Transcript Chemical Equations and Reactions

Chemical Equations and
Reactions
Chapter 8
Describing Chemical
Reactions
Section 1
Chemical Reactions
• A chemical reaction is the process by which one
or more substances are changed into one or
more different substances.
• In any chemical reaction, the original substances
are known as the reactants and the resulting
substances are known as the products.
• According to the law of conservation of mass,
the total mass of reactants must equal the total
mass of products for any given chemical
reaction.
Chemical Reactions
• A chemical equation represents, with
symbols and formulas, the identities and
relative molecular or molar amounts of
the reactants and products in a chemical
reaction.
– example: The following chemical equation
shows that the reactant ammonium
dichromate yields the products nitrogen,
chromium(III) oxide, and water.
(NH4)2Cr2O7(s)
N2(g) + Cr2O3(s) + 4H2O(g)
Visual Concept
• Reaction
Indications of a Chemical
Reaction
•
Certain easily observed changes usually indicate
that a chemical reaction has occurred.
•
Evolution of energy as heat and light
- Magnesium example
2. Production of a gas
- Bubbles – Baking soda and vinegar – CO2
3. Formation of a precipitate.
•
A solid that is produced as a result of a
chemical reaction in solution and that
separates from the solution is known as a
precipitate.
4. Color change
Visual Concept
• Signs of a Chemical Reaction
Characteristics of
Chemical Equations
•
The following requirements will aid you
in writing and reading chemical
equations correctly.
1. The equation must represent known facts.
2. The equation must contain the correct
formulas for the reactants and products.
3. The law of conservation of mass must be
satisfied.
•
A coefficient is a small whole number
that appears in front of a formula in a
chemical equation.
Elements That Normally Exist
as Diatomic Molecules
Word and Formula
Equations
• The first step in writing a chemical
equation is to identify the facts to be
represented.
• A word equation is an equation in which
the reactants and products in a chemical
reaction are represented by words.
– A word equation is qualitative
• example: methane + oxygen
carbon dioxide + water
Word and Formula
Equations
• The next step in writing a correct
chemical equation is to replace the
names of the reactants and products with
appropriate symbols and formulas.
• A formula equation represents the
reactants and products of a chemical
reaction by their symbols or formulas.
• example: The formula equation for the reaction of
methane and oxygen is:
CH4(g) + O2(g)
CO2(g) + H2O(g)
(not balanced)
Word and Formula
Equations
• To complete the process of writing a correct equation,
the law of conservation of mass must be taken into
account.
• The relative amounts of reactants and products
represented in the equation must be adjusted so
that the numbers and types of atoms are the same
on both sides of the equation.
• This process is called balancing an equation
and is carried out by inserting coefficients.
Word and Formula
Equations
• To balance the equation, begin by counting atoms of
elements that are combined with atoms of other
elements and that appear only once on each side of the
equation.
CH4(g) + O2(g)
CO2(g) + 2H2O(g) (not balanced)
• Begin by counting carbon atoms.
• Carbon is already balanced in the equation.
• Two additional hydrogen atoms are needed on the right
side of the equation.
Word and Formula
Equations
CH4(g) + O2(g)
CO2(g) + 2H2O(g) (partially balanced)
• Now consider the number of oxygen atoms.
• Increase the number of oxygen atoms on the left side to four
by placing the coefficient 2 in front of the molecular formula
for oxygen.
• The correct chemical equation, or balanced formula
equation, for the burning of methane in oxygen is
CH4(g) + 2O2(g)
CO2(g) + 2H2O(g)
Symbols Used in Chemical
Equations
Symbols Used in Chemical
Equations
Symbols Used in Chemical
Equations
Methane Combustion
Example
• Write word and formula equations for the
chemical reaction that occurs when solid
sodium oxide is added to water at room
temperature and forms sodium hydroxide
(dissolved in the water). Include symbols
for physical states in the formula
equation. Then balance the formula
equation to give a balanced chemical
equation.
