Chapter 4 Problems

1
Chapter 4 Problems

• 16, 24, 50, 52, 68, 72, 74, 80, 82, 112

2
Chemical Reactions

• Now that we know how to find out what the product
  of a reaction is and how much of it well get,
  we can ask a more interesting question

• How do reactions work?

In other words, how can we tell what were going
to get when we combine chemicals?
3
Chemical Reactions

• First, lets talk about what chemical reactions
  have in common.

• Most of the time, the reactants are compounds
  dissolved in water. We say that such solutions
  are aqueous.

What happens when a compound dissolves?
It depends on what kind of compound it is...
4
Electrolytes

• Many compounds break apart into their ions when
  we put them in water thats what we mean when
  we say a compound dissolves!

Example NaCl (s) ?
Na (aq) Cl- (aq)
solid
aqueous
liquid (l) gas (g)
But why does this happen??
5
Electrolytes

• What are the charges on oxygen and hydrogen in
  water?

2-

So the oxygen side of the molecule has a negative
charge, and the hydrogen end has a positive
charge.
6
Electrolytes

• That means the hydrogens (which have a positive
  charge) get attracted to negative ions, and the
  oxygens get attracted to positive ions

The oxygens pull Na ions off the crystal, and
the hydrogens pull Cl- ions.
7
Electrolytes

• If all the molecules in a compound break apart
  into ions, the compound is a strong electrolyte.

If only some of the molecules break apart, the
compound is a weak electrolyte.
If none of the molecules break apart, the
compound is a nonelectrolyte.
How can we tell if a compound is a strong, weak,
or nonelectrolyte?
8
Electrolytes

• Most molecular compounds are nonelectrolytes.

Examples H2O ? no ions C12H22O11 ? no ions C2H6O
? no ions
The main exception is acids. Most acids are
molecular compounds, but they are electrolytes.
Examples HCl ? H Cl- HNO3 ? H NO3-
9
Electrolytes

• An ionic compound is usually a strong or weak
  electrolyte if it dissolves! Many ionic
  compounds dont

Example NaCl ? Na Cl- PbCl2 ? doesnt dissolve
There are rules that help you decide whether an
ionic compound will dissolve. Well learn these
rules soon...
10
Chemical Reactions

• There are three basic types of chemical reaction

• Precipitation Reactions

• Acid-Base Reactions

• ReductionOxidation (Redox) Reactions

Lets take them one at a time...
11
Precipitation Reactions

• A precipitation reaction occurs when two
  solutions react to form an insoluble solid.

Example AgNO3 (aq) NaCl (aq) ? NaNO3 (aq)
AgCl (s)
the precipitate

• Precipitation reactions are easy to recognize in
  lab, because they either
• Form a solid, or
• Become cloudy

12
Precipitation Reactions

• Notice what happened in the example reaction the
  cations and anions in the two reactants switched
  places

Example AgNO3 (aq) NaCl (aq) ? NaNO3 (aq)
AgCl (s)
A reaction where this happens is called an
exchange or metathesis reaction. Precipitation
reactions almost always do this.
13
Precipitation Reactions

• So how can we tell if something will precipitate
  when we mix two chemicals? Simple!

First, write the reaction that will occur if
there is a precipitate.
Example Ca(OH)2 (aq) (NH4)2CO3 (aq) ?
2
CaCO3 NH4OH
14
Precipitation Reactions
Ca(OH)2 (aq) (NH4)2CO3 (aq) ? CaCO3 2
NH4OH
(s)
(aq)

• Now determine whether any of the products are
  insoluble according to the solubility rules

15
Precipitation Reactions

• If none of the products are solid, there is no
  precipitate, and no reaction occurs.

Zn(C2H3O2)2 (aq) K2SO4 (aq) ?
KC2H3O2 ZnSO4
2
(aq)
(aq)
No Reaction
NaI (aq) Pb(NO3)2 (aq) ?
PbI2 NaNO3
2
2
(s)
(aq)
16
Precipitation Reactions

• So whats going on in a precipitation reaction?

Example CuSO4 (aq) 2 NH4OH (aq) ? (NH4)2SO4
(aq) Cu(OH)2 (s)
This equation doesnt give the whole story!
Remember, ionic compounds dissolve into ions when
theyre aqueous.
Cu2 (aq) SO42- (aq) 2 NH4 (aq) 2 OH- (aq)
? 2 NH4 (aq) SO42- (aq) Cu(OH)2 (s)
17
Spectator Ions
Cu2 (aq) SO42- (aq) 2 NH4 (aq) 2 OH- (aq)
? 2 NH4 (aq) SO42- (aq) Cu(OH)2 (s)

• Notice that several ions appear on both sides of
  this equation.

