Quantities in Chemical Reactions

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Chapter 8

• Quantities in Chemical Reactions

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Chapter 8 Assignments

• Homework, Pages 222-229 3, 5, 7, 49, 51, 55,
  57
• Homework, Pages 252 260 3, 5, 9, 15, 19, 21,
  27, 29a, 31ac, 47
• Blackboard Quiz, Chapter 8

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I. Chemical Equations A. Introduction

• A chemical equation is the representation of a
  chemical reaction (rxn) in terms of chemical
  formulas.
• Example 2 Mg 1 O2 -----) 2 MgO
• Notes - Mg O2 are called reactants
• - MgO is called a product
• - ------) goes to give
• - 2s understood 1s are balancing
  coefficients
• Balancing Coefficients are determined by
  inspection they can be
• molecules or they can represent moles of
  compounds.

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I. Chemical Equations A. Introduction

• Why do rxns occur? Systems generally go to
  produce
• more stable results. i.e. products are more
  stable than reactants.
• How do rxns occur? Molecules or atoms are
  traveling at high speeds in gas or in solution.
  If they collide with enough energy, then the
  valence electrons are rearranged old bonds break
  and new ones form, which results in new
  compounds.
• Evidence that a rxn occurred Production of a
  gas, a solid (precipitate), a color, light, or
  heat are evidence of a chemical reaction.
• Catalyst May need a catalyst to enable a
  chemical reaction to take place. Catalyst
  substance which speeds up a chemical reaction but
  is not consumed.

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I. Chemical Equations B. Balancing

• - Mass is neither lost or gained in a chemical
  reaction so, we need to balance the equation
  without changing the identity of the reactants or
  products - change only coefficients. Only give
  the smallest set of whole s.
• This is done by inspection however, on difficult
  reactions it is best to first balance the
  elements which occur the fewest times.
  Examples
• ___ Mg ___ O2 -----------) ___ MgO
• ___ Al ___ Cl2 -----------) ___
  AlCl3
• ___ Ca ___ H2O ----------) ___
  Ca(OH)2 ___H2
• ___ HCl ___ Al(OH)3 -----) ___ AlCl3
  ___H2O
• ___ C2H4 ___ O2 ------------) ___CO2
  ___H2O

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II. Stoichiometry A. Introduction

• 4 Fe 3 O2 -----------) 2 Fe2O3
• - Balancing coefficients in a RXN can be used as
  molecules or moles. So, 4 moles of Fe reacts
  with 3 moles of O2 to yield 2 moles of Fe2O3
• - The coefficients in a balanced chemical
  reaction can be used as conversion factors in
  calculating the amount of product produced or the
  amount of reactant used.
• - Example How many moles of Fe2O3 will be
  produced from
• 4 moles of Fe? 2 moles? 8 moles?
  1 mole?
• 2 1 4 0.5

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II. Stoichiometry B. Problems

• - The previous problem could be easily solved
  however, more difficult problems can be solved
  using the conversion factor method and the
  following logic
• a b
  c
• g known ------) moles known ------)
  moles unknown ------) g unknown
• Periodic Chart Balanced Chemical Eqn
  Periodic Chart
• - Note that you may be required to enter to
  exit the problem with either grams or moles.

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II. Stoichiometry B. Normal Problems

• - You are given only one amount of reactant and
  you assume that there is enough of the other
  reactants to completely react.
• - You are given 1 CH4 2 O2 -----)
  1 CO2 2 H2O
• How many moles of H2O can be produced from 109
  moles of CH4?
• 109 m CH4 x 2 m H2O 218 mole H2O
• 1 m CH4

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II. Stoichiometry B. Problems Continued

• -You are given 1 C 1 O2 --------)
  1 CO2
• - How many g of O2 will react with 10.0 g of C?

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II. Stoichiometry B. Normal Problems

• Given 2 Al(OH)3 3 H2SO4 -----) 6
  H2O 1 Al2(SO4)3
• 1. How many moles of H2SO4 are needed to produce
  8.0 moles of Al2(SO4)3
• 8.0 moles Al2(SO4)3 3.0 moles H2SO4
  24 moles H2SO4
• 1.0 mole
  Al2(SO4)3
• 2. How many moles of H2O will be produced from
  156 g of Al(OH)3 ?
• 156 g Al(OH)3 1 mole Al(OH)3 6
  mole H2O 6.00 mole H2O
• 
  
  
• 78.0 g Al(OH)3
  2 mole Al(OH)3

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II. Stoichiometry B. Normal Problems

• Given 4 Al 3 O2 -------) 2
  Al2O3
• 3. How many g of Al are needed to react with 64
  g of O2?
• 1 mole O2 4 mole Al
  27 g Al
• 64 g O2 x x
  x 72 g Al
  
• 32 g O2 3 mole O2
  1 mole Al
• Periodic Chart
  
  Periodic Chart
• (1 mole FW in g) Balanced
  Chemical Eqn (1 mole FW in g)

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II. Stoichiometry C. Limiting Reagent
Problems

• If given two reactants, then one reactant will be
  used up first. Use this limiting reagent in the
  problem.
• 109 2.0 - Which is Limiting?
• Example 1 CH4 2 O2 ----) 1 CO2 2
  H2O
• How many moles of CO2 can be produced from a
  mixture of 109 moles of CH4 and 2.0 moles of O2?
• Note will run out of O2 first and this will
  limit the amount of CO2 produced
• 2.0 moles O2 x (1 mole CO2 / 2 moles O2) 1.0
  moles CO2

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II. Stoichiometry C. Limiting Reagent
Problems

• One way to deal with LR problems is to solve for
  the product with each reagent the reagent that
  gives the SMALLEST amount of product is the
  correct answer.
• Example How many moles of aluminum chloride can
  be made by mixing 4.0 moles of Al with 5.0 moles
  of Cl2?
• 2 Al 3 Cl2 -----) 2 AlCl3
• 4.0 moles Al x 2 moles AlCl3 / 2 moles Al
  4.0 moles AlCl3
• 5.0 moles Cl2 x 2 moles AlCl3 / 3 moles Cl2
  3.3 moles AlCl3
• - 3.3 moles AlCl3 is smaller and is the correct
  answer.
• - The Cl2 is the limiting reagent in this
  example.