Solution
sodium oxide + water
Na2O + H2O
Na2O(s) + H2O(l)
sodium hydroxide
NaOH (not balanced)
2NaOH(aq)
Example II
Translate the following chemical equation into a
sentence:
BaCl2(aq) + Na2CrO4(aq)
BaCrO4(s) + 2NaCl(aq)
Aqueous solutions of barium chloride and
sodium chromate react to produce a precipitate
of barium chromate plus sodium chloride in
aqueous solution.
Your Turn I
• Translate these Equations into sentences:
– CS2 (l) + 3O2 (g)
– NaCl (aq) + AgNO3
CO2 (g) + 2SO2 (g)
NaNO3 (aq) + AgCl (s)
Significance of a Chemical
Equation
• Some of the quantitative information revealed by a
chemical equation includes
1. The coefficients of a chemical reaction indicate
relative, not absolute, amounts of reactants and
products.
H2(g) + Cl2(g)
2HCl(g)
1 molecule H2 : 1 molecule Cl2 : 2 molecules HCl
• This ratio shows the smallest possible relative
amounts of the reaction’s reactants and products.
Significance of a Chemical
Equation
2. The relative masses of the reactants and
products of a chemical reaction can be
determined from the reaction’s coefficients.
•
An amount of an element or compound in moles
can be converted to a mass in grams by
multiplying by the appropriate molar mass.
• example:
2.02 g H2
1 mol H2 
 2.02 g H2
mol H2
Interpreting a Chemical
Reaction
Significance of a Chemical
Equation
3. The reverse reaction for a chemical equation has
the same relative amounts of substances as the
forward reaction.
•
An equation gives no indication of whether a reaction
will actually occur.
•
Chemical equations give no information about the
speed at which reactions occur.
•
Equations do not give any information about how
the bonding between atoms or ions changes
during the reaction.
Visual Concept
• Equation
Balancing Chemical
Equations
•
The following procedure demonstrates how to master
balancing equations by inspection using a step-bystep approach.
1. Identify the names of the reactants and the
products, and write a word equation.
water
hydrogen + oxygen
Balancing Chemical
Equations
2. Write a formula equation by
substituting correct formulas for the
names of the reactants and the
products.
H2O(l)
H2(g) + O2(g) (not balanced)
Balancing Chemical
Equations
3. Balance the formula equation according to
the law of conservation of mass.
• 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 of the equation as single units.
• Balance H atoms and O atoms after atoms of
all other elements have been balanced.
Balancing Chemical
Equations
3. Balance the formula equation according to
the law of conservation of mass.
• Balance oxygen atoms by increasing the
number of H2O molecules.
2H2O(l)
H2(g) + O2(g) (partially balanced)
• Balance the hydrogen atoms by placing the
coefficient 2 in front of hydrogen, H2.
2H2O(l)
2H2(g) + O2(g) (balanced)
Balancing Chemical
Equations
4. Count atoms to be sure that the equation is
balanced.
2H2O(l)
(4H + 2O)
•
2H2(g) + O2(g)
=
(4H) + (2O)
If the coefficients do not represent the smallest
possible whole-number ratio of reactants and
products, divide the coefficients by their greatest
common factor in order to obtain the smallest
possible whole-number coefficients.
Never
• Change a subscript to balance an
equation.
• If you change the formula you are
describing a different reaction.
• H2O is a different compound than H2O2
• Never put a coefficient in the middle of a
formula
• 2 NaCl is okay, Na2Cl is not.
Visual Concept
• Balancing Equations
Example
• The reaction of zinc with aqueous
hydrochloric acid produces a
solution of zinc chloride and
hydrogen gas. Write a balanced
chemical equation for the reaction.
Solution
• Write the word equation.
zinc + hydrochloric acid
zinc chloride + hydrogen
• Write the formula equation.
