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Chem 150Chem 150Unit 4 - Chemical Properties IUnit 4 - Chemical Properties I
Chemical ReactionsChemical Reactions
The chemical properties of molecules The chemical properties of molecules describe processes that involve the making describe processes that involve the making and breaking the stronger covalent bonds and breaking the stronger covalent bonds
that hold molecules together. As a that hold molecules together. As a consequence, the compositions of the the consequence, the compositions of the the molecules participating in a chemical molecules participating in a chemical
process change. These processes are called process change. These processes are called chemical reactions and can be expressed chemical reactions and can be expressed
using balanced chemical equations.using balanced chemical equations.
22
IntroductionIntroduction
Chemical reactions involve the rearrangement of the atoms Chemical reactions involve the rearrangement of the atoms within and between molecules that results in the formation of within and between molecules that results in the formation of new molecules.new molecules.• This process involves the making and breaking of covalent This process involves the making and breaking of covalent
bonds.bonds.• An important concept in these processes is that An important concept in these processes is that all of the all of the
atoms present before a reaction are also present after the atoms present before a reaction are also present after the reactionreaction• This a concept allows us to describe chemical reactions using This a concept allows us to describe chemical reactions using chemical chemical
equationsequations
2 H2 (g) + O2 (g) 2H2O (g)
33
IntroductionIntroduction
If you need to review how to create and balance chemical If you need to review how to create and balance chemical equation, take a look at Section 6.1 in Raymond.equation, take a look at Section 6.1 in Raymond.
We will focus on some reactions that are important in We will focus on some reactions that are important in biological chemistry, including:biological chemistry, including:
• Oxidation/Reduction reactionsOxidation/Reduction reactions
• Reactions involving waterReactions involving water
We will also look at the different forms of free energy that can We will also look at the different forms of free energy that can be used to predict the directions and rates of chemical be used to predict the directions and rates of chemical reactions.reactions.
44
QuestionQuestion
When you are driving along in your automobile, octane in the When you are driving along in your automobile, octane in the gasoline is reacting with oxygen from the air to produce gasoline is reacting with oxygen from the air to produce carbon dioxide and water. Write a balance chemical equation carbon dioxide and water. Write a balance chemical equation that can be used to describe this reactionthat can be used to describe this reaction
55
Oxidation and ReductionOxidation and Reduction
In Unit 1 we discussed some of the strategies that atoms use In Unit 1 we discussed some of the strategies that atoms use to obtain 8 valence electrons.to obtain 8 valence electrons.
See Unit ISee Unit IElaboration - The Octet RuleElaboration - The Octet Rule
See Unit 1See Unit 1Elaboration - CompoundsElaboration - Compounds
66
Oxidation and ReductionOxidation and Reduction
When metal atoms combine with non-metal atoms, they When metal atoms combine with non-metal atoms, they transfer electrons from the metal to the non-metal to form transfer electrons from the metal to the non-metal to form ionic compounds:ionic compounds:
• Sodium, Na (Sodium, Na (ss), is a soft grey metal.), is a soft grey metal.
• Chlorine, ClChlorine, Cl22 ( (gg), is toxic green gas.), is toxic green gas.
• Sodium chloride, NaCl (Sodium chloride, NaCl (ss), is a crystalline white solid ), is a crystalline white solid comprising sodium ions, Nacomprising sodium ions, Na++, and chloride ions, Cl, and chloride ions, Cl––..
2 Na (s) + Cl2 (g) 2 NaCl (s)
77
Oxidation and ReductionOxidation and Reduction
Reactions that involve the transfer of electrons from one atom Reactions that involve the transfer of electrons from one atom to another are called to another are called oxidation/reduction reactionsoxidation/reduction reactions..
• The atom losing the electrons is The atom losing the electrons is oxidizedoxidized..• In the previous example, the sodium is oxidized:In the previous example, the sodium is oxidized:
• The atom gaining the electrons is The atom gaining the electrons is reducedreduced..• In the previous example, the chlorine is reduced:In the previous example, the chlorine is reduced:
• While the two processes can be separated, one cannot While the two processes can be separated, one cannot occur without the other.occur without the other.
