chapter8-chemicalequationsandreactions

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CHEMISTRY OF MATCHES
P4S3 + KClO3
tetraphosphorus
trisulfide
potassium
chlorate
D
P2O5 + KCl
diphosphorus
pentoxide
Strike anywhere matches
The substances P4S3 and KClO3 are both
present on the tip of a strike anywhere match.
When the match is struck on a rough surface,
the two chemicals (reactants) ignite and produce
a flame.
+ SO2
potassium
chloride
sulfur
dioxide
Safety matches
The substances P4S3 and KClO3
are separated. The P4S3 is on
the matchbox cover.
Only when the chemicals combine
do they react and produce a flame.
The products from this reaction are P2O5, KCl, and SO2,the
last of which is responsible for the characteristic sulfur smell.
Charles H.Corwin, Introductory Chemistry 2005, page 182
Chemical Equations and
Reactions
CHAPTER 8
Describing Chemical Reactions
SECTION 1

Chemical reaction  process by which one or
more substances are changed into one or more
different substances

Represented by chemical equations  represents,
with symbols and formulas, the identities and
relative amounts of the reactants and products in
a chemical reaction
(NH4)2Cr2O7 (s)  N2(g) + Cr2O3(s) + 4H2O(g)
ORGANIZE YOUR THOUGHTS
Chemical
reactions
Chemical
equations
• Balancing equations
• Predicting products
from reactants
Packard, Jacobs, Marshall, Chemistry Pearson AGS Globe, page 175
Chemical
equations
• Synthesis
• Decomposition
• Single replacement
• Double replacement
• Combustion
SIGNS OF CHEMICAL REACTIONS
There are five main signs that indicate a chemical reaction has taken place:
release
input
change in color
change in odor
production of new
gases or vapor
input or release
of energy
difficult to reverse
CHEMICAL EQUATIONS
aluminum oxide
reactants
Depict the kind of reactants
and products product
and their relative amounts in a reaction.
4 Al(s) + 3 O2(g)
2 Al2O3(s)
The letters (s), (g), and (l) are the
physical states of compounds.
The numbers in the front are called
stoichiometric coefficients.
CHEMICAL EQUATIONS
4 Al(s) + 3 O2(g)
aluminum oxide
sandpaper
2 Al2O3(s)
4 g Al + 3 g O2 yield 2 g Al2O3
This equation means:
4 Al atoms + 3 O2 molecules yield 2 molecules of Al2O3
or
4 Al moles + 3 O2 moles yield 2 moles of Al2O3
4 mol [email protected]/mol
108 g
3 mol [email protected]/mol
+
96 g
2 mol [email protected]/mol
=
204 g
FORMULA EQUATIONS

Formula equation  represents reactants and
products of chemical reaction by their symbols
or formulas
CH4(g) + O2(g)  CO2(g) + H2O(g)

Is the law of conservation of mass satisfied
here?
CH4 + O2  CO2 + H2O
Reactants – 7 atoms
 Products – 6 atoms

Hydrogen = 4 on left, = 2 on right
 Oxygen = 2 on left, = 3 on right


In order to satisfy law of conservation of mass,
must balance the equation by adding coefficients
BALANCING A CHEMICAL EQUATION
1.
Write a formula equation by substituting
correct formulas for the names of the
reactants and products (if you do not start
with a formula equation).
1.
2.
H2O(l)  H2(g) + O2(g)
Balance the formula equation according to the
law of conservation of mass. There are a few
simple guidelines to use for this…..
GUIDELINES FOR BALANCING EQUATIONS

Balance the different types of atoms one at a time.

First balance the atoms of elements that are combined
and that appear only once on each side of the equation.

Balance polyatomic ions that appear on both sides as
single units.

Balance H and O atoms (or any other lone atoms) last.
H2O(l)  H2(g) + O2(g)


2 oxygen on right and only 1 on the left
Start with placing 2 in front of H2O
2H2O(l)  H2(g) + O2(g)



Oxygen is now balance with 2 on left and 2 on right
Now balance hydrogen – 4 on left, 2 on right
Add coefficient 2 to H2
2H2O(l)  2H2(g) + O2(g) ---- balanced!
DO NOT!.....