Zn(s) + HCl(aq)
ZnCl2(aq) + H2(g) (not balanced)
• Adjust the coefficients.
• Balance chlorine first because it is combined on
both sides of the equation.
Zn(s) + 2HCl(aq)
ZnCl2(aq) + H2(g)
• Count atoms to check balance.
Zn(s) + 2HCl(aq)
ZnCl2(aq) + H2(g)
(1Zn) + (2H + 2Cl) = (1Zn + 2Cl) + (2H)
Your Turn II
a) Pb(NO3)2 + K2CrO4  PbCrO4 + KNO3
b) MnO2 + HCl  MnCl2 + H2O+ Cl2
c) C3H6 + O2  CO2 +H2O
d) Zn(OH)2 + H3PO4  Zn3(PO4)2 + H2O
e) CO + Fe2O3  Fe + CO2
f) CS2 + Cl2  CCl4 +S2Cl2
g) CH4 + Br2  CH3Br + HBr
h) Ba(CN)2 + H2SO4  BaSO4 + HCN
Answers
a) Pb(NO3)2 + K2CrO4  PbCrO4 + 2KNO3
b) MnO2 + 4HCl  MnCl2 + 2H2O+ Cl2
c) 2C3H6 + 9O2 6CO2 +6H2O
d) 3Zn(OH)2 + 2H3PO4  Zn3(PO4)2 + 6H2O
e) 3CO + Fe2O3 2Fe + 3CO2
f) CS2 + 3Cl2 CCl4 +S2Cl2
g) CH4 + Br2  CH3Br + HBr
h) Ba(CN)2 + H2SO4  BaSO4 + 2HCN
• If an atom appears more than once
on a side, balance it last.
• If you fix everything except one
element, and it is even on one side
and odd on the other, double the first
number, then move on from there.
• C4H10 + O2  CO2 + H2O
Types of Chemical RxN’s
Section 2
Types of RxN’s
•
•
•
•
•
•
There are millions of reactions.
Can’t remember them all
Fall into several categories.
We will learn 5 types.
Will be able to predict the products.
For some we will be able to predict
whether they will happen at all.
• Will recognize them by the reactants
Types of RxN’s
• The classification scheme described in
this section provides an introduction to
five basic types of reactions:
• Synthesis (Combination)
• Decomposition
• Single-displacement
• Double-displacement
• Combustion reactions
Synthesis Reactions
• In a synthesis reaction, also known as a
composition reaction or combination reaction,
two or more substances combine to form a
new compound.
• This type of reaction is represented by the
following general equation.
A+X
AX
• A and X can be elements or compounds.
• AX is a compound
Synthesis Reactions
• 2 elements, or compounds combine
to make one compound.
• Ca +O2 CaO
• SO3 + H2O  H2SO4
• We can predict the products if they
are two elements.
• Mg + N2 
Reactions of Elements with
Oxygen
• One simple type of synthesis reaction is the
combination of an element with oxygen to produce an
oxide of the element.
• Almost all metals react with oxygen to form oxides.
• example: 2Mg(s) + O2(g)
2MgO(s)
• Group 2 elements react in a similar manner, forming
oxides with the formula MO, where M represents
the metal.
Your Turn III
Ca + Cl2 
Fe + O2  iron (II) oxide
Al + O2 
Remember that the first step is to
write the formula
• Then balance
•
•
•
•
Decomposition Reactions
• In a decomposition reaction, a single compound
undergoes a reaction that produces two or more
simpler substances. s or compounds.
• Decomposition reactions are the opposite of
synthesis reactions.
• They are represented by the following general
equation.
AX
A+X
• AX is a compound.
• A and X can be elements or compounds.
Decomposition of Binary
Compounds
• The decomposition of a substance by an electric
current is called electrolysis.
• example: 2H O(l) electricity

 2H2 (g) + O2 (g)
2
• Oxides of the less-active metals, which are located in
the lower center of the periodic table, decompose into
their elements when heated.
