The Arithmetic of Chemistry

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The Arithmetic of Chemistry

• Stoichiometry and chemical reactions

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A process in which one or more substances is
changed into one or more new substances is a
chemical reaction
A chemical equation uses chemical symbols to show
what happens during a chemical reaction
3.7
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Remember this?

• Law of mass conservation
• Law of definite proportions

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Balancing equations

• Number of atoms conserved atoms in reactants
  equal to atoms in products
• Change coefficients, not subscripts
• Balance atoms that occur in only one reactant and
  one product
• Balance the rest
• Check number of atoms on each side

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Test yourself

• Test yourself ethane gas reacts with oxygen gas
  to produce carbon dioxide and water
• Heating potassium chlorate yields potassium
  chloride and oxygen gas
• Demo reaction Pb(NO3)2 and KI

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• Atoms small scale
• Macroscopic scale the mole

Atomic mass is the mass of an atom in atomic mass
units (amu)
By definition 1 atom 12C weighs 12 amu
On this scale 1H 1.008 amu 16O 16.00 amu
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Average atomic masses weighted averages
Natural lithium is 7.42 6Li (6.015 amu) 92.58
7Li (7.016 amu)
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The mole (mol) is the amount of a substance that
contains as many entities as there are atoms in
exactly 12.00 grams of 12C
3.2
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How Big is Avogadros Number?

• an Avogadros number of softdrink cans would
  cover the surface of the earth to a depth of over
  200 miles
• if we were able to count atoms at the rate of 10
  million per second, it would take about 2 billion
  years to count an Avogadros number of atoms

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eggs
shoes
Molar mass is the mass of 1 mole of
in grams
marbles
atoms
molecules
1 mole 12C atoms 6.022 x 1023 atoms 12.00 g 1
12C atom 12.00 amu
1 mole 12C atoms 12.00 g 12C 1 mole lithium
atoms 6.941 g of Li
For any element atomic mass (amu) molar mass
(grams)
3.2
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One Mole of
S
C
Hg
Cu
Fe
3.2
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Converting small scale to large (and back)

• Dimensional analysis method of calculation
  utilizing a knowledge of units.
• Conversion factors are used to manipulate units

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Test yourself

• How many atoms are in 0.551 g of K?
• How many molecules of water are in a 15g sample?
  How many individual H atoms? O atoms?

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Back to chemical equations

• 2H2 O2 ? 2H2O can be read as
• 2 molecules H2 of react with 1 molecule of O2 to
  produce 2 molecules of water, OR
• 2 moles of H2 react with 1 mole O2 to form 1 mole
  of water
• Convert to grams.

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Limiting reagent
3.9
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In one process, 124 g of Al are reacted with 601
g of Fe2O3
Calculate the mass of Al2O3 formed.
367 g Fe2O3
124 g Al
Have more Fe2O3 (601 g) so Al is limiting reagent
3.9
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General types of simple chemical reactions

1. Combination A Z ? AZ
2. Decomposition AZ ? A Z
3. Single replacement (substitution) A BZ ? AZ
   B
4. Double replacement/displacement
5. Neutralization (acids and bases)
6. Combustion reaction with O2, energy produced

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1. Combination Reactions

• Two or more substances (elements or compounds)
  react to produce one compound
• Also known as synthesis or addition
• A Z ? AZ
• Metal oxygen metal oxide
• Nonmetal oxygen nonmetal oxide
• Metal nonmetal salt
• Water metal oxide base
• Metal oxide nonmetal oxide salt

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2. Decomposition

• ONE substance reacts to form two or more
  substances (elements or compounds).
• AZ ? A Z
• Compound compound/element oxygen
• Metal carbonate metal oxide carbon dioxide
• Hydrated salt salt water
• Compound compound/element water

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3. Single Replacement

• Also known as single displacement or
  substitution.
• A metal (A) replaces a metal ion (Bmetal) in its
  salt or a hydrogen ion (BH) in an acid
• A BZ ? AZ B
• Ex. Fe(s) CuSO4(aq) ? FeSO4(aq) Cu(s)
• Ex. Sn(s) HCl(aq) ? SnCl2(aq) H2(g)
• A nonmetal (X) replacing another nonmetal (Z) in
  its salt (Bmetal) or acid (BH)
• Cl2(g) NaBr (aq) ? _____________________________
  
• Bromine Potassium Iodide ?______________________

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4. Double Replacement

• Also known as Double displacement,
  Metathesis, or Double decomposition.
• Two compounds are involved with the cation of
  one compound EXCHANGING with the cation of
  another compound.
• AX BZ ? AZ BX
• These reactions proceed if one of the ff. is
  satisfied
• An insoluble/slightly soluble product is formed
  (PRECIPITATE formation)
• A weakly ionized species is produced. The most
  common species of this type is water.
• A gas is produced as a product.

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5. Neutralization

• Reaction of an acid and a base that usually
  produces a salt and water.
• HX MOH ? MX HOH
• Hydrochloric acid and sodium hydroxide.
• Hydrochloric acid and magnesium hydroxide.
• Sulfuric acid and Barium hydroxide
• To be discussed in a separate lesson

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6. Combustion

• Reactions involving oxygen (metal oxygen),
  (nonmetal oxygen), (organic compounds oxygen)
  are sometimes called combustion
  reactions.
• Energy is given off in combustion reactions.
• For organic compounds, water and carbon dioxide
  are usual byproducts.
• Ex. Metabolism of food, fuel combustion
• Sucrose (C22H12O11) combusts
• Combustion of propane
• Combustion of hydrogen

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Quiz

• Insoluble silver chloride is produced in the
  reaction between silver nitrate and hydrochloric
  acid.
• What type of reaction is this?
• Balance the equation.

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Quiz

• Sodium azide produces nitrogen gas and sodium
  metal.
• Type of reaction?
• Balanced equation?
• How many grams of nitrogen gas is produced from
  60.0g sodium azide?

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Quiz

• Working at 273.15K and 1atm (STP), I have 10.0g
  of carbon and 56L of oxygen. Under these
  conditions, I know 1mol of any gas has a volume
  of 22.4L (Molar volume at STP). What is the
  limiting reagent? If carbon monoxide made sure to
  be formed, what will be its volume?