Reactions and Stoichiometry

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Reactions and Stoichiometry

• Chapters 11-12

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Reactions
Reactants
Products
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Balancing Reactions

• Reactions must maintain conservation of mass,
  charge, and energy
• Reactants and Products must have the same number
  of atoms of each element
• 2H2 O2 ? 2H2O

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Balancing Reactions

• Reactions must maintain conservation of mass,
  charge, and energy
• Reactants must have the same total charge as
  Products
• Cu1 Fe3 ? Cu2 Fe2

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Balancing Reactions

• To balance a reaction
• Do NOT change chemistry (compounds, subscripts)
• Only change coefficients (big numbers in front of
  chemicals)
• Coefficients can only be whole numbers
• Must be reduced
• 2H2 O2 ? 2H2O

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Balancing Reactions

• 4Na O2 ? 2Na2O
• 2Al 3Br2 ? 2AlBr3
• 4Ni 3O2 ? 2Ni2O3
• 2HNO3 Ca(OH)2 ? Ca(NO3)2 2H2O

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Reaction Types

• Synthesis
• Decomposition
• Single Replacement
• Double Replacement
• Combustion

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Synthesis

• Chemical change in which two or more substances
  react to form a single new substance (1 product)
• Also called Combination
• 2Mg O2 ? 2MgO

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Decomposition

• Chemical change in which a single compound breaks
  down into two or more simpler products
• 1 reactant
• 2NaCl ? 2Na Cl2

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Single Replacement

• Chemical change in which one element replaces a
  second element in a compound
• Metal replaces metal (hydrogen included)
• Nonmetal replaces nonmetal
• Zn 2HCl ? ZnCl2 H2

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Double Replacement

• Chemical change involving an exchange of positive
  ions between compounds
• AgNO3 NaCl ? NaNO3 AgCl

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Combustion

• Chemical change in which an element or a compound
  reacts with oxygen, often producing energy in the
  form of heat and light
• 2Mg O2 ? 2MgO

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Combustion

• Complete combustion of a hydrocarbon produces
  carbon dioxide and water
• 2C2H6 7O2 ? 4CO2 6H2O

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Spontaneous Reactions

• A single replacement reaction will only occur if
• The single element in the reactants is more
  active than the element it replaces in the
  compound

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Table J

• The more active element
• Does not want to be alone
• Wants to be combined with someone else

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Spontaneous or not?

• ZnCO3 Mg ? MgCO3 Zn
• PbSO4 Mn ? MnSO4 Pb
• 3KNO3 Al ? Al(NO3)3 3K
• 2HCl Zn ? ZnCl2 H2
• 2NaBr I2 ? 2NaI Br2

YES
YES
NO
YES
NO
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Spontaneous Reactions

• A double replacement reaction will only occur if
• A precipitate (solid) is produced
• A liquid is produced
• H2O(l)
• A gas is produced

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Table F
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Spontaneous or not?

• NaCl AgNO3 ? NaNO3 AgCl
• K2CO3 MgSO4 ? K2SO4 MgCO3
• NH4OH NaNO3 ? NaOH NH4NO3

Yes
Yes
No
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Stoichiometry

• Calculations of quantities in chemical reactions

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Food Example
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X
amount
4
2
1
10
ratio
X 20 pancakes

• How many pancakes can be made with 8 cups flour,
  excess eggs and milk?

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Process for Solving

• Balance reaction
• Copy coefficients from reaction into ratio row
• Place numbers from question in row above ratio
  row (Setting up proportion)
• Solve for X using a proportion

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Food Example
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X
amount
4
2
1
10
ratio
X 5 eggs

• How many eggs are needed to make 25 pancakes?

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Chemical Example
6
X
amount

1
3
2
ratio
X 4 mol NH3

• How many moles of NH3 can be made with 6 moles H2
  and excess N2?

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Chemical Example
7
X
amount
1
3
2
ratio
X 3.5 mol N2

• How many moles of N2 are needed to produce 7
  moles of NH3?

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Chemical Example
X
9
amount
1
5
3
4
ratio
X 11.25 mol O2

• How many moles of O2 are needed to produce 9
  moles of H2O?

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Conservation of Energy

• Reactions must maintain conservation of energy
• Energy term written in reaction

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Endo/Exothermic

• Endothermic Energy is absorbed
• Energy term is on the left side
• Exothermic Energy is released
• Energy term is on the right side
• Treat just like a coefficient

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Example
6
X
amount
ratio
1
3
2
91.8
X 183.6 kJ

• How much energy is produced when 6 moles H2
  reacts with excess N2?