• example: 2HgO(s) 
 2Hg(l) + O2 (g)
Decomposition Reactions
Decomposition of Metal Carbonates

CaCO3 (s) 
 CaO(s) + CO2 (g)
Decomposition of Metal Hydroxides

Ca(OH)2 (s) 
 CaO(s) + H2O(g)
Decomposition of Metal Chlorates

2KClO3 (s) MnO

 2KCl(s) + 3O2 (g)
(s)
2
Your Turn IV

• NiCO3  
• H2CO3(aq)
Single-Replacement
Reactions
• In a single-replacement reaction, also known as a
displacement reaction, one element replaces a similar
element in a compound.
• Many single-replacement reactions take place in
aqueous solution.
• Single-replacement reactions can be represented by
the following general equations.
A + BX
AX + B or Y + BX
BY + X
• A, B, X, and Y are elements. AX, BX, and BY are compounds.
Single-Replacement
Reactions
• One element replaces another
• Reactants must be an element and a
compound.
• Products will be a different element
and a different compound.
• Na + KCl  K + NaCl
• F2 + LiCl  LiF + Cl2
Single-Replacement
Reactions
•
•
•
•
•
Metals replace metals (and hydrogen)
K + AlN 
Zn + HCl 
Think of water as HOH
Metals replace one of the H, combine
with hydroxide.
• Na + HOH 
Activity Series
• We can tell whether a reaction will
happen
• Some are more active than other
• More active replaces less active
• There is a list on page 286
• Higher on the list replaces lower.
• If the element by itself is higher, it
happens, if lower it doesn’t
Activity Series
• H can be replaced in acids by
everything higher
• Only the first 6 (Li - Na) react with
water.
• Fe + CuSO4 
• Pb + KCl 
• Al + HCl 
Activity Series
• What does it mean that Au And Ag are on
the bottom of the list?
• Nonmetals can replace other nonmetals
• Limited to F2 , Cl2 , Br2 , I2
• The order of activity is that on the table.
• Higher replaces lower.
• F2 + HCl 
• Br2 + KCl 
Double-Replacement
Reactions
• In double-replacement reactions, the ions of two
compounds exchange places in an aqueous solution to
form two new compounds.
• One of the compounds formed is usually a precipitate,
an insoluble gas that bubbles out of the solution, or a
molecular compound, usually water.
• The other compound is often soluble and remains
dissolved in solution.
Double-Replacement
Reactions
• A double-displacement reaction is represented by the
following general equation.
AX + BY
AY + BX
• A, X, B, and Y in the reactants represent ions.
• AY and BX represent ionic or molecular
compounds.
Double-Replacement
Reactions
• The formation of a precipitate occurs when the
cations of one reactant combine with the anions of
another reactant to form an insoluble or slightly
soluble compound.
• example:
2KI(aq) + Pb(NO3)2(aq)
PbI2(s) + 2KNO3(aq)
• The precipitate forms as a result of the very strong attractive
forces between the Pb2+ cations and the I anions.
Double-Replacement
Reactions
• Two things replace each other.
• Reactants must be two ionic compounds
or acids.
• Usually in aqueous solution
• NaOH + FeCl3 
• The positive ions change place.
• NaOH + FeCl3 Fe+3 OH- + Na+1Cl-1
• NaOH + FeCl3 Fe(OH)3 + NaCl
Double-Replacement
Reactions
• Will only happen if one of the
products
– doesn’t dissolve in water and forms a
solid
– or is a gas that bubbles out.
– or is a covalent compound usually
water.
Your Turn V
• assume all of the reactions take
place.
• CaCl2 + NaOH 
• CuCl2 + K2S 
• KOH + Fe(NO3)3 
• (NH4)2SO4 + BaF2 
Combustion Reactions
• In a combustion reaction, a substance combines with
oxygen, releasing a large amount of energy in the form
of light and heat.
