Chapter Eight Chemical Reactions

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Chapter Eight Chemical Reactions
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Section 8.1Chemical Equations
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Chemical Reactions

• A chemical reaction is simply a chemical change
• Atoms rearrange themselves to form new compounds

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Chemical Indicators

• Production of a gas (bubbles)
• Production of a solid (precipitate)
• Color Change
• Production of heat/light (energy)
• Noise (ex sizzling)
• Only the evidence of a NEW substance after
  chemical analysis is 100 proof of chemical
  reaction

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Chemical Equations

• A systematic way to write a chemical reaction
  using symbols
• Shows compounds involved in the reaction
• States of matter
• Changes in energy (endo or exo)
• Quantifies reaction (ratios between compounds)

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Chemical Equations

• Ex Hydrogen gas and oxygen gas react together to
  produce water
• Ex
• 2H2(g) O2(g) ? 2H2O(l)
• Reactants Products

and (Separates reactants or products)
Yields or Produces
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Another Example

• Cu(s) 2AgNO3(aq) ? 2Ag(s) Cu(NO3)2(aq) ?H
  -484
• kJ
• States of matter
• (s) solid, (l) liquid, (g) gas, (aq)
  aqueous
• Coefficients
• Tell us the mole ratio between reactants/products.
  
• Enthalpy
• Tells if the reaction gives off energy (exo) or
  needs energy (endo)

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Some Special Notes

• At standard conditions, here are the following
  states of matter
• Metal atoms are solids
• Ionic Compounds are typically solid or aqueous
  (dissolved in water)
• Nonmetals are gases
• Bromine/Mercury are liquid

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Some Special Notes

• Metals are listed as a single element
• (i.e. Iron ? Fe)
• Carbon can be listed as C(graphite) or C(diamond)
• Remember diatomics (i.e. F2, Cl2, H2)
• Phosphorus can be P4 and Sulfur can be S8
• A species written over the arrow indicates a
  helping substance, though doesnt affect
  chemicals involved
• Ex

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Types of Reactions

• Six general types of reactions
• Synthesis/Combination
• Decomposition
• Combustion
• Single Replacement (Displacement)
• Double Replacement (Displacement)
• Acid/Base Neutralization

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Synthesis/Combination

• Reaction between two or more substances
  (reactants) and making ONE PRODUCT (compound)
• Form
• A B ? AB
• Example
• 2Na Cl2 ? 2NaCl

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Decomposition

• Reaction of ONE REACTANT breaking down into its
  elementary substances
• Form
• AB ? A B
• Example
• H2O ? H2 O2

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Combustion

• Carbon-based compound reacting w/ oxygen
  (burning)
• CXY could be a hydrocarbon or also contain
  oxygen
• Products are ALWAYS CO2 and H2O
• Form
• CXY O2 ? CO2 H2O
• Example
• CH4 2O2 ? CO2 2H2O

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What are the following reactions?

• What type of reaction are the following?
• N2 H2 ? NH3
• C2H8 O2 ? CO2 H2O
• H2 O2 ? H2O
• KClO3 ? KCl O2
• Ag2O ? Ag O2
• S8 O2 ? SO3
• CH4 O2 ? CO2 H2O
• H2O ? H2O O2

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Single Replacement

• Chemical reaction where one element replaces
  another element in a compound
• Like replaces like (ex metals switch)
• An ELEMENT and a COMPOUND for REACTANTS
• Form
• A BC ? B AC
• Example
• Al CuCl2 ? AlCl3 Cu

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Single Displacement
A
B
C
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Not Every SR reaction will happen

• You must look at the Activity Series to determine
  if a cationic replacement reaction will happen.
• Ex
• Zn(s) HCl(aq) ?
• H2 ZnCl2
• Cu(s) HCl(aq) ?
• No Reaction

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Double Displacement/Precipitation

• Chemical reaction where two elements (or
  polyatomic ions) in different compounds switch
  places
• TWO COMPOUNDS for REACTANTS
• Form
• AB CD ? CB AD
• Example
• Na2SO4 AlCl3 ? Al2(SO4)3 NaCl

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Double Displacement/Precipitation
A
C
D
B
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Acid Base Neutralization

• Special type of Double Replacement
• Where an Acid (containing H) and a Base
  (containing OH-) react to produce
• A Salt (ionic compound) and
• Water (HOH or H2O)
• Ex HCl NaOH ? NaCl HOH (H2O)

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What type is it?