2 Na 2 Na+ + 2 e –
2 Cl–+ 2 e –Cl2
88
Oxidation and ReductionOxidation and Reduction
Reactions that involve the transfer of electrons from one atom Reactions that involve the transfer of electrons from one atom to another are called oxidation/reduction reactions.to another are called oxidation/reduction reactions.
• The reactant that takes away the electrons is the The reactant that takes away the electrons is the oxidizing oxidizing agentagent..• In the previous example, the chlorine is the oxidizing agent.In the previous example, the chlorine is the oxidizing agent.
‣ The chlorine took the electrons away from the sodium.The chlorine took the electrons away from the sodium.
• The reactant that donates the electrons is the The reactant that donates the electrons is the reducing reducing agentagent..• In the previous example, the sodium is the reducing agent.In the previous example, the sodium is the reducing agent.
‣ The sodium gave the electrons to the chlorine.The sodium gave the electrons to the chlorine.
99
Oxidation and ReductionOxidation and Reduction
In oxidation and reduction, metals can also transfer electrons In oxidation and reduction, metals can also transfer electrons between themselves:between themselves:
• Copper, Cu (Copper, Cu (ss), is a reddish metal.), is a reddish metal.
• Silver(I) nitrate, AgNO3 Silver(I) nitrate, AgNO3 ((aqaq)), a colorless aqueous solution , a colorless aqueous solution containing silver(I) ions, Agcontaining silver(I) ions, Ag++ ions and nitrate ions, NO ions and nitrate ions, NO3-3-..
• copper(II) nitrate, Cu(NOcopper(II) nitrate, Cu(NO33))22 ( (aqaq), a green aqueous solution ), a green aqueous solution containing copper(II) ions, Cucontaining copper(II) ions, Cu2+2+, ions and nitrate ions, NO, ions and nitrate ions, NO3-3-..
• Silver, Ag (Silver, Ag (ss), a silvery metal), a silvery metal
Cu (s) + 2 AgNO3 (aq) 2 Cu(NO3)2 (aq) + 2 Ag (s)
1010
Oxidation and ReductionOxidation and Reduction
Oxidation and Reduction Oxidation and Reduction • The atom losing the electrons is The atom losing the electrons is oxidizedoxidized..• In the previous example, the copper is oxidized:In the previous example, the copper is oxidized:
• The atom gaining the electrons is The atom gaining the electrons is reducedreduced..• In the previous example, the silver(I) ion is reduced:In the previous example, the silver(I) ion is reduced:
+ 2 e –Cu2+Cu
2 Ag+ 2 e –2 Ag+
1111
Oxidation and ReductionOxidation and Reduction
Oxidation/reduction reactions can also occur when no ions or Oxidation/reduction reactions can also occur when no ions or metals are involved.metals are involved.• This can occur when molecular compounds composed of This can occur when molecular compounds composed of
nonmetals react with one another to form other molecular nonmetals react with one another to form other molecular compounds,compounds,
• And:And:• Polar covalent bonds are produced or eliminatedPolar covalent bonds are produced or eliminated
‣OrOr
• Double or triple bonds are produced or eliminated Double or triple bonds are produced or eliminated
See Unit 1See Unit 1Elaboration - PolarityElaboration - Polarity
1212
Oxidation and ReductionOxidation and Reduction
The combustion of an organic molecule to produce carbon The combustion of an organic molecule to produce carbon dioxide and water is an example:dioxide and water is an example:• The products of this reaction contain polar covalent bonds The products of this reaction contain polar covalent bonds
in which the electrons are being drawn away fromin which the electrons are being drawn away from• The carbon atom in COThe carbon atom in CO22
• The hydrogen atoms in HThe hydrogen atoms in H22OO
• The carbons and the hydrogens are being The carbons and the hydrogens are being oxidizedoxidized..• The oxygen is being The oxygen is being reducedreduced..
