Chem 150 Unit 4 - Chemical Properties I Chemical Reactions

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


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


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


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


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 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/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

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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


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

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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


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.

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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

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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


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


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


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


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


HO C

O

C C

H

C

O

OH


FAD FADH2 H

H

HO C

O

C C

H H

C

OH H

OH


FAD HO C

O

C C

H

C

O

OH


FADH2+ +

2222


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


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


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


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


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


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


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


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


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 > 0nonspontaneousnonspontaneous


3535


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)


(G)(G)

Progress ofProgress ofreactionreaction


Α Α → → BBΑ Α → → BB

AAAA

ΒΒΒΒ




(G)(G)


(G)(G)




AAAA

ΒΒΒΒΔΔGG > 0 > 0nonspontaneousnonspontaneous


3636


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?


(G)(G)


(G)(G)



Diamond Diamond → → GraphiteGraphiteDiamond Diamond → → GraphiteGraphite

DiamondDiamondDiamondDiamond

GraphiteGraphiteGraphiteGraphite



3737


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.


(G)(G)


(G)(G)




AAAA

ΒΒΒΒ

3838


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.


(G)(G)


(G)(G)




AAAA

ΒΒΒΒ



EEactact > 0 > 0EEactact > 0 > 0

3939


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.


(G)(G)


(G)(G)





EEactact > 0 > 0EEactact > 0 > 0

Diamond Diamond → → GraphiteGraphiteDiamond Diamond → → GraphiteGraphite

DiamondDiamondDiamondDiamond

GraphiteGraphiteGraphiteGraphite

4040


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


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


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.


(G)(G)


(G)(G)




AAAA

ΒΒΒΒ



EEactact > 0 > 0without catalystwithout catalyst

--with catalystwith catalyst

EEactact > 0 > 0without catalystwithout catalyst

--with catalystwith catalyst

4343


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


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.

4545


ExampleExample

• HexokinaseHexokinase

SeeSeeJmol ModelJmol Model

ofofHexokinaseHexokinase

SeeSeeJmol ModelJmol Model

ofofHexokinaseHexokinase

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Chem 150 Unit 4 - Chemical Properties I Chemical Reactions