• Determine the type of reaction. Predict the
  products for the starred reactions
• Na2S ZnCl2 ?
• Mg(OH)2 ?
• H2 Cl2 ?
• C6H6 O2 ?
• K Fe(OH)3 ?
• F2(aq) KI(aq) ?
• HF(aq) NaOH(aq) ?
• C10H8 O2 ?

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Balancing Equations

• H2 O2 ? H2O
• Law of Conservation of Matter/Mass Matter
  (atoms) cannot be created or destroyed
• In other words, same of atoms on both sides
• Ex Balance the above chemical equation

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DIVIDE CONQUER! Steps for Balancing Equations

• Step 1 List elements on both sides
• Step 2 Write down the number of atoms for all
  the elements
• Step 3 Use coefficients to balance atoms
• DO NOT CHANGE THE SUBSCRIPTS!!!!!
• Ex __FeS __HCl ? __FeCl2 __H2S

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Tips That Make Balancing Easier

• Here are some tips to balancing some difficult
  equations
• Are there any 23 ratios? Do those elements
  first
• Ex AlCl3 ZnBr2 ? AlBr3 ZnCl2
• Balance metals first
• If you have the same polyatomic on both sides,
  treat the polyatomic as ONE
• Ex Na3PO4 CaCl2 ? Ca3(PO4)2 NaCl
• Na2CO3 ? Na2O CO2
• Try to balance Oxygen and Hydrogen at the end.

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Tips Specific to Combustion

• Balance carbons first
• Balance hydrogens second
• Balance oxygens last
• If you end up having an odd of oxygens on the
  product side, put a decimal coefficient in front
  of oxygen on reactant side that will make it
  balance, THEN DOUBLE EVERY COEFFICIENT to make
  whole s
• Ex ___C2H6 ___O2 ? ___CO2 ___H2O

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Some practice

• __Li3PO4 __SrCl2 ? __Sr3(PO4)2 __LiCl
• __Fe2O3 __H2SO4 ? __Fe2(SO4)3 __H2O
• __C5H12 __O2 ? __CO2 __H2O

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Group Quiz 1

• Balance the following equations and state what
  kind of reaction it is
• _____ N2 _____ O2 ? _____ N2O
• ____KClO3 ? ____KCl ____ O2
• __ Al(OH)3 __ H2SO4 ? __ Al2(SO4)3 __ H2O

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Section 8.2Combustion Analysis
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Combustion Analysis

• Determining the empirical formula of an organic
  compound

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Steps

• 1) Convert your known quantities of CO2 and H2O
  into grams of C H only
• 2) Determine how many grams of oxygen (if any)
  are present in the compound as well.
• 3) Turn your grams of C, H, O into moles of C,
  H, O, respectively.
• 4) Compare your 3 answers from step 3 and divide
  ALL 3 answers by the lowest you get (this is
  your ratio, and therefore your subscripts)

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Example

• 18.8 g of an unknown organic substance produced
  27.6 g of carbon dioxide and 11.3 g of water.
• What is the empirical formula?

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Section 8.3Calculations with Balanced Chemical
Equations
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Stoichiometry

• Used to predict amount of product produced or how
  much reactants were used
• Remember Coefficients tell us MOLE ratio
• N2(g) 3H2(g) ? 2NH3(g)
• How many moles of ammonia are created when 2
  moles of nitrogen are used? 6 moles of hydrogen?
• How many moles of nitrogen are needed to make 4
  moles of ammonia?

