Chemical Reactions

Chapter 8

Chemical Reactions

• Chemical reactions represent the formation of a new substance (remember chemical changes).

• We know a chemical change has occurred if:• Give some examples---• Some can be found on pages 261-262.

Chemical Equations

• A chemical equation is a representation of a chemical reaction.

• The starting material(s) are the reactants and they react to form products.

• Reactants Products• We can use words to describe the reaction or

use the formulas of the substances involved.• YOU MUST KNOW THE CORRECT FORMULAS TO

WRITE THE CORRECT EQUATION

Chemical Equations

• For example: methane combines with oxygen to produce carbon dioxide and water.

• Methane + oxygen carbon dioxide + water• CH4 + O2 CO2 + H2O

• This is sometimes called a skeleton equation• We have written the correct formulas for the

substances involved. Note oxygen gas is diatomic-what are the other diatomic molecules?

Chemical Equations

• If we know the physical state of the reactants and products we can add them to our equation.

• s-l-g-aq are commonly used symbols.• Since methane is a gas we can rewrite our

skeleton equation as:• CH4(g) + O2(g) CO2(g) + H2O(l)

Chemical Equations

• Equations must conform to the Law of Conservation of Mass which we already know (?)-In writing equations the atoms of the reactants must be balanced by the atoms of the products.

• Lets look at our equation again • CH4 + O2 CO2 + H2O

• On the reactant side there are: 4-H,1C and 2O

CH4 + O2 CO2 + H2O

• On the product side 2H,1C and 3O. Things are not in balance! We know the formulas are correct so we can’t change them but we can use coefficients before the formula .

• Consider the H-4 on the left and 2 on the right. So

• CH4 + O2 CO2 + 2H2O

• Now we need to balance the O’s-4 on the right and 2 on the left, so

• CH4 + 2O2 CO2 + 2H2O

• Now things are balanced, so our final equation would be:

• CH4(g) + 2O2(g) CO2(g) + 2H2O(l)

Chemical equations

• Some other examples:• Iron(III) oxide reacts with hydrogen gas to

produce metallic iron and water.

Another example

• Ammonia reacts with oxygen gas to produce nitrogen monoxide gas and water vapor.

Types of Reactions

• 1. Combustion reactions occur when substances react with oxygen and usually produce a flame in the process. We will restrict our discussion for reactions of oxygen with hydrocarbons (compounds containing carbon and hydrogen only). The previous reaction of methane fits this description. Combustion of hydrocarbons always produce carbon dioxide and water.

An example:

• Combustion of butane C4H10

• Hint: when you have an even number of atoms on one side of the equation and an odd number on the other-multiply everything by 2

Reaction Types

• Combination or synthesis reactions:• You do not have to predict products of these

reactions, but you must be able to identify them and be able to balance them!

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Combination or Synthesis Reactions• Combination reactions occur when two or more

substances combine to form a compound.• There are three basic types of combination

reactions.1. Two elements react to form a new compound2. An element and a compound react to form one new

compound3. Two compounds react to form one compound

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Combination Reactions

1. Element + Element CompoundA. Metal + Nonmetal Binary Ionic Compound

sg2s NaCl ClNa

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Combination Reactions1. Element + Element Compound

A. Metal + Nonmetal Binary Ionic Compound

s32s AlBr Br Al

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Combination Reactions1. Element + Element Compound

B. Nonmetal + Nonmetal Covalent Binary Compound

3g2s4 PCl Cl P

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Combination Reactions

2. Compound + Element Compound

s5g2s3 AsClClAsCl

g6g2s4 SFFSF

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Combination Reactions

3. Compound + Compound Compound– gaseous ammonia and hydrogen chloride

– lithium oxide and sulfur dioxide

s4gg3 ClNH HClNH

3222 SO LiSOOLi

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Decomposition Reactions

• Decomposition reactions occur when one compound decomposes to form:

1. Two elements2. One or more elements and one or more

compounds3. Two or more compounds

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Decomposition Reactions

1. Compound Element + Element– decomposition of dinitrogen oxide

• decomposition of calcium chloride

g2g2g2 ON 2ON 2

g2yelectricit

2 ClCaCaCl

2sh

s BAg 2AgBr 2 r

decomposition of silver halides

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Decomposition Reactions

2. Compound One Element + Compound(s)

– decomposition of hydrogen peroxide

g22or Mn or Feνh

aq22 OO H2O H23

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Decomposition Reactions

3. Compound Compound + Compound– decomposition of ammonium hydrogen

carbonate

g2g2g3s34 COO HNHHCONH

Disassociation Reactions

• These are not true chemical reactions (no new substance is produced) , but they are important to understand.

