Unit 7 Equations and Solutions

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Unit 7 Equations and Solutions

• Chemical Reactions and Equations Chapter 11
  p321 to 347
• Chemical Equation symbolic representation of a
  chemical change (reaction).
• Reactants Products

starting substances makes or ending
substances
yields
? heat
ex. CH3OH(l) O2(g) ? CO2(g)
H2O(g)
A compound an element
phase separates
substances
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B. Types of Reactions

• Synthesis Rxn makes only one product.

to make
General Form A B ? C
ex. 2Fe(s) O2(g) ? 2FeO(s)
only one kind of product, no sign
2. Decomposition one reactant breaks apart.
General Form A ? B C
ex. 2KClO3(s) ? 2KCl(s) 3O2(g)
note coefficient (number in front of substance)
tells how many of that whole substance you have.
only one kind of reactant
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• 3. Single Replacement Rxn a more reactive
  element replaces a less reactive element in a
  compound. Look for a single element on each side
  of the reaction.

If C is a nonmetal.
General Form AB C ? AC B
If C is a metal.
or AB C ? CB A
ex. 2NaI Cl2 ? 2NaCl I2
Cl replaces I because Cl is more reactive than I.
ex. LiF K ? KF Li
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• Double Replacement Rxn A reaction in which
  ionic compounds in water (aq) switch partners
  to make a product that leaves the water (an
  insoluble solid or gas).
• General Form AB CD ? AD CB

ex. K2S(aq) ZnCl2(aq) ? 2 KCl(aq) ZnS(s)
5. Combustion Rxn The reaction of a carbon
compound with Oxygen to form Carbon Dioxide and
Water.
(show video glencoe chap6 3)
General Form CxHyOz O2(g) ? CO2(g) H2O
ex. 2CH3OH(g) 3O2(g) ? 2CO2(g) 4H2O(g)
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C. How To Tell if a Reaction Will Go

• A Rxn is said to go if, when reactants are
  combined and activation energy is supplied,
  products are made a Spontaneous Rxn.
• Single Replacement Rxn A more reactive element
  replaces a less reactive element. Table J lists
  elements in order from most to least reactive.
• In general AB C ? CB A goes if element C
  is above element A in Table J.
• ex. Will the rxn CuSO4 Zn ? ZnSO4 Cu go?

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2. Double Replacement Rxn

• Double Replacement Rxns will go if a product is
  made that leaves the water solution either an
  insoluble salt (a solid precipitate) or a gas.
• Table F lists Guidelines for telling if a salt is
  soluble (dissolves) in water.
• If one or both products is insoluble the reaction
  will go and the insoluble product will form a
  precipitate.
• ex. Pb(NO3)2(aq) KI(aq) ?
• Will this rxn go and if so what is the ppt.?

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D. Balanced Chemical Equations

• What must be equal between the reactants and
  products in a chemical equation?
• Mass of reactants Mass of products
• The number of atoms of each element on reactant
  and product sides must be equal.
• This is because of the Law of Conservation of
  Mass (Matter).
• A balanced equation has equal number of atoms of
  each element on each side.
• ex. H2(g) Cl2(g) ? 2HCl(g)

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2. Balancing Equations

• Write out the equation.
• List all elements and the amount on each side
  under the equation.
• Decide what to multiply by to make amounts of
  each element on each side equal write these
  multiples in as coefficients.
• ex. Li(s) CaCl2(aq) ? LiCl(aq) Ca(s)
• Li 1 1
• Ca 1
  1
• Cl 2 1

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E. Stiochiometry Problems based on Equations

• Converting From Amount of One Substance to Amount
  of Another Substance.
• Recipe for French Toast 3 eggs
• 1
  cup milk etc.
• We can adjust the recipe for amounts we have
  using a ratio
• ex. If we only have 2 eggs, how much milk
  do we use?

