# Chapter 8 � Chemical Equations and Reactions by rz4zjFQ

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Chapters 9 – Chemical Reactions and
Equations
Goals for Chapter 9

• Describe, compose, and balance chemical equations

• Recognize the main types of chemical reactions – predict products

• Use Activity series to predict reactions

• Understand the solubility chart
Describing Chemical Reactions
• OK, so what is a chemical reaction?
– What are the main parts of a chemical reaction?
Reactants and products
• Define chemical equation:
Chemical equations use symbols and formulas that represent the
identities and relative amounts (moles and molecules) of the
reactants and products in a chemical reaction

C6H14 (l) +   O2 (g)            CO2 (g) + H2O (l) (not balanced)
What type of reaction is this?
• Law of Conservation of mass tells us:
The total mass of reactants MUST equal the total mass of
products for any given chemical reaction

What can’t a chemical equation tell us?
LCM
How can we tell a reaction has taken place?

• heat or light
– can also happen with
physical changes
• gas bubbles
– means a gas is being
created as product
• precipitate
– solid is being created
• color change
Chemical Equations

The equation must represent known facts

It is vital you know what you are working with!

The equation must contain the correct formulas!

Chapter 7 is quite important – naming and writing formulas

The law of conservation of mass must be satisfied
“balancing act”
Word and Formula Equations
The coefficient:

C6H14 (l) + O2 (g)            CO2 (g) + H2O (l) (not balanced)

C6H14 (l) + 9.5O2 (g)            6CO2 (g) + 7H2O (l) (almost)

2C6H14 (l) + 19O2 (g)            12CO2 (g) + 14H2O (l) (balanced)

What’s the significance of the arrow?
Lettuce get the formulas right first!

solid zinc sulfide + oxygen gas yields solid zinc oxide and sulfur dioxide gas

ZnS (s) + O2 (g)                 ZnO (s) + SO2 (g)

The reverse approach

Zn (s) + HCl (aq)                   ZnCl2 (aq) + H2 (g)

solid zinc + hydrochloric acid yields
aqueous zinc chloride and hydrogen gas

Hydrochloric acid? We need to go back to page 248-249 for a moment.
You need to know all of the common acids in Figure 7-27.
Symbols used in Chemical Equations

“Yields”

Reversible

(s), (g), (aq)        Solid, gas, aqueous solution

or
precipitate or gas formation

D                           heat

x atm or pressure   Reaction proceeds at defined pressure
or above atmospheric pressure
x oC                A temperature at which the
reaction is carried out
cat
A catalyst is employed
Balancing Chemical Equations
What is the point? What are the rules?

1. Identify the names of the reactants and products

Potassium iodide + lead (II) nitrate yields lead (II) iodide + potassium nitrate

2. Write the formula equation

Pb(NO3)2 (aq) + KI (aq)      PbI2 (s) + KNO3 (aq)

EXAMPLES
Let’s try some
Given the following word equations suggest the chemical equation

Water yields hydrogen and oxygen

H2O   (l)     H2   (g)   +     O2 (g)

Zinc + hydrochloric acid yields zinc chloride and hydrogen

Zn (s) + HCl (aq)              ZnCl2 (aq)      + H2 (g)

Nitric acid + magnesium hydroxide yields magnesium nitrate and water

HNO3 (aq) + Mg(OH)2 (s)                  Mg(NO3)2 (aq)     + H2O (l)

Silver nitrate + sodium chloride yields silver chloride and sodium nitrate

AgNO3 (aq) + NaCl (s)                 AgCl    (s)   + NaNO3 (aq)
Balancing the equations – Rules Continued
3. Balance the formulas according to the law of conservation of mass
Use the coefficients only! DO NOT CHANGE SUBSCRIPTS

1. Balance the atoms one at a time
2. Balance the atoms that are combined and appear once on each side
of the equation
3. Balance polyatomic ions that appear on both sides of the
equation as single units
4. Lastly, balance H and O atoms
5. Simplify if you can – do not leave fractions.

4. Count the atoms and be sure the equation is balanced

Yes – some of these will be trial and error – but if you
follow the rules you will be able to quickly balance the equations.
Try some!
H2O   (l)   H2   (g)     +       O2 (g)

2H2O   (l)   2H2    (g)       +    O2 (g)

Zn (s) + HCl (aq)              ZnCl2 (aq)         + H2 (g)

Zn (s) + 2HCl (aq)              ZnCl2 (aq)         + H2 (g)

HNO3 (aq) + Mg(OH)2 (s)                Mg(NO3)2 (aq)         + H2O (l)

2HNO3 (aq) + Mg(OH)2 (s)                Mg(NO3)2 (aq)         + 2H2O (l)

AgNO3 (aq) + NaCl (s)                 AgCl    (s)   + NaNO3 (aq)

AgNO3 (aq) + NaCl (s)                 AgCl    (s)   + NaNO3 (aq)
More?
Al2(SO4)3    (aq)       + Ca(OH)2      (aq)       Al(OH)3   (s)          +   CaSO4 (s)

Al2(SO4)3   (aq)   + 3Ca(OH)2          (aq)       2Al(OH)3     (s)        +   3CaSO4 (s)

Fe (s) + Cl2 (g)            FeCl3    (s)

2Fe (s) + 3Cl2 (g)               2FeCl3        (s)

NH4Cl (aq) + Ca(OH)2 (aq)                      CaCl2 (aq)    + NH3 (g) + H2O (l)

2NH4Cl (aq) + Ca(OH)2 (aq)                      CaCl2 (aq)    + 2NH3 (g) + 2H2O (l)

C23H48        (l)   + O2 (g)          CO2 (g)    + H2O (l)

C23H48      (l)   + 35O2 (g)             23CO2 (g)      + 24H2O (l)

Let’s try a lot more
Types of Chemical Reactions
• Recognize the 5 main types of reactions:
– Example: Synthesis reaction between an element and
oxygen:

Mg (s) +    O2 (g)     MgO (s)

Balanced?