CH4 + 2 O2 CO2 + 2 H2O
C
H
H
H
H +O O2 CO O
--
2 H O H
- +
1313
Oxidation and ReductionOxidation and Reduction
An easier way to assess whether a reaction is an An easier way to assess whether a reaction is an oxidation/reduction reaction or not is to look for the following:oxidation/reduction reaction or not is to look for the following:
OxidationOxidation ReductionReduction
An atom An atom losesloses electronselectrons
An atom An atom gainsgains electronselectrons
An atom An atom gainsgains a a bond to oxygenbond to oxygen
An atom An atom losesloses a a bond to oxygenbond to oxygen
An atom An atom losesloses a a bond to hydrogenbond to hydrogen
An atom An atom gainsgains a a bond to hydrogenbond to hydrogen
1414
Oxidation and ReductionOxidation and Reduction
Applying these rules to the combustion of methane:Applying these rules to the combustion of methane:
• The carbon is being The carbon is being oxidizedoxidized because it because it gainsgains bonds to bonds to oxygen.oxygen.
• The carbon is also being The carbon is also being oxidizedoxidized because it is because it is losinglosing bonds to hydrogen.bonds to hydrogen.
• The hydrogens are being The hydrogens are being oxidizedoxidized because they because they gaingain bonds to oxygen.bonds to oxygen.
C
H
H
H
H +O O2 CO O
--
2 H O H
- +
1515
Oxidation and ReductionOxidation and Reduction
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch19/oxred_1.php#assign
Rules for Assigning Oxidation Numbers
* The oxidation number of an atom is zero in a neutral substance that contains atoms of only one element. Thus, the atoms in O2, O3, P4, S8, and aluminum metal all have an oxidation number of 0. * The oxidation number of monatomic ions is equal to the charge on the ion. The oxidation number of sodium in the Na+ ion is +1, for example, and the oxidation number of chlorine in the Cl- ion is -1. * The oxidation number of hydrogen is +1 when it is combined with a nonmetal. Hydrogen is therefore in the +1 oxidation state in CH4, NH3, H2O, and HCl. * The oxidation number of hydrogen is -1 when it is combined with a metal. Hydrogen is therefore in the -1 oxidation state in LiH, NaH, CaH2, and LiAlH4. * The metals in Group IA form compounds (such as Li3N and Na2S) in which the metal atom is in the +1 oxidation state. * The elements in Group IIA form compounds (such as Mg3N2 and CaCO3) in which the metal atom is in the +2 oxidation state. * Oxygen usually has an oxidation number of -2. Exceptions include molecules and polyatomic ions that contain O-O bonds, such as O2, O3, H2O2, and the O2
2- ion. * The nonmetals in Group VIIA often form compounds (such as AlF3, HCl, and ZnBr2) in which the nonmetal is in the -1 oxidation state. * The sum of the oxidation numbers of the atoms in a molecule is equal to the charge on the molecule. * The most electronegative element in a compound has a negative oxidation number.
1616
Oxidation and ReductionOxidation and Reduction
An easier way to assess whether a reaction is an An easier way to assess whether a reaction is an oxidation/reduction reaction or not is to look for the following:oxidation/reduction reaction or not is to look for the following:
OxidationOxidation ReductionReduction
An atom An atom losesloses electronselectrons
An atom An atom gainsgains electronselectrons
An atom An atom gainsgains a a bond to oxygenbond to oxygen
An atom An atom losesloses a a bond to oxygenbond to oxygen
An atom An atom losesloses a a bond to hydrogenbond to hydrogen
An atom An atom gainsgains a a bond to hydrogenbond to hydrogen
1717
Oxidation and ReductionOxidation and Reduction
HydrogenationHydrogenation• Another type of oxidation/reduction reaction is the Another type of oxidation/reduction reaction is the
hydrogenationhydrogenation reaction: reaction:• In this example, an alkene is reduced to an alkane.In this example, an alkene is reduced to an alkane.