2 eggs
3 eggs

1 cup milk
x cup milk
3x 2
x 2/3 cup milk
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• 2. Converting from Moles of One Substance to
  Moles of Another Substance.
• Chemical Equations are like recipes, coefficients
  are the amounts!
• ex. Given the balanced equation
• H2(g) Cl2(g) ? 2HCl(g)
• How many molecules of HCl(g) can be made from one
  molecule of H2(g) assuming excess Cl2?
• How many moles of HCl(g) molecules can be made
  from one mole of H2(g) molecules assuming excess
  Cl2?

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Ex. How many moles of HCl can be made from 5.0
moles of H2(g), assuming excess Cl2(g)?
H2 Cl2 ? 2 HCl
Ex. Given the balanced equation
N2(g)3H2(g)?2NH3(g), how many moles of ammonia
can be made from 5.0 moles of H2?
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II. SOLUTIONS Chapter 16, pgs 320-333

• Terms to know
• Solution a homogenous mixture.
• Solvent substance in which other substances
  dissolve the substance in greater amount.
• Solute a substance that is dissolved the
  substance in lesser amount.
• Concentration amount of solute dissolved per
  unit of solvent. Dilute small
  amount Concentrated
  relatively large amount

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B. Saturation and Solubility

• A saturated solution has the maximum amount of
  solute dissolved in it.
• Unsaturated less than maximum.
• Supersaturated more than maximum?!
• Solubility concentration of a saturated soln.
• Soluble ? Slightly Soluble ? Insoluble
• Solubility changes with temp. and press.(gas)
• Solid solute inc. Temp. inc. Solubility
• Gaseous solute inc. Temp. dec. Solubility
• inc. Press.
  inc. Solubility

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C. Example Regents Problems Based on Table G

• According to Table G, which solution is saturated
  at 30oC?
• (1) 6 g of KClO3 in 100g of H2O
• (2) 24g of KClO3 in 200g of H2O
• (3) 15g of NaCl in 100g of H2O
• (4) 30g of NaCl in 200g of H2O
• 100g of H2O is saturated with NH4Cl at 50oC.
  According to Table G, if the temperature is
  lowered to 10oC, what is the total amount of
  NH4Cl that will precipitate?
• (1) 5.0g (2) 30.g (3) 17g
  (4) 50.g

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D. Molarity Calculations

• Molarity, a concentration unit the number of
  moles of solute dissolved per liter of solution.

Moles of solute
n

• Morality

or
M
Liters of solution
L
Ex. What is the molarity of a solution that
contains 4 moles of solute dissolved in 8,000ml
of solution?
4 moles
M x
x
8.0 L
n 4 moles
x 0.5M
L 8,000ml
8.0 L
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• Ex. What is the molarity of a 4.0 L solution made
  from mixing 29g of sodium chloride in water?

moles of solute
M
x
Molarity
Liters of solution
L
4.0 L
0.50moles
n
29g
/(58.5g/mole)
x
4.0 L
0.50mol
x 0.13 M
Ex. What mass of NaOH is needed to make 0.5L of a
6.0M NaOH(aq) solution.
n
M
L
M 6.0M
x
L 0.5 L
6.0M
0.5L
x
3.0
n x
40
x 3.0 moles
m
n
x 120 g
gfm
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E. Other Methods of Indicating Concentration
volume of solute

• by Volume x 100
• ex. What is the percent by volume concentration
• of 10.ml of ethanol dissolved in 50.ml of water?
• volume of solute 10.ml
• volume of solution

volume of solution
50ml
10.ml 60.ml
10.ml
vol x 100
60.ml
vol 17
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mass of solute

• by Mass x 100
• ex. What is the percent by mass concentration
• of a solution where 5.0g of salt are dissolved
  in 80.g of water?
• mass of solute 5.0g
• mass of solution 85g
  

mass of solution
5.0g
mass x 100
85g
mass 5.9
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3. Parts per Million ppm
x 1,000,000
mass of solute
mass of solution
ex. Find the concentration in ppm if 0.100 g of
sucrose are dissolved in 500. g of water.
mass of solute 0.100 g
mass of solution 500.1 g
0.100 g
ppm x 1,000,000
500.1 g
ppm 200. ppm