2Mg (s) +    O2 (g)      2MgO (s)

Let’s go through the rest.
#1 - Synthesis Reactions
• Two or more substance combine to form a new
compound: A + X     AX
Reactions of elements with O2 and S8
Rb (s) + S8 (s)           Rb2S (s)    (balanced?)
16Rb (s) + S8 (s)           8Rb2S (s)    (balanced)
General formula for Group 1 and Group 2?
M2O and MO, same with sulfur (M2S and MS)

Nonmetals also react with oxygen
S8 (s) + O2 (g)           SO2 (g)     (balanced?)

S8 (s) + 8O2 (g)           8SO2 (g)    (balanced)

Keep in mind that some metals can react with oxygen and
form two different oxides – we have learned about these.
Synthesis reactions continued
– Metals reacting with Halogens

Rb (s) + Cl2 (g)          RbCl (s)    (balanced?)

2Rb (s) + Cl2 (g)          2RbCl (s)    (balanced)

– Reactions with oxides

CaO (s) + H2O (l)           Ca(OH)2 (s)    (balanced?)

#2 - Decomposition Reactions
• A single compound (reactant) undergoes a process where two
or more simpler substances are produced: AX      A + X

electricity
2H2O (l)                   2H2 (g)     + O2 (g)

Name this type of decomposition reaction

electrolysis

• Oxides of less active metals undergo decomposition reactions when heated:

D
2HgO (s)                   2Hg (l)     + O2 (g)
Other types of Decomposition Reactions

• Decomposition of metal carbonates:
D
CaCO3 (s)                       CaO (s)     + CO2 (g)

• Decomposition of metal hydroxides:

D
Ca(OH)2 (s)                      CaO (s)      + H2O (g)

• Decomposition of metal chlorates:

D
2KClO3 (s)                     2KCl (s)      + 3O2 (g)

• Decomposition of acids:

D
H2SO4 (aq)                      SO3 (g)     + H2O (l)

What would you predict for the gas product of decomposing carbonic acid?
#3 - Single Replacement Reaction
• AKA “displacement reactions”, is where a single element
replaces a similar element in a compound. General equations:

A + BX                    AX + B
Y + BX                    BY + X
What is the difference?

• Replacement of a metal in a compound:

Al    (s)   + Pb(NO3)2 (aq)          Pb (s) + Al(NO3)3 (aq)

Balanced?

3Al   (s)   + 2Pb(NO3)2 (aq)           2Pb (s) + 3Al(NO3)3 (aq)
Single Replacements Continued

• Replacement of hydrogen in water by a metal:

2K (s) + 2H2O (aq)       2KOH (aq) + H2 (g)

• Replacement of hydrogen in acid by a metal:

Mg (s) + 2HCl (aq)       MgCl2 (aq) + H2 (g)

• Replacement of Group 17 elements:

Cl2 (g) + 2KBr (aq)      2KCl (aq) + Br2 (g)
How to predict SRR

#4 - Double Replacement Reactions
• Ions of two compounds exchange places in solution to form
two new compounds:
AX + BY                 AY + BX

Characteristics of this type of reaction?

• Formation of a precipitate:

Pb(NO3)2 (aq) + 2KI (aq)     PbI2 (s) + 2KNO3 (aq)

• Formation of a gas:

FeS (s) + 2HCl (aq)      H2S (g) + FeCl2 (aq)

How do you know if a compound is soluble?
Solubility Table
Lastly, #5 - Combustion Reactions
• Substances combining with oxygen releasing energy in
the form of heat and light, and water, and mainly CO2:
Write and balance the combustion reaction of propane in excess oxygen

C3H8(g)   + 5 O2(g)        3CO2(g)+ 4H2O(g)

Let me show you an easy way to balance these!
Name some reactions
Mg (s) + 2HCl (aq)             MgCl2 (aq) + H2 (g)

Single replacement

Cl2 (g) + 2KBr (aq)            2KCl (aq) + Br2 (g)
Single replacement

CaO (s) + H2O (l)                   Ca(OH)2 (s)
Synthesis
D
CaCO3 (s)                   CaO (s)     + CO2 (g)
Decomposition

C3H8(g)     + 5O2(g)             3CO2(g)+ 4H2O(g)
Combustion

Pb(NO3)2 (aq) + 2KI (aq)       PbI2 (s) + 2KNO3 (aq)

Double Replacement
What’s Next? Chapters 10 and 11 – The Mole and
The Mathematics of Chemical Equations
• Please R&O Chapters 10 and 11 – print off all
materials listed for these chapter.

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