‣ This is considered reduction, because the hydrogen is bringing in additional This is considered reduction, because the hydrogen is bringing in additional electrons to the molecule.electrons to the molecule.
• The alkane that is produced in this reaction is considered “saturated” because it The alkane that is produced in this reaction is considered “saturated” because it can no longer absorb any more hydrogen atoms.can no longer absorb any more hydrogen atoms.
saturatedsaturatedsaturatedsaturatedunsaturatedunsaturatedunsaturatedunsaturated
C C
H
H
H
H
H2 C
H
H
H
CH
H
H
+platinumcatalyst
1818
Oxidation and ReductionOxidation and Reduction
Often chemist use a shorthand method of writing equations Often chemist use a shorthand method of writing equations like these:like these:• The equation shown on the previous slide can be written The equation shown on the previous slide can be written
as follows:as follows:
• One of the reactants, HOne of the reactants, H22, is placed above the reaction arrow, is placed above the reaction arrow
‣ Technically, this equation is no longer balancedTechnically, this equation is no longer balanced
• The shorthand method of writing a chemical equation is used to emphasize what The shorthand method of writing a chemical equation is used to emphasize what happens to a key component of the reactionhappens to a key component of the reaction
‣ In this case it is the alkene.In this case it is the alkene.
C CH
H
H
H
H2
PtC
H
H
H
CH
H
H
1919
Oxidation and ReductionOxidation and Reduction
Saturated Saturated vsvs Unsaturated Fats Unsaturated Fats
C
O
O
C
O
O
C
O
OH
H
H
H
CH2
CH
CH2
saturated fat
trans unsaturated fat
cis unsaturated fat
Fat (Triacylglyceride)Fat (Triacylglyceride)Fat (Triacylglyceride)Fat (Triacylglyceride)
2020
Oxidation and ReductionOxidation and Reduction
DehydrogenationDehydrogenation• Oxidation/reduction also occurs when hydrogens are taken Oxidation/reduction also occurs when hydrogens are taken
away from a molecule. This is called away from a molecule. This is called dehydrogenation.dehydrogenation.
• The oxidation of succinic acid to fumaric acid:The oxidation of succinic acid to fumaric acid:• This reaction takes place in the Citric Acid Cycle.This reaction takes place in the Citric Acid Cycle.
‣ We will discuss the Citric Acid Cycle in Unit 12.We will discuss the Citric Acid Cycle in Unit 12.
• The FAD is an abbreviation for a large organic molecule called The FAD is an abbreviation for a large organic molecule called Flavin Adenine Flavin Adenine DinucleotideDinucleotide..
HO C
O
C C
H H
C
OH H
OH
succinic acid(saturated)
HO C
O
C C
H H
C
O
OH
fumaric acid(unsaturated)
FAD FADH2
2121
Oxidation and ReductionOxidation and Reduction
The reaction equation on the previous slide also illustrates The reaction equation on the previous slide also illustrates another shorthand method of writing equations, which used another shorthand method of writing equations, which used multiple reaction arrows.multiple reaction arrows.• The longhand form of this reaction equation isThe longhand form of this reaction equation is
HO C
O
C C
H H
C
OH H
OH
succinic acid(saturated)
HO C
O
C C
H
C
O
OH
fumaric acid(unsaturated)
FAD FADH2 H
H
HO C
O
C C
H H
C
OH H
OH
succinic acid(saturated)
FAD HO C
O
C C
H
C
O
OH
fumaric acid(unsaturated)
FADH2+ +
2222
Oxidation and ReductionOxidation and Reduction
Dehydration exampleDehydration example
• The oxidation of ethanol to form acetaldehyde:The oxidation of ethanol to form acetaldehyde:• This reaction occurs in liver after consuming alcohol.This reaction occurs in liver after consuming alcohol.
• The NADThe NAD++ is an abbreviation for a large organic molecule named is an abbreviation for a large organic molecule named Nicotinamide Nicotinamide Adenine DinucleotideAdenine Dinucleotide..
