• Half-reactions
• Oxidation Zn(s) ? Zn2(aq)
• Reduction NO3-(aq) ? NO2(g)
• Balancing steps
• Already done ?
• NO3-(aq) ? NO2(g) H2O(l)
• 2H(aq) NO3-(aq) ? NO2(g) H2O(l)
• e- 2H(aq) NO3-(aq) ? NO2(g) H2O(l) AND
  Zn(s) ? Zn2(aq) 2e-

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Example (continued..)
Zn(s) NO3-(aq) ? Zn2 NO2(g)

• Which half-reaction do we need to multiply, and
  by what integer?
• 2 e- 2H(aq) NO3-(aq) ? NO2(g) H2O(l)
  2e- 4H(aq) 2NO3-(aq) ? 2NO2(g) 2H2O(l)
• Zn(s) ? Zn2(aq) 2e-
• Add the two equations together
  4H(aq) 2NO3-(aq) Zn(s) ? 2NO2(g) 2H2O(l)
  Zn2(aq)
• Check the balance of elements and charges!

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Try This One
MnO4-(aq) Zn(s) ? Mn2(aq) Zn2(aq)

• Write the half-reactions.
• Balancing steps
• Mn and Zn
• Add __________ to balance O.
• Add __________ to balance H.
• Add __________ to balance any charges.
• Multiply to get equal number of ____ in both
  half-reactions.
• Add them together.
• Check the elements and charges!

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Steps in Basic Solutions

1. Start with the steps used for acidic solutions.
2. Add OH- (equal to the final amount of H) to BOTH
   sides of the equation.
3. Eliminate equal amounts of water from both sides
   of the equation.
4. Check balance of elements and charges!

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Example (65(b))
Cl2(g) ? Cl-(aq) OCl-(aq)

• Acidic steps get you to 2H2O(l) 2Cl2(g) ?
  2Cl-(aq) 2OCl-(aq) 4H(aq)
• Add OH- to BOTH sides. How many???
  4OH-(aq) 2H2O(l) 2Cl2(g) ? 2Cl-(aq)
  2OCl-(aq) (4H(aq) 4OH-)
• 4OH-(aq) 2Cl2(g) ? 2Cl-(aq) 2OCl-(aq)
  2H2O(l)
• Check balance of elements and charges!
• What is one more thing we can do to this final
  equation????

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Assignment

• P.184 64, 66