Chemical Reactions
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What is a chemical reaction?
A chemical equation uses symbols to
represent what happens at the atomic level.
What happens when you boil water? What
happens during the electrolysis of water?
Chemical equations follow the Law of
Conservation of Mass
◦ Conservation of atoms, not conservation of Equation
molecules (molecular structures can change)
Reaction
2
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Adjust coefficients, not subscripts, to
balance an equation
Coefficients are whole-number integers in
the lowest ratio
Reactants
 Products
H2
+
O2

H2O
3
Coefficients vs. Subscripts
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Draw structures for the following
substances:
H2, Br2, HBr, CO, CO2, Cl2, AlCl3, N2, H2, NH3
Use your drawings to balance the equations
below. Add more substances as needed by
adding coefficients.
H2 (g) + Br2 (l)  HBr (g)
CO (g) + O2 (g)  CO2 (g)
Al (s) + Cl2 (g)  AlCl3 (g)
N2 (g) + H2 (g)  NH3 (g)
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Write correct formulas for reactants and
products.
Begin balancing with the most complex
formula.
Balance polyatomic ions as a single unit.
Check each reactant and product to verify the
coefficients.
Mg(NO3)2 + Cr2(SO4)3  MgSO4 + Cr(NO3)3
6
Gummy Bear
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Balance the following equations:
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Na + H2O
Mg + O2  MgO
H2O2  H2O + O2
NaN3  Na + N2
Alkali metals in H2O
CaCO3  CaO + CO2
Mg3(PO4)2 + NaOH  Mg(OH)2 + Na3PO4
KClO3  KCl + O2
Na + H2O  NaOH + H2
AgNO3 + MgCl2  AgCl + Mg(NO3)2
CH3OH + O2  CO2 + H2O
Burn methanol
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Balanced equations:
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2 Mg + O2  2 MgO
2 H2O2  2 H2O + O2
2 NaN3  2 Na + 3 N2
CaCO3  CaO + CO2
Mg3(PO4)2 + 6 NaOH  3 Mg(OH)2 + 2 Na3PO4
2 KClO3  2 KCl + 3 O2
2 Na + 2 H2O  2 NaOH + H2
2 AgNO3 + MgCl2  2 AgCl + Mg(NO3)2
2 CH3OH + 3 O2  2 CO2 + 4 H2O
8
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Balance the following equations:
__Fe2(SO4)3 + __K3PO4  __K2SO4 + __FePO4
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__Hg(NO3)2 + __ KI  __HgI2 + __ KNO3
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__Li2O(s) + __ H2O  __LiOH
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__HBr + __Ba(OH)2  __BaBr2 + __H2O
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__K2PtCl4 + __NH3  __Pt(NH3)2Cl2 + __KCl
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Section 2.6
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In lab, we want to predict amount of product
we can expect from a reaction
Have to use stoichiometry: quantitative
study of reactants and products in a
reaction (mole to mole ratio)
Use the mole to relate one substance in a
reaction to another (equation coefficients,
see Table 3.1)
◦ 2H2 (g) + O2 (g)  2H2O (g)
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We use coefficients in a balanced equation to
determine the mole ratio in which substances
need to react.
MgCl2(aq) +2AgNO3(aq) 2AgCl(s) + Mg(NO3)2(aq)
◦ 1 molecule of MgCl2 reacts with 2 molecules of
AgNO3 to produce 2 molecules of AgCl and 1
molecule of Mg(NO3)2
◦ Molecules are not practical! Scale up to moles!
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Use molar mass:
◦ Mass ÷ molar mass = moles
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Concentration: amount of solute present in
a given amount of solution
Molarity: moles of solute in 1 L of solution
◦ moles solute / liters solution; M = mol / L
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If I have 1.50 mL of a 0.100 M solution of
NaCl, how many moles are in the sample?
◦ M * V = moles
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Mass
Mole
Mole
Particles
Molarity
Courtesy: www.unit5.org/christjs/Stoichiometry/Mole%20Island%20Diagram.ppt
Mole to mole conversions
 MgCl2(aq) +2AgNO3(aq) 2AgCl(s) + Mg(NO3)2(aq)
1.00 mol
xs
?
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How many moles of solid product do you
predict to form?
Cannot compare grams of one to grams of
another!!
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Predict products (with phases) and balance
each equation.
Calculate the moles of precipitate (ppt) that
can be made in the following reactions.
