Transcript Chapter 13: Chemical Reactions

Chemical Reactions
(Chapter 13)
coefficients
reactants
products
Student Learning Objectives
• Utilize chemical equations to determine
the amounts of reactants, products, and
energy change.
• Characterize acids and bases.
• Describe reactions involving oxygen.
How is a chemical equation balanced?
A chemical reaction is the process of forming new
substances.
Atoms are combined to form molecules.
Molecules separate into atoms.
The chemical equation shows definite proportions of
each substance. (same numbers of atoms on each side)
Al
+
Br2

Al2Br6
Practice
1)
Pb
2)
C3H8
3)
Al
4)
BaO
+ PbO2
+
+
+
+ H2SO4

PbSO4
O2

CO2
+
H2O
HCL

AlCl3
+
H2
H2O

Ba(OH)2
+
H2O
There are always 6.022 x 1023 atoms or molecules in
1 mole of a substance.
Avogadro’s Number
6.022 x 1023 particles
mole
Hydrogen & Water
Practice
1) Calculate the amount of grams and number of
particles in one mole.
a) Gold
b) NaCl
c) Cu(NO3)2
2) Calculate the number of moles, and the number of
molecules, in 2 grams of carbon dioxide.
More Practice
3) How many grams of each reactant are needed and
how many grams of product are produced in the
reaction?
Al + Br2  Al2Br3
What are exothermic and endothermic reactions?
The amount of chemical potential energy may change
during a chemical reaction.
Exothermic
Reaction
Endothermic
Reaction
Energy released
Reactants → higher PE
Energy absorbed
Products → higher PE
Activation Energy
A spike of energy is
needed to break existing
bonds.
H2 + Cl2  2HCl
Bond Strength
Bond Energy is the energy required to break 1 mole
of bonds. (Same energy released when bond forms)
Bond Energy to Break = Bond Energy Released
The net bond energy indicates whether the process
was exothermic or endothermic.
Examples
1) N2 + H2  N2H4
2) H4C2 + O2  CO2 + H2O
What can cause reaction rates to vary?
1. Temperature
2. Concentration
3. Surface area
4. Catalysts
What is this?
Catalysts
A catalyst accelerates
the reaction.
 Provides different
pathway
 Lowers activation energy
 Reappears as a product
Progression of Reaction
With catalyst
Practice
1) Why are there warnings on some medications not to
break, crush, or chew the pill?
2) Why do you think some laundry soaps contain
enzymes?
3) Why are photographic prints processed in the dark?
What is an acid? What is a base?
Acids contain H+ and
H30+ ions, and are
substances that donate
hydrogen ions.
Proton Donor
Bases contain hydroxide
(OH−) ions, or are
substances that accept a
hydrogen ions.
Proton Acceptor
Examples
1) Baking soda (NaHCO3)
2) HCl + NaHCO3  H2CO3 + NaCl
Practice
Identify the acid substances and the base substances.
CH3CO2H + H2O  CH3CO2- + H3O+
Strong Acids & Bases
Strong acids and bases completely ionize in water.
Some Strong Acids
HNO3
HCl
H2SO4
nitric acid
hydrochloric
acid
sulfuric acid
Some Strong Bases
LiOH
NaOH
KOH
lithium
hydroxide
sodium
hydroxide
potassium
hydroxide
The neutralization reaction of an acid and a base
produces water and a salt.
Acid
Base
Water
Salt
HCl
+
NaOH

H2O
+
NaCl
HBr
+
KOH

H2O
+
KBr
pH
A substance that has an equal number of acid and
base ions is neutral. (pH balanced)
pH
1 to 6
7
8 to 14
Solution
Acidic
Neutral
Basic
Substance
pH
Battery Acid
Stomach Acid
Cola
Orange Juice
Coffee
Milk
Human Blood
Hand Soap
Ammonia
Bleach
0.5
1.5 – 2.0
2.5
3.5
5.0
6.5
7.34 – 7.45
9.0 – 10.0
11.5
12.5
Practice
1) If the water you drink has a pH of 6.8, is it acidic,
basic, or neutral?
2) Blood has buffers to help control the natural pH your
body needs. The amount of CO2 in the blood stream
can change pH. What do we do naturally that can
change the pH of blood?
What is “acid rain”?
Acid Rain is moisture
that has a pH of less
than 5.
 Rain, snow, sleet, or fog
 Rainwater normally has a
pH of 5.6 to 6.2
 Acids may combine with
dust (dry deposition)
Water naturally reacts
with CO2 in the
atmosphere, forming a
weak acid.
water + carbon dioxide
 carbonic acid (H2CO3)
Strong Acids in Our Atmosphere
Human-produced exhaust emissions of sulfur and
nitrogen oxides form strong acids.
sulfates + water  sulfuric acid (H2SO4)
nitrates + water  nitric acid (HNO3)
Location Question
Where would you expect a higher level of acidity
in the atmosphere, WY or CA? Why? What are
the factors?
Trees killed by
acid rain
Czech Republic
Image Credit
Simon Fraser/Science
Photo Library
What is an oxidation-reduction reaction?
 Redox reactions involve a transfer of electrons, and often
the addition or subtraction of oxygen.
Oxidation
Reduction
Reactant loses electrons Reactant gains electrons
Oxygen Added
Oxygen Removed
Redox Reactions
Corrosion is a redox reaction between a metal and
oxygen (O2).
Combustion is a redox reaction between a non-metal
and oxygen (O2).
Redox Examples
Corrosion is a redox reaction between a metal and
oxygen (O2).
Combustion is a redox reaction between a non-metal
and oxygen (O2).
Practice
1) Which element is oxidized and which is reduced?
a) HgS + O2  Hg + SO2
b) SnO2 + 2C  Sn + 2CO2
2) Is the iron oxidized or reduced in the formation of rust?
4Fe(s) + 3O2(g)  2Fe2O3(s)
More Practice
3) Is a combustion reaction, like the combustion of
methane, exothermic or endothermic?
CH4(g) + 2O2(g)  CO2(g) + 2H2O(l)
How do batteries and fuel cells work?
Batteries have 2 diodes; one that is oxidized and one
that is reduced.
Acidic solution reacts with metals
One metal is reduced (+ end)
One metal is oxidized (– end)
Reactions free electrons
Battery
Cathode (+)
• Metal gains e−
• Metal is reduced
Battery
Anode (–)
• Metal loses e−
• Metal is oxidized
Fuel Cells
A fuel cell uses a membrane to separate the
reactants.
Hydrogen gas is oxidized.
Oxygen gas is reduced.
Product is H2O vapor.
Practice
1) Where do you store your batteries? Why?
2) What is the difference between a dry cell battery and
an alkaline battery?
3) What is the primary difference between a battery
and a fuel cell?
Practice: Where do you store your
batteries? Why?
 A fuel cell uses a membrane to separate the
reactants.
Hydrogen gas is oxidized.
Oxygen gas is reduced.
Product is pure water vapor.
What is electrolysis?
Electrolysis is the use of electric current to cause
chemical change.
Reverses chemical reactions
Splits compounds
Al2O3 + Energy  Al2 + O2
What is electrolysis?
Electrolysis is the use of electric current to cause
chemical change.
Reverses chemical reactions
Splits compounds
Example:
Al2O3 + Energy  Al2 + O2
Question: How do you recharge your cell
phone battery?
Practice
1) Is electrolysis an exothermic or endothermic
process?
2) Describe the chemical process of recharging your
mobile phone device.
3) What other devices operate in the same way?
4) Can you think of other examples of electrolysis?