C C O
HH
H
H
H
H
C C O
H
H
H
H
NAD+ NADH + H+
ethanol acetaldehyde
2323
Reactions Involving WaterReactions Involving Water
While the major role for water in biology is a physical one as While the major role for water in biology is a physical one as the primary solvent in living cell, it also plays a chemical role the primary solvent in living cell, it also plays a chemical role as a reactant or product in some chemical reactions.as a reactant or product in some chemical reactions.
• Reactions involving water as a reactant or productReactions involving water as a reactant or product
• Acid-catalyzed hydrolysisAcid-catalyzed hydrolysis
• Base-catalyzed hydrolysisBase-catalyzed hydrolysis
• HydrationHydration
• DehydrationDehydration
2424
Reactions Involving WaterReactions Involving Water
HydrolysisHydrolysis• In the In the hydrolysishydrolysis reaction, water ( reaction, water (hydrohydro) is used to split ) is used to split
((lyselyse) another molelcule.) another molelcule.
• In this case, water is being used to split an ester into a In this case, water is being used to split an ester into a carboxylic acid plus and alcohol.carboxylic acid plus and alcohol.
CH2 C
O
OH CH2 CH3+
ethylbutanoate(an ester)
butanoic acid(a carboxylic acid)
ethanol(an alcohol)
HOCH3 CH2 CH2 C
O
O CH2 CH3 CH2CH3H OH+acid
catalyzed
2525
Reactions Involving WaterReactions Involving Water
Hydrolysis exampleHydrolysis example• The hydrolysis of the ester bond in the neurotransmitter The hydrolysis of the ester bond in the neurotransmitter
acetylcholine.acetylcholine.• Again, shorthand notation is being used:Again, shorthand notation is being used:• The HThe H22O reactant is placed above the reaction arrow,O reactant is placed above the reaction arrow,
• The HThe H++ below the arrow indicates an acid catalyst is used. below the arrow indicates an acid catalyst is used.
CH3 C
O
O CH2 CH2 N
CH3
CH2
CH3
H2O
H+CH3 C
O
OH CH2 CH2 N
CH3
CH2
CH3
+
acetylcholine(an ester)
acetic acid(a carboxylic acid)
choline(an alcohol)
HO
2626
Reactions Involving WaterReactions Involving Water
HydrolysisHydrolysis• HydrolysisHydrolysis can also be catalyzed using a base (OH can also be catalyzed using a base (OH--):.):.
• Because one of the products of the hydrolysis is a carboxylic acid, in base Because one of the products of the hydrolysis is a carboxylic acid, in base catalyzed hydrolysis the base undergoes a second acid/base reaction with the catalyzed hydrolysis the base undergoes a second acid/base reaction with the carboxylic acid to produce a carboxylate ion.carboxylic acid to produce a carboxylate ion.
• The base catalyzed hydrolysis of esters is also called The base catalyzed hydrolysis of esters is also called saponificationsaponification
• We will be discussing acids and bases in Unit 6We will be discussing acids and bases in Unit 6
H2O
OH-CH2 C
O
O CH2 CH3+
ethylbutanoate(an ester)
butanoate ion(a carobxylate ion)
ethanol(an alcohol)
HOCH3 CH2 CH2 C
O
O CH2 CH3 CH2CH3
2727
Reactions With WaterReactions With Water
Hydrolysis example:Hydrolysis example:• The base catalyzed hydrolysis of fats produces soap and The base catalyzed hydrolysis of fats produces soap and
glycerolglycerolC
O
O
C
O
O
C
O
OH
H
H
H
CH2
CH
CH2
3 H2O
OH-
FatFatFatFat
2828
Reactions With WaterReactions With Water
Hydrolysis example:Hydrolysis example:• The base catalyzed hydrolysis of fats produces soap and The base catalyzed hydrolysis of fats produces soap and
glycerolglycerol
SoapSoapSoapSoap GlycerolGlycerolGlycerolGlycerol
3 H2O
OH-
C
O
O
C
O
O
C
O
OH
H
H
H
CH2
CH
CH2
HO
HO
HO
+
2929
Reactions Involving WaterReactions Involving Water
HydrationHydration• In the In the hydration hydration reaction water is also split, but instead of reaction water is also split, but instead of
being used to split another molecule, it is added to another being used to split another molecule, it is added to another molecule to produce a single product.molecule to produce a single product.