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Stoichiometry

• What mass of lithium nitride is produced when
  75.0 g of lithium metal react with excess
  nitrogen?
• 6Li(s) N2(g) ? 2Li3N(s)
• What is the mass of O2 necessary to react with
  5.71 g Al?
• 4Al(s) 3O2(g) ? 2Al2O3(s)

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Group Quiz 2

• Given the following equation
• 2 KClO3 ---gt 2 KCl 3 O2
• How many moles of O2 can be produced by letting
  12.00 moles of KClO3 react? (watch sig figs!!!)
• Given the following equation 2 K Cl2 ---gt 2
  KCl
• How many grams of KCl are produced from 2.50 g of
  K and excess Cl2 ?

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Section 8.4Limiting Reactants
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Limiting Reactants

• Although we want all reactants to be completely
  used up, in reality there is usually one reactant
  that is used up before the other and thus LIMITS
  the reaction and how much can be produced.

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Limiting Reactants

• Limiting Reactant The reactant that is
  completely consumed (runs out first)
• Limits the reaction
• Nothing is left over
• Excess Reatant The reactant left over
• Not completely consumed

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Stoichiometric Steps

• Though every stoichiometry problem is unique, the
  same general steps are used
• Convert given chemical(s) into MOLES
• Use MOLE RATIO to convert from given chemical to
  desired chemical
• After this step is where you would determine
  limiting reactant IF it applies to the problem
• Convert desired chemical into DESIRED UNIT (ex
  grams, liters, etc.)

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Example Problems

• How many moles of NH3 can be produced from the
  combination of 3.0 moles N2 and 1.5 moles H2?
• What mass of water is produced when you react
  12.4 g of H2 with 13.5 g of oxygen?
• If you have 11.5 L of nitrogen reacting with 16.5
  L of hydrogen, what volume of ammonia is produced
  at STP? (At STP 1 mole of any gas 22.41 L)

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Group Quiz 3

• If you have 1.22 g of O2 reacting with 1.05 g of
  H2, what mass of water will you produce?

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Reaction Yields

• When amount of product is calculated using
  stoichiometry, this is known as theoretical yield
• What you theoretically SHOULD get
• When you perform the experiment in the lab, you
  typically get less, this is known as actual yield

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Percent Yield

• Actual Yield x 100 Yield
• Theoretical Yield
• Ex Magnesium burns in air. If you burn 6.73
  grams of Magnesium
• How many grams of product would you produce?
• What would be your percent yield if you did the
  experiment and got 10.7g of the product?

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Group Quiz 4

• Suppose you react sulfur dioxide with oxygen to
  produce sulfur trioxide. If you react 12.4 g of
  sulfur dioxide with 3.45 g of oxygen, how many
  grams of sulfur trioxide will you produce?
• If you did this lab and yielded 13.4 grams of
  sulfur trioxide, what is your percent yield?

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Section 8.5Periodic Trends in Reactivity of the
Main Group Elements
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Trends of Reactivity

• Group 1A metals tend to be highly reactive toward
  oxygen, water, and acid.
• M H2O ? MOH H2
• Lithium forms oxides (Li O2 ? Li2O)
• The other alkali metals form oxides or peroxides
  (O22- ion) (Na O2 ? Na2O2)
• K, Rb, and Cs can also form superoxides
  (containing the superoxide ion O2-)
• Group 2A metals are less reactive

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Group 2A Continued

• Be doesnt react with water, Mg reacts slowly to
  steam, and Ca, Sr Ba react strongly with cold
  water to form hydrogen gas and hydroxides
• Ca H2O ? Ca(OH)2 H2
• React w/ oxygen (Be and Mg only at high temps)
• React with acids form hydrogen gas

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3A elements

• Boron (a metalloid) is unreactive to oxygen and
  water all other elements are metals and tend to
  be reactive
• Aluminum will readily form aluminum oxide when
  exposed to air (oxygen)