• When an ionic compound dissolves in water it does so by breaking up or dissociating into its component ions. So….

• NaCl(aq) Na+(aq) + Cl-(aq)

• Some other examples:

• CaBr2

• Mg3(PO4)2

• Al2(SO4)3

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Displacement Reactions• Displacement reactions occur when one

element displaces another element from a compound.– These are reactions in which the more active

metal displaces the less active metal or hydrogen from a compound in aqueous solution.

– Activity series is given on page 281. The most active metals are at the top and will displace those lower on the list.

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Displacement Reactions1. [More Active Metal + Salt of Less Active Metal] [Less Active

Metal + Salt of More Active Metal]– Molecular or formula unit equation

(s)aq3(s) aq3 Ag CuNO Cu +AgNO

• Note that the silver nitrate and copper(I) nitrate are in solution, so we can write an ionic equation by showing these compounds in their dissociated form:

• Did any compound or ion remain the same during this reaction?

• An ion that does not undergo a change is called a spectator ion.

• These ions can be eliminated from the equation producing a net ionic equation.

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Displacement Reactions2. [Active Metal + Nonoxidizing Acid] [Hydrogen +

Salt of Acid]– Common method for preparing hydrogen in the laboratory.

• Molecular or formula unit equation

g2aq342aq42(s) H + )(SOAl SOH + Al

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Displacement Reactions

• Total ionic equationYou do it!

• Net ionic equationYou do it!

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Displacement Reactions• The following metals are active enough to

displace hydrogen– K, Ca, Na, Mg, Al, Zn, Fe, Sn, & Pb

Non-metals can displace less active non-metals, we will just consider the halogens (?). The activity series for halogens is simple-its their order on the periodic table. Fluorine is the most active, iodine the least.

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Displacement Reactions3. [Active Nonmetal + Salt of Less Active Nonmetal] [Less Active

Nonmetal + Salt of More Active Nonmetal]

• Molecular equation

(aq)s2aqg2 NaCl 2 I NaI 2 + Cl Total ionic equation

-aqaqs2

-aqaqg2 Cl 2 +Na 2 I I 2 + Na 2 +Cl

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Displacement Reactions• Net ionic equation

-aqs2

-aqg2 Cl 2 I I 2 +Cl

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Double-Displacement or Metathesis Reactions• Metathesis reactions occur when two ionic

aqueous solutions are mixed and the ions switch partners.

AX + BY AY + BX• Metathesis reactions remove ions from solution in

two ways:1. form predominantly unionized molecules like H2O2. form an insoluble solid

• Ion removal is the driving force of metathesis reactions.

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Metathesis Reactions

1. Acid-Base (neutralization) Reactions– Formation of the nonelectrolyte H2O – acid + base salt + water– a nonelectrolyte is a compound that does not

produce ions-With the exception of organic acids and some other organic compounds , all other molecular compounds are nonelectrolytes.

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Metathesis Reactions• Molecular equation

)(2 (aq)(aq)(aq) OH + KBr KOH + HBr Total ionic equation

Net ionic equation

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Metathesis Reactions2. Precipitation reactions are metathesis

reactions in which an insoluble compound is formed.

– The solid precipitates out of the solution much like rain or snow precipitates out of the air.

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Metathesis Reactions• Precipitation Reactions • Molecular or formula unit equation

(s)3)aq(3aq)(32(aq)23 CaCO +KNO 2 COK + )Ca(NO Total ionic reaction

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Metathesis Reactions• Net ionic reaction

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Metathesis Reactions• Molecular or formula unit equation

2(s)43)aq(aq)(43(aq)2 POCa +NaCl PONa + CaCl

Total ionic reaction

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Metathesis Reactions• Net ionic reaction