Download Know how to balance a chemical reaction

Lecture 15
Patterns of reactivity
Know how to balance a chemical reaction
• (Complete) Combustion reactions (Chapter 3)
C3H8(g) + 5O2 (g) → 3CO2 (g) + 4H2O(g)
• Combination reactions (Chapter 3)
2Mg(s) + O2(l) → 2MgO(s)
• Decomposition reactions (Chapter 3)
PbCO3(s) →PbO(s) + CO2(g)
• Single displacement reactions (Chapter 4)
2Ca(s) + O2(g) → 2CaO(s)
• Exchange reactions (Chapter 4)
Precipitation
Pb(NO3)2(aq) + 2KI(aq) → PbI(s) ↓ + 2KNO3(aq)
Neutralization
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
Gas formation
2HCl(aq) + Na2S(aq) → H2S(g) ↑ + 2NaCl(aq)
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Chem 6 Study Guide
Lecture 15
Net Ionic Equations
Before you begin, you need to know:
1. Nomenclature of molecules and ions.
2. How to identify strong, weak and non electrolytes
3. Solubility rules
-----------------------------------------------------------------------------------------SOLUBILITY RULES FOR COMMON IONIC COMPOUNDS IN WATER
1.
Almost all ammonium and alkali
metal salts are soluble.
2.
Most nitrates, acetates, chlorides,
bromides, and sulfates are soluble.
Exceptions: silver halides
sulfates of Ca, Ba, Pb, Ag
3.
Most sulfides, carbonates, phosphates
and hydroxides are insoluble.
Exceptions: alkali salts
ammonium salts.
(See #1 above.)
------------------------------------------------------------------------------------------------------------------------
How to write a net ionic equation
A.
B.
Write the molecular equation. (Balance it.)
Write the ionic equation.
a) Identify strong electrolytes and write them in ionic form ions unless it is an insoluble salt
E.g. NaCl ⇒Na+(aq) + Cl−(aq)
b) Identify weak and non electrolytes and write them in molecular form.
E.g. H2CO3(aq) Weak electrolyte
CH3OH(aq) Non electrolyte
c) Insoluble salts are written in molecular form. (Need to memorize solubility rules.)
E.g. AgCl(s).
C.
D
Eliminate the spectator ions to get the net ionic equation.
The net ionic equation describes the chemistry that is occurring.
driving forces for a reaction
a)
b)
c)
d)
formation of precipitate
formation of gas
formation of weak or no electrolyte
oxidation/reduction
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Chem 6 Study Guide
Lecture 15
Group Problems
Name ____________________
Examples of Net ionic equations
1. Write the net ionic equation for the precipitation of an insoluble compound when
MgCl2 and AgNO3 are mixed.
What is the driving force? What are the spectator ions?
2. Write molecular, ionic and net ionic equations for the complete neutralization
reaction of the strong acid H2SO4(aq) and the strong base Ba(OH)2(aq).
What is the driving force? What are the spectator ions?
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Chem 6 Study Guide
Lecture 15
Rules for determining Oxidation States
1. Oxidation state of atom in elemental form is zero.
Cl2 O2
P4
C(s)
S8
e.g.
2. The oxidation number of a monatomic ion equals its charge.
3. Some elements have “common” oxidation numbers that can be used as
reference in determining the oxidation numbers of other atoms in the
compound.
Alkali metals
Alkaline earth metals
Fluorine
O
usually
+1
+2
–1
–2
(peroxides (-1) & superoxides possible)
H
usually
+1
(Hydrides: metal-H compounds (–1))
Cl, Br, I
almost always
–1
4. Sum of oxidation numbers is equal to overall charge of molecule or ion:
• For a neutral compound the sum of oxidation numbers equals zero.
• For a polyatomic ion, the sum of the oxidation numbers is equal to the
charge on the ion.
5. Shared electrons are assigned to the more electronegative atom of the
pair:
• more electronegative atom will have a negative oxidation number.
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Chem 6 Study Guide
Lecture 15
Group Problems
Oxidation Numbers
Name ____________________
Use the rules for determining oxidation states found on page 12 of your Chem 6
Study Guide to find the oxidation states of the elements listed in each of these
examples.
1.
Cl2
Cl
2.
S2−
S
3.
KHCO3
K
C
4.
CaSO4
Ca
S
5.
HCl
Cl
6.
ClF3
F
Cl
7.
P2O5
O
P
8.
Na2O2
Na
O
9.
H2O2
H
O
10.
NaAlH4
H
What are the oxidation states of the elements in this reaction?
Fe2O3(s) + 3CO(g) → 3Fe(s) + 3CO2(g)
What is oxidized? What is reduced?
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Chem 6 Study Guide
Lecture 16
These are some of the basic connections needed to solve problems in
chemistry.
•
•
•
•
Avogadro’s number
memorize it!
definition of mole
Formula weight (molar mass)
the following connections
(or conversions)
gram ⇔ mole
gram ⇔ molecules
• what is meant by:
empirical formula
molecular formula
To solve problems in chemistry :
1. Write the balanced equation (or process).
2. Make a table
Note connections wherever possible.
Enter what you know
3. Fill in the table until you are able to solve the problem.
4. TRY things.
5. Make sure your answer is REASONABLE!
In other words: Make connections between experimentally measured properties and
the balanced equation.
OR
Given information such as mass, volume, pressure and temperature, how can one
determine quantities of moles/molecules?