• The water it is added to either an alkene or alkyne:The water it is added to either an alkene or alkyne:• The hydration of an alkene produces an alcohol.The hydration of an alkene produces an alcohol.
C CH
H H
H + H OHacid
catalyst
C CH
H H
H
H OH
ethene(an alkene)
ethanol(an alcohol)
3030
Reactions Involving WaterReactions Involving Water
HydrationHydration• This can also be written in shorthand as:This can also be written in shorthand as:• The HThe H++ below the reaction arrow is used to indicate that this is an acid-catalyzed below the reaction arrow is used to indicate that this is an acid-catalyzed
reaction.reaction.• The shorthand is used to emphasize what happens to the key reactant.The shorthand is used to emphasize what happens to the key reactant.
C CH
H H
H C CH
H H
H
H OH
H2O
H+
ethene(an alkene)
ethanol(an alcohol)
3131
Reactions Involving WaterReactions Involving Water
Hydration exampleHydration example• On an earlier slide a reaction from the Citric Acid Cycle On an earlier slide a reaction from the Citric Acid Cycle
was shown, which involved the dehydrogenation of was shown, which involved the dehydrogenation of succinic acid to produce fumaric acid.succinic acid to produce fumaric acid.
• The sequent reaction in the Citric Acid Cycle is an example The sequent reaction in the Citric Acid Cycle is an example of a hydration reaction:of a hydration reaction:
HO C
O
C C
H H
C
O
OH
fumaric acid(an alkene)
H2O
HO C
O
C C
H H
C
O
OH
malic acid(an alcohol)
H OH
3232
Reactions Involving WaterReactions Involving Water
DehydrationDehydration• In the In the dehydration dehydration reaction is the reverse of the hydration reaction is the reverse of the hydration
reaction.reaction.
• The water it is removed from an alcohol:The water it is removed from an alcohol:• The dehydration of an alcohol produces an alkene.The dehydration of an alcohol produces an alkene.
C CH
H H
H + H OHC CH
H H
H
H OH
ethene(an alkene)
ethanol(an alcohol)
H+
3333
Reactions Involving WaterReactions Involving Water
Dehydration exampleDehydration example• The Citric Acid Cycle also provides a good example of a The Citric Acid Cycle also provides a good example of a
dehydration reaction.dehydration reaction.
• A dehydration reaction followed by a hydration reaction is used to move a A dehydration reaction followed by a hydration reaction is used to move a hydroxyl group from one carbon to an adjacent carbon in citric acid:hydroxyl group from one carbon to an adjacent carbon in citric acid:
CH
C
C
O OH
HO C
O
OH
H
CH2
C
O H
citric acid(both a carboxylic acid
and an alcohol)
CH
C
C
O OH
C
O
OH
CH2
C
O H
cis aconntic acid(both a carboxylic acid
and an alkene
CH
C
C
O OH
H C
O
OH
HO
CH2
C
O H
isocitric acid(both a carboxylic acid
and an alcohol)
H2O H2O
Dehydration Hydration
3434
Free Energy and Reaction RatesFree Energy and Reaction Rates
In Unit 3 we discussed how changes in the free energy can In Unit 3 we discussed how changes in the free energy can be used to predict whether a process is spontaneous be used to predict whether a process is spontaneous (favorable) or nonspontaneous (not favorable)(favorable) or nonspontaneous (not favorable)
ΔΔGG < 0 < 0spontaneousspontaneous
ΔΔGG < 0 < 0spontaneousspontaneous
ΔΔGG > 0 > 0nonspontaneousnonspontaneous