These are called spectator ions, because
they dont actually do anything in the reaction.
Since they dont do anything, we can drop them
out of the equation
Cu2 (aq) 2 OH- (aq) ? Cu(OH)2 (s)
18
Types of Equation
Molecular Equation
CuSO4 (aq) 2 NH4OH (aq) ? (NH4)2SO4 (aq)
Cu(OH)2 (s)
Ionic Equation
Cu2 (aq) SO42- (aq) 2 NH4 (aq) 2 OH- (aq)
? 2 NH4 (aq) SO42- (aq) Cu(OH)2 (s)
Net Ionic Equation
Cu2 (aq) 2 OH- (aq) ? Cu(OH)2 (s)
19
Types of Equation

• Write the ionic and net ionic equations

LiBr (aq) AgNO3 (aq) ? AgBr (s) LiNO3 (aq)
Li (aq) Br- (aq) Ag (aq) NO3- (aq) ? AgBr
(s) Li (aq) NO3- (aq)
Br- (aq) Ag (aq) ? AgBr (s)
20
Types of Equation

• Write the ionic and net ionic equations

2 KCl (aq) Zn(NO3)2 (aq) ? ZnCl2 (aq) 2 KNO3
(aq)
2 K (aq) 2 Cl- (aq) Zn2 (aq) 2 NO3- (aq)
? Zn2 (aq) 2 Cl- (aq) 2 K (aq) 2 NO3- (aq)
NO REACTION!
21
AcidBase Reactions

• An acidbase reaction occurs when we combine an
  acid and a base (duh).

Example HCl NaOH ? NaCl H2O
Acidbase reactions always produce a salt and
water.
H2SO4 KOH ?
K2SO4 H2O
2
2
22
Acids and Bases

• An acid produces H ions in water.
• A base produces OH- ions in water.

These ions combine during the acidbase reaction
HCl NaOH ? NaCl H2O
Because the acid and base are turned into water,
acidbase reactions are also called
neutralization reactions.
23
Concentration

• Were almost ready to use what weve learned to
  do some calculations.
• First, we need to talk about the unit for
  concentration, molarity (M)

24
Concentration
Suppose you dissolve 30.00 g of NaCl in 250 mL of
water. What is the concentration of this
solution?
30.00 g
0.5133 mol
First, find the moles.
Now find the molarity.
2.05 M
25
Concentration
Suppose you want to make 150 mL of a 3.00 M K3PO4
solution. How much solid K3PO4 will you need?
This time we know the molarity and volume. What
we dont know (yet) is the moles.
moles M L of solution
moles 3.00 M 0.150 L
0.450 mol
Now find the grams.
0.450 mol
95.5 g
26
Stoichiometric Calculations
Theres a sequence of steps you can always follow
to do a stoichiometry problem
27
Calculations
What volume of 2.50 M H3PO4 will be needed to
react with 30.0 mL of 5.00 M KOH?
The first thing we must do is figure out what the
reaction is
H3PO4 KOH ?
K3PO4 H2O
3
3
28
Calculations
H3PO4 3 KOH ? K3PO4 3 H2O
0.0300 L
0.0200 L
20.0 mL H3PO4
Now for the calculation. Well start with the
volume we know.
Next, convert to moles of KOH.
Now convert from moles of KOH to moles of H3PO4.
Finally, convert to volume of H3PO4.
29
Dilution
Theres one more (very important!) calculation we
can run into. We often have a very concentrated
solution that needs to be watered down (diluted).
The equation we use for this is easy to
remember M1V1 M2V2
The molarity and volume before dilution
The molarity and volume after dilution
30
Dilution
M1V1 M2V2
Suppose we have 20.0 mL of a 6.00 M H3PO4
solution. We dilute this to 100 mL. What is the
new concentration?
6.00 M 20.0 mL M2 100 mL
M2 1.20 M
Suppose we wish to make 250 mL of a 0.0100 M NaCl
solution. How much 2.00 M NaCl should we start
with?
2.00 M V1 0.0100 M 250 mL
V1 1.25 mL
31
Redox Reactions

• The third type of reaction is reductionoxidation
  (redox) reactions.

Example 2 Mg SiO2 ? Si 2 MgO
All these are solids, so its not a precipitation
reaction, and none of them are acids or bases.
Whats going on?
32
Redox Reactions
2 Mg SiO2 ? Si 2 MgO

• To understand this reaction, we need to know the
  charge on each atom.

There are a few rules to help us figure out the
charges of the atoms

• An element all by itself has a charge of 0.

• Oxygen has a charge of -2.

• Hydrogen has a charge of 1.

• The charges on a neutral molecule must add up to
  0.

• The charges on the atoms in an ion must add up to
  the overall charge of the ion.

33
Redox Reactions

• Lets give each atom a charge.

2 Mg SiO2 ? Si 2 MgO
0
0
-2
-2
4
2
Notice what happened! Magnesiums charge
increased from 0 to 2...
and silicons charge decreased from 4 to 0.
What does this tell us? The magnesium atom lost
electrons, which made the charge go up.
The silicon atom gained electrons, which
made the charge go down.
34
Redox Reactions
2 Mg SiO2 ? Si 2 MgO
oxidation
reduction
When an atom loses electrons, we say it is
oxidized. When it gains electrons, we say it is
reduced. Thats why these reactions are called
redox (reductionoxidation) reactions.
GER!
Theres an easy way to remember this LEO the
lion says GER
Loses Electrons Oxidation
Gains Electrons Reduction
35
Redox Reactions

• Write the charges on the atoms and determine what
  is oxidized and what is reduced

Al CuSO4 ? Cu Al2(SO4)3
0
0
-2
-2
2
3
6
6
oxidation
reduction
CH4 2 O2 ? CO2 2 H2O
-4
4
-2
0
1
-2
1
oxidation
reduction