chap 06a-chemical reactions 51 le… · formaldehyde formic acid oxidation (gain of oxygen) formic...

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Chapter 6A

Chemical

Reactions

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CHAPTER OUTLINE��

§  Chemical Reactions §  Chemical Equation §  Balancing Equations §  Types of Chemical Reactions §  Double Replacement Reactions §  Oxidation-Reduction Reactions §  Redox in Biological Systems §  Activity Series of Metals

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CHEMICAL��REACTIONS��

q  A chemical reaction is a rearrangement of atoms in which some of the original bonds are broken and new bonds are formed to give different chemical structures.

q  In a chemical reaction, atoms are neither created, nor destroyed.

q  A chemical reaction, as described above, is supported by Dalton’s postulates.

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6 oxygen atoms 6 oxygen atoms =In a chemical reaction, atoms are neither created, nor destroyed

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q  A chemical reaction can be detected by one of the following evidences:

1. Change of color (formation of a solid)

2. Formation of a gas

3. Exchange of heat with surroundings

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CHEMICAL��EQUATIONS��

q  A chemical equation is a shorthand expression for a chemical reaction.

Word equation:

Aluminum combines with ferric oxide to form iron and aluminum oxide.

Chemical equation:

Al + Fe2O3 → Fe + Al2O3

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q  Reactants are separated from products by an arrow.


q  Coefficients are placed in front of substances to balance the equation.

2 Al + Fe2O3 → 2 Fe + Al2O3

Subscripts

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q  Reaction conditions are placed over the arrow.


q  The physical state of the substances are indicated by the symbols (s), (l), (g), (aq).

2 Al (s) + Fe2O3 (s) → 2 Fe (l) + Al2O3 (s)

Δ

heat

solid liquid

Δ

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BALANCING��EQUATIONS��

q  A balanced equation contains the same number of atoms on each side of the equation, and therefore obeys the law of conservation of mass.

q  Many equations are balanced by trial and error; but it must be remembered that coefficients can be changed in order to balance an equation, but not subscripts of a correct formula.

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q  The general procedure for balancing equations is:

Write the unbalanced equation:

CH4 + O2 → CO2 + H2O

Make sure the formula for each

substance is correct

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q  The general procedure for balancing equations is:

Balance by inspection:

CH4 + O2 → CO2 + H2O

Count and compare each element on both sides of the

equation

1 C = 1 C

4 H 2 H

2 O 3 O

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q  Balance elements that appear only in one substance first. Balance H

CH4 + O2 → CO2 + H2O

1 CH4 + O2 → CO2 + 2 H2O

4 H present on each side

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Balance O

1 CH4 + O2 → CO2 + 2 H2O

1 CH4 + 2 O2 → CO2 + 2 H2O

4 O present on each side

When finally done, check for the smallest coefficients possible

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Examples:��

AgNO3 + H2S → Ag2S + HNO3

Al(OH)3 + H2SO4 → Al2(SO4)3 + H2O

Fe3O4 + H2 → Fe + H2O

C4H10 + O2 → CO2 + H2O

2 AgNO3 + H2S → Ag2S + 2 HNO3

2 Al(OH)3 + 3 H2SO4 → Al2(SO4)3 + 6 H2O

Fe3O4 + 4 H2 → 3 Fe + 4 H2O

2 C4H10 + 13 O2 → 8 CO2 + 10 H2O

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CONCEPT��CHECK��

q  If red spheres represent oxygen atoms and blue spheres represent nitrogen atoms, write a balanced equation for the reaction shown below.

2 NO + O2 → 2 NO2

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TYPES OF��CHEMICAL REACTIONS��

q  Chemical reactions are classified into five types:

1. Synthesis or combination

2. Decomposition

3. Single replacement

4. Double replacement

5. Combustion

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SYNTHESIS or��COMBINATION��

q  In these reactions, 2 elements or compounds combine to form another compound.

A + B → AB

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DECOMPOSITION��

q  In these reactions, a compound breaks up to form 2 elements or simpler compound.

AB → A +B

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SINGLE��REPLACEMENT��

q  In these reactions, a more reactive element replaces a less reactive element in a compound.

A + BC → B + AC

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DOUBLE��REPLACEMENT��

q  In these reactions, two compounds combine to form two new compounds.

AB + CD → AD + CB

q  The cation from one compound replaces the cation in another compound.