DO NOT WRITE INCORRECT FORMULAS, THIS
WILL MESS UP YOUR BALANCING

DO NOT CHANGE SUBSCRIPTS IN FORMULAS
TO BALANCE THE EQUATION!
H2O(l)  H2(g) + O2(g)
H2O(l)  H2(g) + O(g)
DO!!!!.....

When you think you have balanced the
equation, COUNT THE NUMBERS OF EACH TYPE
OF ATOM ON EITHER SIDE OF THE EQUATION
2H2O(l)  2H2(g) + O2(g)
H
O
Reactant side
4
2
Product side
4
2
SAMPLE PROBLEM

The reaction of zinc with aqueous
hydrochloric acid produces a solution of zinc
chloride and hydrogen gas. Write a balanced
equation for the reaction.
1. Write the word equation
Zinc + hydrochloric acid  zinc chloride + hydrogen
2. Write the formula equation
Zn(s) + HCl(aq)  ZnCl2(aq) + H2(g)
ADJUST THE COEFFICIENTS
Zn(s) + HCl(aq)  ZnCl2(aq) + H2(g)
Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g)
Count atoms:
Zn – 1 on left, 1 on right
H – 2 on left, 2 on right
Cl – 2 on left, 2 on right
PRACTICE PROBLEM 1A

Write word, formula, and balanced chemical
equations for magnesium and hydrochloric acid
react to produce magnesium chloride and
hydrogen.
Magnesium + hydrochloric acid  magnesium
chloride + hydrogen
 Mg + HCl  MgCl2 + H2
 Mg + 2HCl  MgCl2 + H2

PRACTICE PROBLEM 1B

Write word, formula, and balanced chemical
equations for silicon dioxide and hydrofluoric
acid reacting to produce silicon tetrafluoride
and water.
Silicon dioxide+ hydrofluoric acid  silicon
tetrafluoride + water
 SiO2+ HF  SiF4 + H2O
 SiO2+ 4HF  SiF4 + 2H2O

PRACTICE PROBLEM 2

Write word, formula and balanced equations for
aqueous nitric acid reacts with solid magnesium
hydroxide to produce aqueous magnesium nitrate
and water.
Nitric acid + magnesium hydroxide  magnesium
nitrate + water
 HNO3(aq) + Mg(OH)2(s)  Mg(NO3)2(aq) + H2O(l)
 2HNO3(aq) + Mg(OH)2(s)  Mg(NO3)2(aq) + 2H2O(l)

PRACTICE PROBLEM 3


Ammonium sulfate crystals are made by
treating ammonia gas, often a by-product
from coke-ovens, with aqueous sulfuric acid:
2 NH3(g) + H2SO4(aq) → (NH4)2SO4(s)
PRACTICE PROBLEM 4

Aluminum sulfate and calcium hydroxide are
used in a water-purification process. When
added to water, they dissolve and react to
produce 2 insoluble products, aluminum
hydroxide and calcium sulfate. Write a
balanced equation for this reaction.
Al2(SO4)3(aq) + 3Ca(OH)2(aq)  2Al(OH)3(s) + 3CaSO4(s)
PRACTICE PROBLEM 5

Write balanced chemical equations for the
following reaction: Solid sodium combines with
chlorine gas to produce solid sodium chloride.

2Na(s) + Cl2(g) → 2NaCl(s)
PRACTICE PROBLEM 6

When solid copper reacts with aqueous silver
nitrate, the products are aqueous copper(II)
nitrate and solid silver.
Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)
PRACTICE PROBLEM 7

In a blast furnace, the reaction between solid
iron(III) oxide and carbon monoxide gas
produces solid iron and carbon dioxide gas.

Fe2O3(s) + 3CO(g) → 2Fe(s) + 3CO2(g)
Types of Chemical Reactions
SECTION 2
1. DECOMPOSITION REACTIONS

Decomposition reaction  a single compound
has a reaction that makes two or more simpler
substances

General equation

Most happen when energy (light/heat) is added
AX  A + X
2. SYNTHESIS REACTIONS

Synthesis (composition) reaction  two or
more substances combine to form a new
compound

General equation
A + X  AX
3. SINGLE-REPLACEMENT REACTIONS

Single-replacement reaction  one
element replaces a similar element in a
compound

General equation
A + BX  AX + B
Y + BX  BY + X
4. DOUBLE REPLACEMENT REACTIONS

Double-replacement reaction  ions of two
compounds exchange places in an aqueous
solution to form two new compounds