ΔΔGG > 0 > 0nonspontaneousnonspontaneous
3535
Free Energy and Reaction RatesFree Energy and Reaction Rates
The same principles can be applied to chemical reactions to The same principles can be applied to chemical reactions to predict whether they are favorable or not:predict whether they are favorable or not:
FreeFreeEnergyEnergy
(G)(G)
FreeFreeEnergyEnergy
(G)(G)
Progress ofProgress ofreactionreaction
Progress ofProgress ofreactionreaction
Α Α → → BBΑ Α → → BB
AAAA
ΒΒΒΒ
ΔΔGG < 0 < 0spontaneousspontaneous
ΔΔGG < 0 < 0spontaneousspontaneous
FreeFreeEnergyEnergy
(G)(G)
FreeFreeEnergyEnergy
(G)(G)
Progress ofProgress ofreactionreaction
Progress ofProgress ofreactionreaction
Α Α → → BBΑ Α → → BB
AAAA
ΒΒΒΒΔΔGG > 0 > 0nonspontaneousnonspontaneous
ΔΔGG > 0 > 0nonspontaneousnonspontaneous
3636
Free Energy and Reaction RatesFree Energy and Reaction Rates
Just because a reaction is Just because a reaction is spontaneous, does not spontaneous, does not mean that it will occur at an mean that it will occur at an observable rate.observable rate.• For example, diamond For example, diamond
and graphite are two and graphite are two different forms of pure different forms of pure carbon. The reaction that carbon. The reaction that converts diamond to converts diamond to graphite is actually a graphite is actually a favorable onefavorable one• This does not make diamonds a This does not make diamonds a
bad investment for fear that they bad investment for fear that they will turn into pencil lead.will turn into pencil lead.
• Why?Why?
FreeFreeEnergyEnergy
(G)(G)
FreeFreeEnergyEnergy
(G)(G)
Progress ofProgress ofreactionreaction
Progress ofProgress ofreactionreaction
Diamond Diamond → → GraphiteGraphiteDiamond Diamond → → GraphiteGraphite
DiamondDiamondDiamondDiamond
GraphiteGraphiteGraphiteGraphite
ΔΔGG < 0 < 0spontaneousspontaneous
ΔΔGG < 0 < 0spontaneousspontaneous
3737
Free Energy and Reaction RatesFree Energy and Reaction Rates
There is is a hill that for most reactions the reactants must There is is a hill that for most reactions the reactants must climb and go over to before they can go on to become climb and go over to before they can go on to become product.product.
FreeFreeEnergyEnergy
(G)(G)
FreeFreeEnergyEnergy
(G)(G)
Progress ofProgress ofreactionreaction
Progress ofProgress ofreactionreaction
Α Α → → BBΑ Α → → BB
AAAA
ΒΒΒΒ
3838
Free Energy and Reaction RatesFree Energy and Reaction Rates
The height of this hill is called the The height of this hill is called the activation energyactivation energy, , EEactact..• The activation energy has no effect on the overall change The activation energy has no effect on the overall change
in the free energy for the reaction.in the free energy for the reaction.
FreeFreeEnergyEnergy
(G)(G)
FreeFreeEnergyEnergy
(G)(G)
Progress ofProgress ofreactionreaction
Progress ofProgress ofreactionreaction
Α Α → → BBΑ Α → → BB
AAAA
ΒΒΒΒ
ΔΔGG < 0 < 0spontaneousspontaneous
ΔΔGG < 0 < 0spontaneousspontaneous
EEactact > 0 > 0EEactact > 0 > 0
3939
Free Energy and Reaction RatesFree Energy and Reaction Rates
Diamonds are still good investment because the activation Diamonds are still good investment because the activation energy for the conversion of diamond to graphite is very high.energy for the conversion of diamond to graphite is very high.