+ +

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COMBUSTION��

q  A reaction that involves oxygen as a reactant and produces large amounts of heat is classified as a combustion reaction.

CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (g)

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Examples:��

Classify each of the reactions below: 1.  Mg + CuCl2 → MgCl2 + Cu 2.  CaCO3 → CaO + CO2 3.  2 HCl + Ca(OH)2 → CaCl2 + 2 H2O 4.  4 Fe + 3 O2 → 2 Fe2O3

Single replacement

Mg is more reactive than Cu

Decomposition

Double replacement

Synthesis

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DOUBLE REPLACEMENT��REACTIONS��

q  Double replacement reactions can be subdivided into one the following subgroups:

1. Precipitation: formation of a solid

2. Neutralization: formation of water

3. Unstable product: formation of a gas

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PRECIPITATION��REACTIONS��

q  In these reactions one of the products formed is an insoluble solid called a precipitate.

q  For example, when solutions of potassium chromate, K2CrO4 , and barium nitrate, Ba(NO3)2 , are combined an insoluble salt barium chromate, BaCrO4 , is formed.

K2CrO4 (aq) + Ba(NO3)2 (aq) BaCrO4 (s) + 2 KNO3 (aq) precipitate

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NEUTRALIZATION��REACTIONS��

q  The most important reaction of acids and bases is called neutralization.

q  In these reactions an acid combines with a base to form a salt and water.

2HCl (aq) + NaOH (aq) NaCl (aq) + H O (l)æ æÆ

Acid Base Salt

q  Salts are ionic substances with the cation donated from the base and the anion donated from the acid.

q  In the laboratory, neutralization reactions are observed by an increase in temperature (exothermic reaction).

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UNSTABLE PRODUCT��REACTIONS��

q  Some chemical reactions produce gas because one of the products formed in the reaction is unstable.

q  Two such products are:

H2CO3 (aq) → CO2 (g) + H2O (l) Carbonic acid:

Sulfurous acid: H2SO3 (aq) → SO2 (g) + H2O (l)

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UNSTABLE PRODUCT��REACTIONS��

q  When either of these products appears in a chemical reaction, they should be replaced with their decomposition products.

2 HCl + Na2CO3 → 2 NaCl + H2CO3

2 HCl + Na2CO3 → 2 NaCl + CO2 (g) + H2O (l)

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Complete and balance each neutralization reaction below:

Examples:��

HNO3 + Ba(OH)2 → Ba(NO3)2 + H2O 2 2

H2SO4 + NaOH → Na2SO4 + H2O 2 2

HC2H3O2 + KOH → KC2H3O2 + H2O

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OXIDATION-REDUCTION��REACTIONS��

q  Reactions known as oxidation and reduction (redox) have many important applications in our everyday lives.

q  Rusting of a nail or the reaction within your car batteries are two examples of redox reactions.

q  In an oxidation-reduction reaction, electrons are transferred from one substance to another.

q  If one substance loses electrons, another substance must gain electrons.

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q  Oxidation is defined as loss of electrons, and reduction is defined as gain of electrons.

q  One way to remember these definitions is to use the following mnemonic:

q  Combination, decomposition, single replacement and combustion reactions are all examples of redox reactions.

Oxidation Is Loss of electrons

Reduction Is Gain of electrons

OIL

RIG

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q  In general, atoms of metals lose electrons to form cations, and are therefore oxidized, while atoms of non-metals gain electrons to form anions, and are therefore reduced.

Ca + S CaS

Ca Ca2+ + 2 e- S + 2 e- S2-

Oxidation

Reduction

q  For example, in the formation of calcium sulfide from calcium and sulfur

q  Therefore, the formation of calcium sulfide involves two half-reactions that occur simultaneously, one an oxidation and the other a reduction.

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Mg + 2 HCl MgCl2 + H2

Mg Mg2+ + 2 e- 2 H+ + 2 e- H2

Oxidation

Reduction

q  Similarly, in the reaction of magnesium metal with hydrochloric acid

In every oxidation-reduction reaction, the number of electrons lost must be equal to the number of electrons gained.