General equation
AX + BY  AY + BX
5. COMBUSTION REACTIONS

Combustion reaction  substance
combines with oxygen, releasing large
amount of energy as heat and light

2H2(g) + O2(g) → 2H2O(g)
REACTIONS OF ELEMENTS WITH OXYGEN AND
SULFUR

One simple type of synthesis reaction is
combination of element with oxygen to form an
oxide of the element

Almost all metals react with oxygen to form
oxides

Ex. Magnesium burned  magnesium oxide
2Mg(s) + O2(g)  2MgO(s)
GROUP 2 ELEMENTS
Group 2 elements react to form oxides with
general formula MO
 M represents metal


Group 1 metals form oxides with general
formula M2O

Li + O2  Li2O
REACTIONS WITH SULFUR

Groups 1 and 2 react with sulfur to make
sulfides of the element
Group 1  M2S
 Group 2  MS

16Rb(s) + S8(s) → 8Rb2S(s)
 8Ba(s) + S8(s) → 8BaS(s)


Some metals (usually transition metals)
combine with O2 to make two different oxides

Ex. Fe – can be +2 or +3
2Fe(s) + O2(g) → 2FeO(s)
 4Fe(s) + 3O2(g) → 2Fe2O3(s)

NONMETALS
Nonmetals also react with oxygen to make
oxides
 Sulfur reacts with oxygen to make sulfur dioxide
 When carbon is burned, it makes carbon
dioxide
 S8(s) + 8O2(g) → 8SO2(g)
 C(s) + O2(g) → CO2(g)


Hydrogen reacts with oxygen to make
dihydrogen monoxide

2H2(g) + O2(g) → 2H2O(g)

Most metals react with halogens to make
either ionic or covalent compounds
Ex. Group 1 reacts with halogens to form
ionic compounds with formula MX
 M = metal, X = halogen

2Na(s) + Cl2(g) → 2NaCl(s)
 2K(s) + I2(g) → 2KI(s)

REACTIONS OF METALS & HALOGENS
GROUP 2 METALS & HALOGENS

Formula MX2
Mg(s) + F2(g) → MgF2(s)
 Sr(s) + Br2(l) → SrBr2(s)

Active metals  highly reactive metals
 Oxides of active metals react with water to
make metal hydroxides


CaO(s) + H2O(l) → Ca(OH)2(s)

CaO = lime

Ca(OH)2 is important in setting cement
SYNTHESIS REACTIONS WITH OXIDES
OXYACIDS

Many oxides of nonmetals react with water to
make oxyacids
SO2(g) + H2O(l) → H2SO3(aq)
 In air polluted with SO2, reacts with oxygen in
air to form sulfuric acid (acid rain)
 2H2SO3(aq) + O2(g) → 2H2SO4(aq)

PRACTICE PROBLEMS
Identify the products in each of the following
reactions:
 a. hydrogen burned in oxygen
 H2O
 b. H2(g) + N2(g) 
 NH3
 c. CaO(s) + H2O(l) 
 Ca(OH)2(aq)

DECOMPOSITION OF BINARY COMPOUNDS

Simplest kind of decomposition reaction is
binary compound into its elements

Ex. Passing electricity through water

2H2O(l)
electricity
2H2(g) + O2(g)

Electrolysis  decomposition of a substance by
electricity

Oxides of less-active metals (lower center of PT)
decompose into elements when heated
Δ
 2HgO(s)  2Hg(l) + O2(g)
DECOMPOSITION OF METAL CARBONATES

When heated, metal carbonates break down to
make a metal oxide and CO2
Δ
 CaCO3(s)  CaO(s) + CO2(g)
DECOMPOSITION OF METAL HYDROXIDES

All except with Group 1 metals decompose
when heated to make metal oxides and water
Δ
 Ca(OH)2(s)  CaO(s) + H2O(g)
DECOMPOSITION OF METAL CHLORATES

When heated, metal chlorates decompose to
make metal chloride and oxygen
Δ

2KClO3(s)
MnO2(s)
2KCl(s) + 3O2(g)
DECOMPOSITION OF ACIDS

Certain acids decompose into nonmetal oxides
and water

H2CO3(aq) → CO2(g) + H2O(l)
Δ
 H2SO4(aq)  SO3(g) + H2O(l)
PRACTICE PROBLEMS
Predict the products for these decomposition
reactions
 a. sodium chlorate 
Sodium chloride + oxygen
 b. calcium carbonate 
Calcium oxide + carbon dioxide
 c. potassium bromide 
Potassium + bromine