NaCl (aq) + Hg(NO3)2 (aq) 
◦ 0.500 mol NaCl
FeCl3 (aq) + NaOH (aq) 
◦ 2.35 mol NaOH
Li2SO4 (aq) + AgCH3COO (aq) 
◦ 0.1098 mol AgCH3COO
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We need to be able to determine amounts of
chemicals to use so they react in the exact
ratio needed.
For example, we need 2 molecules of H2 and
1 molecule of O2 to make 2 molecules of H2O.
Do you want to count individual atoms and
molecules?
Size of atoms
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Who wants to count:
Peppers?
Water
Molecules?
Grains of rice?
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We can measure the mass of a pure sample
but how do we know how many atoms or
molecules are in that sample?
Mole: amount of a substance that contains a
specific number of particles.
A mole (mol) is a quantity that tells us two
things:
◦ the number of particles in a substance
◦ the mass of that substance (containing a specific
number of particles)
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1 12C atom weighs 12 amu, 1 mole of 12C
atoms weighs 12 grams.
1 dozen = 12 objects; 1 mole = 6.022 x
1023 objects. This is Avogadro’s number.
The moral: 1 mole of carbon-12 weighs 12
grams and contains 6.022 x 1023 atoms.
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How big IS Avogadro’s number?
◦ An Avogadro's number of standard soft drink
cans would cover the surface of the earth to a
depth of over 200 miles.
◦ If you had Avogadro's number of unpopped
popcorn kernels, and spread them across the
US, the country would be covered in popcorn
to a depth of over 9 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 the atoms in one mole.
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What do these substances
have in common?
Atoms measured in amu,
moles measured in grams
amu and g are the same value, different
units
◦ 1 atom 12C = 12 amu; 1 mol 12C atoms = 12 g
◦ 1 atom 4He = 4 amu; 1 mol 4He atoms = 4 g
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1 mole contains 6.0221415 x 1023 particles
◦ Particles can be atoms, ions, molecules, or
formula units
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Determine what type of particle Avogadro’s
number will describe for the following:
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Kr
C6H12O6
MgCO3
SF6
CaBr2
P
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Atomic, molecular, and formula masses are
not practical in lab. We can scale up to
calculate the mass of a mole of substances.
Molar mass is the same value as atomic
mass, just with different units. Molar mass
(g/mol: (mass per 1 mole of substance)
What is molar mass of C6H6?
What is molar mass of Mg(OH)2?
Molar masses have 6.022 x 1023 particles in
the substance.
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Al
CO2
Cl2
SO2
CaCl2
H2SO4
AlCl3
Mg3(PO4)2
Na2CO3
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Conversions:
◦ If we know the mass of a substance, how can we
calculate number of moles?
 mass  moles (use molar mass)
◦ If we know how many moles of a substance, how
can we calculate number of particles?
 moles  number of particles (use Avogadro’s
number)
1 mol
molar mass (in g)
1 mol
6.02 x 10 23 particles
Mass
Mole
Particles
Courtesy: www.unit5.org/christjs/Stoichiometry/Mole%20Island%20Diagram.ppt
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How many moles are in a 2.67 g sample of Ag?
How many grams are in a 0.45 mol sample of
Mg3(PO4)2?
How many atoms are in a 16.3 g sample of S?
How many formula units are in 0.81 g of MgCl2?
How many ions are in 0.81 g of MgCl2? (Hint:
How many ions are in 1 formula unit of MgCl2?)
How many moles of ions are in 0.81 g of MgCl2?
How many C atoms are in 3.64 g of C6H12O6?
How many O atoms are in 0.0765 g of CO2
Answers: 0.0248 mol Ag, 1.2 x 102 g Mg3(PO4)2, 3.06 x 1023 S
atoms, 5.12 x 1021 f. un., 1.54 x 1022 ions, 0.026 mol ions,
7.30 x 1022 C atoms, 2.09 x 1021 O atoms
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Calculate the number of atoms that are in a
6.00 g sample of carbon.
Calculate the number of moles in 12.04 x
1023 atoms of uranium.
Calculate the mass of 12.04 x 1023 atoms of
uranium.
If you have 1 g samples of NaCl, CO2, and
H2O, which sample will contain the fewest
particles?
Answers: 4.34 x 1025 C atoms, 2 mole Uranium,
476.1 g Uranium, NaCl (fewest moles because
largest MM)
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How many atoms are in 12.987 g of iron?
How many H atoms are in a 3.4215 gram
sample of H2O?
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Atom/molecule/mole ratios in C6H12O6