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OXIDATION-REDUCTION IN��BIOLOGICAL SYSTEMS��

q  Many important biological reactions involve oxidation and reduction.

q  In these reactions, oxidation involves addition of oxygen or loss of hydrogen, and reduction involves loss of oxygen or gain of hydrogen.

q  For example, poisonous methyl alcohol is metabolized by the body by the following reaction:

CH3OH H2CO + 2H•

methyl alcohol formaldehyde

Oxidation (loss of

hydrogen)

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q  The formaldehyde is further oxidized to formic acid and finally carbon dioxide and water by the following reactions:

2 H2CO + O2 2H2CO2 formic acid formaldehyde

Oxidation (gain of oxygen)

formic acid

2 H2CO2 + O2 CO2 + H2O

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q  In many biochemical oxidation-reduction reactions, the transfer of hydrogen atoms produces energy in the cells.

q  For example, cellular respiration is an oxidation-reduction process that transfers energy from the bonds in glucose to form ATP.


C6H12O6 + 6 O2 6 CO2 + 6 H2O + ATP + Heat

Glucose + Heat

C6H12O6 O2 CO2 H2O ATP 6 6 6

Loss of hydrogen atoms (becomes oxidized)

Gain of hydrogen atoms (becomes reduced)


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q  The oxidation of a typical biochemical molecule can involve the transfer of hydrogen atoms to a proton acceptor such as coenzyme FAD to produce its reduced form FADH2.

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REDOX IN��BIOLOGICAL SYSTEMS��

q  In summary, the particular definition of oxidation-reduction depends on the process that occurs in the reaction.

q  A summary of these definitions appears below:

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Example 1:��Linoleic acid, an unsaturated fatty acid, can be converted to a saturated fatty acid by the reaction shown below. Is linoleic acid oxidized or reduced in this reaction?

C18H32 O2 + 2 H2 C18H36 O2

Gain of hydrogen Reduction

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Example 2:��The reaction of succinic acid (C4H6O4) provides energy for the ATP synthesis and is shown below: a)   Is succinic acid oxidized or reduced? b)  Is FAD oxidized or reduced?

C4H6 O4 C4H4 O4 + 2 H•

Loss of hydrogen Gain of

hydrogen

FAD + 2 H• FADH2

oxidation reduction

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ENZYMES IN��BIOLOGICAL SYSTEMS��

q  In biochemical reactions, enzymes are necessary to oxidize glucose and other foods.

q  For example, oxidation of glucose involves the transfer of hydrogen atoms and electrons to an enzyme, such as NAD+ to produce its reduced form NADH.

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q  Similarly, oxidation of methanol involves transfer of 2 hydrogen atoms and 2 electrons to NAD+ to form the reduced form NADH.

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q  Molecules such as NAD+ are called “electron carriers” since they carry electrons in their reduced form.

q  The electron carriers collectively are called electron transport chain.

q  As electrons are transported down the chain, ATP is generated.

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ACTIVITY SERIES��OF METALS��

q  Activity series is a listing of metallic elements in descending order of reactivity.

q  Hydrogen is also included in the series since it behaves similar to metals.

q  Activity series tables are available in textbooks and other sources.

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q  Elements listed higher will displace any elements listed below them.

q  For example Na will displace any elements listed below it from one of its compounds.

2 Na (s) + MgCl2 (aq) → 2 NaCl (aq) + Mg (s)

Na (s) + AgCl (aq) → NaCl (aq) + Ag (s)

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q  Elements listed lower will not displace any elements listed above them.

q  For example Ag cannot displace any elements listed above it from one of its compounds.

Ag (s) + CuCl2 (aq) → No Reaction

Ag (s) + HCl (aq) → No Reaction

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Example 1:��Use activity series to complete each reaction below. If no reaction occurs, write “No Reaction”.

Pb (s) + HCl (aq) → Metals Fe Ni Sn Pb H

Cu Ag

Pb is more reactive than H

PbCl2 (aq) + H2 (g) 2

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Zn (s) + MgCl2 (aq) → Metals Na Mg Al Zn Fe Ni

Zn is less reactive than Mg

No Reaction

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Ni (s) + CuCl2 (aq) → Metals Fe Ni Sn Pb H

Cu Ag

Ni is more reactive than Cu

NiCl2 (aq) + Cu (s)

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Mg (s) + AlCl3 (aq) → Metals Na Mg Al Zn Fe Ni

Mg is more reactive than Al

MgCl2 (aq) + Al (s) 2 3 3 2

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THE END

chap 06a-chemical reactions 51 le… · formaldehyde formic acid oxidation (gain of oxygen) formic...

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