FreeFreeEnergyEnergy
(G)(G)
FreeFreeEnergyEnergy
(G)(G)
Progress ofProgress ofreactionreaction
Progress ofProgress ofreactionreaction
ΔΔGG < 0 < 0spontaneousspontaneous
ΔΔGG < 0 < 0spontaneousspontaneous
EEactact > 0 > 0EEactact > 0 > 0
Diamond Diamond → → GraphiteGraphiteDiamond Diamond → → GraphiteGraphite
DiamondDiamondDiamondDiamond
GraphiteGraphiteGraphiteGraphite
4040
Free Energy and Reaction RatesFree Energy and Reaction Rates
The reaction rate (speed) of a reaction is determined by the The reaction rate (speed) of a reaction is determined by the height of the hill.height of the hill.
• The higher the activation energy, the slower the reaction The higher the activation energy, the slower the reaction rate.rate.
4141
Free Energy and Reaction RatesFree Energy and Reaction Rates
There are several ways that reactants can be pushed over There are several ways that reactants can be pushed over the hill to speed up the reaction rate. Two of these include:the hill to speed up the reaction rate. Two of these include:
• Increase the temperatureIncrease the temperature of the reactant molecules. of the reactant molecules.• This increases the kinetic energy, which increases the motion of the reactant This increases the kinetic energy, which increases the motion of the reactant
molecules. This increases the frequency with which they will collide with one molecules. This increases the frequency with which they will collide with one another to react.another to react.
• Increase the concentrationIncrease the concentration of the reactant molecules. of the reactant molecules.• This increases the number of reactant molecules. This also increases the This increases the number of reactant molecules. This also increases the
frequency with which they will collide with other reactant molecules.frequency with which they will collide with other reactant molecules.
4242
Free Energy and Reaction RatesFree Energy and Reaction Rates
There is a third way to speed up the reaction rate and that is There is a third way to speed up the reaction rate and that is to lower the height of the hill.to lower the height of the hill.
• This is done using This is done using catalystscatalysts, which provide an alternative , which provide an alternative pathway over the hill for the reactants.pathway over the hill for the reactants.
FreeFreeEnergyEnergy
(G)(G)
FreeFreeEnergyEnergy
(G)(G)
Progress ofProgress ofreactionreaction
Progress ofProgress ofreactionreaction
Α Α → → BBΑ Α → → BB
AAAA
ΒΒΒΒ
ΔΔGG < 0 < 0spontaneousspontaneous
ΔΔGG < 0 < 0spontaneousspontaneous
EEactact > 0 > 0without catalystwithout catalyst
--with catalystwith catalyst
EEactact > 0 > 0without catalystwithout catalyst
--with catalystwith catalyst
4343
Free Energy and Reaction RatesFree Energy and Reaction Rates
Catalysts speed up a reaction, but are not produced or Catalysts speed up a reaction, but are not produced or consumed in a reaction.consumed in a reaction.
• In the reaction equation, their presence in indicated above In the reaction equation, their presence in indicated above or below the reaction arrow.or below the reaction arrow.
• They have not effect on the change in free energy for the They have not effect on the change in free energy for the reaction, Δreaction, ΔGG..• They cannot be used to make an unfavorable reaction favorable.They cannot be used to make an unfavorable reaction favorable.
C C
H
H
H
H
H2 C
H
H
H
CH
H
H
+platinumcatalyst
4444
Free Energy and Reaction RatesFree Energy and Reaction Rates
In biological systems, catalysts are called In biological systems, catalysts are called enzymesenzymes..
• Most enzymes are proteins.Most enzymes are proteins.
• Nearly every reaction that takes place in a living cell has an Nearly every reaction that takes place in a living cell has an enzyme associated with.enzyme associated with.
• Enzymes bind the reactants, facilitate the reaction, and Enzymes bind the reactants, facilitate the reaction, and then release the products.then release the products.
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Free Energy and Reaction RatesFree Energy and Reaction Rates
ExampleExample
• HexokinaseHexokinase
SeeSeeJmol ModelJmol Model
ofofHexokinaseHexokinase
SeeSeeJmol ModelJmol Model
ofofHexokinaseHexokinase
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