• example: combustion of hydrogen
2H2(g) + O2(g)
2H2O(g)
• example: combustion of propane
C3H8(g) + 5O2(g)
3CO2(g) + 4H2O(g)
Combustion Reactions
• Combustion
• A compound composed of only C H
and maybe O is reacted with oxygen
• If the combustion is complete, the
products will be CO2 and H2O.
• If the combustion is incomplete, the
products will be CO and H2O.
Your Turn VI
• C4H10 + O2  (complete)
• C4H10 + O2  (incomplete)
• C6H12O6 + O2  (complete)
• C8H8 +O2  (incomplete)
Your Turn VI Part II
Distinguish between complete an incomplete
combustion.
Write a balanced equation for the complete
combustion of each of these compounds.
a) acetic acid, HC2H3O2 c) glycerol, C3H8O3
b) decane, C10H22
d) sucrose,
C12H22O11
Write a balanced equation for the incomplete
combustion of each of these compounds.
a) glycerol, C3H8O3
c) acetic acid,
HC2H3O2
b) glucose, C6H12O6
d) acetylene, C2H2
How to recognize which
type
•
•
•
•
•
Look at the reactants
E + E Combination
C
Decomposition
E + C Single replacement
C + C Double replacement
Your Turn VII
• Determine the type of reaction and the
products and balance.
• H2 + O2 
• H2O 
• Zn + H2SO4 
• HgO 
• KBr +Cl2 
• AgNO3 + NaCl 
• Mg(OH)2 + H2SO3 
Let’s Review
• Write out the balanced equation for
Copper reacts with chlorine to form
copper (II) chloride.
• Write out the balanced equation for Zinc
reacts with Hydrochloric acid to form Zinc
chloride and hydrogen.
• Write out the balanced equation for
Calcium carbonate reacts with Zinc nitride
to form Calcium nitride and Zinc
carbonate.
Let’s Review
• Balance the following equations
– C7H16 + O2 ---> CO2 + H2O
– Na2O2 + H2O ---> NaOH + O2
– Al2(SO4)3+Ca(OH)2----> Al(OH)3+CaSO4
– NH3 +HCl ----> NH4Cl
– Ca3(PO4)2 +SiO2 + C ----> CaSiO3 + CO + P
Let’s Review
• What type of reaction are the
following
–
–
–
–
–
–
Rb + S8 ---> Rb2S
NH3 + O2 ---> N2 + H2O
C + SO2 ---> CS2 + CO
C3H11 + O2 ---> CO2 + H2O
C5H12 +O2 ---> CO + H2O
FeCl3 + NH4OH --->Fe(OH)3 + NH4Cl
Let’s Review
• Balance them
–
–
–
–
–
–
Rb + S8 ---> Rb2S
NH3 + O2 ---> N2 + H2O
C + SO2 ---> CS2 + CO
C3H11 + O2 ---> CO2 + H2O
C5H12 +O2 ---> CO + H2O
FeCl3 + NH4OH --->Fe(OH)3 + NH4Cl
Let’s Review
• Predict the products and balance
– Sodium phosphate reacts with Lithium
metal.
– Butane (C4H10) is completely combusted.
– Sodium Chloride reacts with Silver
Hydroxide
– Aluminum Bromate reacts with Copper
– Potassium reacts with Sulfur
– Octane (C8H18) is incompletely combusted
Let’s Review
• Predict the products and balance
–
–
–
–
–
–
–
H2 + O2 
Al2Te3 
Zn + H2SO4 
HgO 
KBr +Cl2 
AgNO3 + NaCl 
Mg(OH)2 + H2SO3 
Let’s Review
• Using the table on Pg 286. Predict if
these reactions will occur
–
–
–
–
–
–
–
–
Li + BaS 
Fe + KCl 
Au + HgS 
Mn + CuS 
Ba + LiS 
Na + KCl 
HCl + Pb 
PbCl2 + H2 