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Chem 6 Study Guide
Lecture 16
Group Problems
Name ____________________
Methane burns in oxygen to produce water, carbon dioxide, and heat. 4.0 g of
methane requires 16.0 g of oxygen. The amount of carbon dioxide produced is:
Styrene is a hydrocarbon. If 0.438g of the compound is burned and produces 1.481 g
of CO2 and 0.303 g of H2O, what is the empirical formula of styrene?
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Chem 6 Study Guide
Group Problems
Name ____________________
Stoichiometric Problem Solving
1. 50.00 g of hydrated calcium sulfate is strongly heated to drive off all the water of hydration. The
remaining mass is 39.54 g. What is the empirical formula of the hydrate?
(a) CaSO4 • H2O
(b) CaSO4 • 2H2O
(c) 2CaSO4 • H2O
(d) CaSO4 • 7H2O
(e) None of the above is correct.
HINTS
Write out the equation for the process that is occurring. Use a variable (x) to represent the number of H2O
molecules in the hydrate. (The • is a symbol used to indicate the amount of water bonded to the salt in the
hydration process, so CaSO4 • H2O means one H2O per CaSO4; CaSO4 • 2H2O means there are 2 H2O
molecules per each CaSO4, etc.)
The formula of the substance that remains after the hydrate is heated is CaSO4. How many moles of CaSO4 remain
after the hydrate is heated?
Calculate the grams of H2O driven off (using conservation of mass). How many moles of H2O is this?
2. Styrene is a hydrocarbon. If 0.438g of the compound is burned and produces 1.481
g of CO2 and 0.303 g of H2O, what is the empirical formula of styrene?
(a)
(b)
(c)
(d)
(e)
CH
CH2
CH3
C2H5
C3H2
HINTS:
1.
What does it mean when it says that styrene is a hydrocarbon?
2.
Write out the chemical equation for the process that is occurring. (Note in this case, you cannot write a
balanced equation, but you can write a process with the proper reactants and products.) What are the reactants?
What are the products?
3.
From the given information, determine the mass of C in the original sample. (What information do you need to
do this?)
4.
From the given information, determine the mass of H in the original sample. (What information do you need to
do this?)
5.
Use the masses of H and C to find the mole ratio and then the empirical formula.
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Chem 6 Study Guide
Group Problems
Name ____________________
3. If 45.0 g of Pb(NO3)2 is reacted with excess KX, 49.9 g of PbX2 is produced. What is the
molecular weight of PbX2?
Pb(NO3)2 + 2KX → PbX2 + 2KNO3
(a)
(b)
(c)
(d)
(e)
278.2
367.3
461
322.7
414.1
HINTS: Set up a table under the balanced reaction as shown:
Pb(NO3)2 + 2KX →
PbX2 +
2KNO3
mass
M.W.
moles
Enter what you know [e.g. mass of Pb(NO3)2 and PbX2, MW of KNO3 and Pb(NO3)2].
Try to fill in as many of the blank spots in the table as you can until you see a way to get the molecular weight of PbX2.
4. 1.0 mol of NH3 and 1.0 mol of O2 are allowed to react to produce NO and H2O. If the reaction
goes to completion which one of the following is true? (NOTE: you do not need to calculate any masses,
or use MW to solve this problem, try following the HINTs below BEFORE looking at the answer choices. And keep
in mind that more than one answer could be correct.)
4NH3 + 5 O2 → 4NO + 6H2O
(a) All of the NH3 is consumed.
(b) 1.0 mol of NO is produced.
(c) 1.5 mol of H2O is produced.
(d) 4.0 mol of NO is produced.
(e) All of the O2 is consumed.
HINTS:
• Rewrite the balanced equation so that the coefficient in front of oxygen is one. Then rewrite the equation again
such that the coefficient in front of the ammonia is one. (Since the equation is already balanced, make changes to
the coefficients in front of the reactants and products. The resulting equations should be balanced with noninteger coefficients.)
• Using the two new forms of the balanced equation, decide which is the limiting reagent NH3 or O2
•
Now look at the answer choices and pick out the statement that is true.
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Chem 6 Study Guide
Group Problems
Additional Problems
1.
Name ____________________
What is the molarity of the solution produced by dissolving 5.00 g of KBr in water such that the total
solution volume is 300.0 mL?
(a) 0.140 M
(b) 0.397 M
(c) 16.7 M
(d) 4.20 × 10−2 M
(e) 6.00 × 10−2 M
2.
What is the net ionic equation for NH3(aq) + HBr(aq) →?
(a)
(b)
(c)
(d)
(e)
NH3(aq) + HBr(aq) → NH4+(aq) + Br−(aq)
NH3(aq) + HBr(aq) → NH4Br(aq)
NH3(aq) + H+(aq) + Br−(aq) → NH4Br(aq)
NH3(aq) + H+(aq) → NH4+(aq)
NH3(aq) + H+(aq) + Br−(aq) → NH4+(aq) + Br−(aq)
FOLLOW UPS:
What is the driving force for the reaction?
Which one of the choices is the molecular equation?
Which choice is the ionic equation?
3.
What is the molarity of an aqueous HBr solution if 35.00 mL of it is neutralized with 70.00 mL of a
0.500 M NaOH solution?
(a)
(b)
(c)
(d)
0.250 M
0.500 M
0.750 M
1.00 M
HINTS: Write out the balanced reaction for the neutralization. Make a table underneath the balanced equation. The rows
will be:
volume
concentration (M)
moles
Fill in the table until you are able to determine the concentration of the original HBr solution.
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Chem 6 Study Guide