REPLACEMENT OF A METAL IN A COMPOUND BY
ANOTHER METAL
Aluminum is more active than lead
 When solid aluminum is placed in aqueous
lead(II) nitrate, the aluminum replaces the lead

2Al(s) + 3Pb(NO3)2(aq) → 3Pb(s) + 2Al(NO3)3(aq)

Based on activity series of metals
REPLACEMENT OF HYDROGEN IN WATER BY
METAL

Most-active metals (Group 1) react strongly
with water to make metal hydroxides and
hydrogen

2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)

Less-active metals react with steam or other
form of energy
REPLACEMENT OF HYDROGEN IN ACID BY METAL
More-active metals react with certain acidic
solutions and replace hydrogen
 Reaction products are metal compound (salt)
and hydrogen gas


Mg(s) + 2HCl(aq) → H2(g) + MgCl2(aq)
REPLACEMENT OF HALOGENS

One halogen replaces another
Fluorine is most reactive
 Can replace any other halogen

Cl2(g) + 2KBr(aq) → 2KCl(aq) + Br2(l)
 F2(g) + 2NaCl(aq) → 2NaF(aq) + Cl2(g)
 Br2(l) + KCl(aq) → no reaction

PRACTICE PROBLEMS
For the following equations, predict what the
products will be:
 a. Ag + KNO3 →
No reaction
 b. Zn + AgNO3 →
Zn(NO3)2 + Ag
 c. Cl2 + KI →
I2 + 2KCl

d. Cu + FeSO4 →
No reaction
 e. Fe + Pb(NO3)2 →
Pb + Fe(NO3)2
 f. Cu + Al2(SO4)3 →
No reaction
 g. Al + Pb(NO3)2 →
Pb + Al(NO3)3
 h. Cl2 + NaI →
I2 + NaCl

i. Fe + AgC2H3O2 →
Fe(C2H3O2)2 + Ag
 j. Al + CuCl2 →
Cu + Al2Cl3
 k. Br2 + CaI2 →
I2 + CaBr2
 l. Fe + CuSO4 →
FeSO4 + Cu
 m. Cl2 + MgI2 →
I2 + MgCl2

FORMATION OF A PRECIPITATE

Occurs when cations of one reactant combine
with anions of another to form insoluble (or
slightly soluble) compound

2KI(aq) + Pb(NO3)2(aq) → PbI2(s) + 2KNO3(aq)
FORMATION OF A GAS

In some D-R reactions, one product is insoluble
gas that bubbles out of mixture

FeS(s) + 2HCl(aq) → H2S(g) + FeCl2(aq)
FORMATION OF WATER

In some D-R reactions, water is one of the
products

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
PRACTICE PROBLEMS
Classify each of the following reactions as
synthesis, decomposition, single-replacement,
double-replacement, or combustion:
 a. N2(g) + 3H2(g) → 2NH3(g)
 synthesis
 b. 2Li(s) + 2H2O(l ) → 2LiOH(aq) + H2(g)
 single-replacement
 c. 2NaNO3(s) → 2NaNO2(s) + O2(g)
 decomposition

d. 2C6H14(l ) + 19O2(g) →12CO2(g) + 14H2O(l )
 combustion
 e. NH4Cl(s) → NH3(g) + HCl(g)
 decomposition
 f. BaO(s) + H2O(l ) → Ba(OH)2(aq)
 synthesis
 g. AgNO3(aq) + NaCl(aq) →AgCl(s) + NaNO3(aq)
 double-replacement

PRACTICE PROBLEM
For each of the following reactions, identify the
missing substances, then balance the final
equation. Each slot may be one OR MORE
substances.
 a. synthesis: _____ → Li2O
 4Li + O2  2Li2O
 b. decomposition: Mg(ClO3)2 → _____
 Mg(ClO3)2  MgCl2 + 3O2

c. single-replacement: Na + H2O → _____
 2Na + 2H2O  2NaOH + H2
 d. double-replacement: HNO3 + Ca(OH)2 → _____
 2HNO3 + Ca(OH)2  Ca(NO3)2 + 2H2O
 e. combustion: C5H12 + O2 → _____
 C5H12 + 8O2  5CO2 + 6H2O

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