chemical reactions chapter 5 dr. victor vilchiz. types of chemical reactions acid-base reactions...

Chemical Chemical ReactionsReactions

Chapter 5Chapter 5

Dr. Victor Vilchiz

Types of Chemical Types of Chemical ReactionsReactionsAcid-Base Reactions

• Neutralization ReactionsNeutralization ReactionsIn a reaction involving HCN(aq), a weak acid, and KOH(aq), a strong base, the product is KCN, a strong electrolyte

Referring to Tables 4.1, 4.2 and 4.3, we obtain this net ionic equation:

)l(OH)aq(CN)aq(OH)aq(HCN 2

H+Note the proton transfer.


• Acid-Base Reactions with Gas FormationAcid-Base Reactions with Gas Formation

Carbonates react with acids to form CO2, carbon dioxide gas.

2232 COOHNaCl2HCl2CONa

Sulfites react with acids to form SO2, sulfur dioxide gas.

2232 SOOHNaCl2HCl2SONa

Gas Production in Gas Production in Neutralization ReactionsNeutralization Reactions

• The previous two reactions are overall The previous two reactions are overall reactions of the actual molecular events.reactions of the actual molecular events.

NaClNaCl

ClOHHClOH

COOHHCO

HCONaNaHCO

NaHCONaClHClCONa

2

23

33

332

2232 COOHNaCl2HCl2CONa


• Acid-Base Reactions with Gas FormationAcid-Base Reactions with Gas Formation

• The Driving Force of Neutralization The Driving Force of Neutralization reactions like that in precipitation reactions like that in precipitation reactions is the removal of ions from reactions is the removal of ions from solution in this case to form water.solution in this case to form water.

Sulfides react with acids to form H2S, hydrogen sulfide gas.

SHNaCl2HCl2SNa 22

Working with SolutionsWorking with Solutions

• The majority of chemical reactions The majority of chemical reactions discussed so far occur in discussed so far occur in aqueous aqueous solutionsolution..

When you run reactions in liquid solutions, it is convenient to dispense the amounts of reactants by measuring out volumes of reactant solutions and not mass.

Solution StoichiometrySolution Stoichiometry• MolarityMolarity is the measurement of the is the measurement of the

concentration of a chemical in solution.concentration of a chemical in solution.– The unit of molarity is the Molar (M).The unit of molarity is the Molar (M).

solution of L

solute of molesolarityM

Example: Calculate the molarity of a solution made by dissolving 12.94g of Ca(OH)2 in enough water to make 1.23L of solution.

22

22

22

Ca(OH) 0.142Msolution of 1.23L

Ca(OH) 0.175molMolarity

Ca(OH) 0.175molCa(OH) 74.1g

Ca(OH) 1molxCa(OH) 12.94g

(see Figure 4.19)

MolarityMolarity

ExampleExample: how many grams of ammonium : how many grams of ammonium nitrate are in a 172.7mL sample of nitrate are in a 172.7mL sample of 1.21M NH1.21M NH44NONO33 solution? solution?

34

343434

34

3434

34

3434

3434

NONH 16.7g

NONH 80.0gx NONH 0.209molesNONH mass

NONH 80.0g

NONH massNONH 0.209moles

NONH 0.209moles

solution of 0.1727Lx NONH 1.21MNONH molessolution of 0.1727L

MNONH molesNONH 1.21M

Diluting SolutionsDiluting Solutions

• When diluting a solution the When diluting a solution the number of moles is constant.number of moles is constant.

(Molarity)(Volume) =moles(Molarity)(Volume) =moles

MM11xVxV11 = n = M = n = M22xVxV22

•So, as water is added, increasing the final volume, V2, the final molarity, M2, decreases.

MM22=M=M11xVxV11/V/V22

Acid Base TitrationsAcid Base Titrations

• A A titrationtitration is a laboratory technique is a laboratory technique used to determine the concentration used to determine the concentration of a solution sample from the volume of a solution sample from the volume of a known concentration solution of a known concentration solution required to complete a given reaction.required to complete a given reaction.

• Titrations are usually used to Titrations are usually used to determine the concentration of acids determine the concentration of acids or bases.or bases.

Acid/Base IndicatorsAcid/Base Indicators

• Most acids and bases as well as the Most acids and bases as well as the resulting salt solution are colorless.resulting salt solution are colorless.– In order to determine when the reaction is In order to determine when the reaction is

complete, we must use chemical complete, we must use chemical indicators.indicators.•Chemical IndicatorsChemical Indicators in the case of acid/base in the case of acid/base

reactions are weak acids that have the reactions are weak acids that have the property of changing colors when going from property of changing colors when going from basic to acidic solutions or vice-versa.basic to acidic solutions or vice-versa.

•The most used acid/base indicator is The most used acid/base indicator is phenolphthalein. phenolphthalein.

IndicatorsIndicators

• The job of the indicator is to signal to you The job of the indicator is to signal to you the point when you are done with the the point when you are done with the experiment. The point when the color experiment. The point when the color changes is defined as the changes is defined as the end pointend point..

• The The equivalence pointequivalence point is not the same as is not the same as the end point, ideally it should be but those the end point, ideally it should be but those occasions are rare. The occasions are rare. The equivalence equivalence pointpoint is when the amount of is when the amount of titranttitrant added added is exactly the amount needed to is exactly the amount needed to “neutralize” the “neutralize” the analyteanalyte in the flask. in the flask.

Types of Chemical Types of Chemical ReactionsReactions• Oxidation-Reduction (RedOx) ReactionsOxidation-Reduction (RedOx) Reactions

RedOx reactions are by far the most important type of reactions.

RedOx reactions involve the transfer of electrons from one species to another.

Oxidation is defined as the loss of electrons.

Reduction is defined as the gain of electrons.

Oxidation and reduction always occur simultaneously, since the electrons lost in the Oxidation must go somewhere.

Types of Chemical Types of Chemical ReactionsReactions

• Oxidation-Reduction ReactionsOxidation-Reduction ReactionsThe reaction of an iron nail with a solution of copper(II) sulfate, CuSO4, is an oxidation- reduction reaction. (see Figure 4.11)

The molecular equation for this reaction is:

)s(Cu)aq(FeSO)aq(CuSO)s(Fe 44


• Oxidation-Reduction ReactionsOxidation-Reduction ReactionsThe net ionic equation shows the reaction of iron metal with Cu2+

(aq) to produce iron(II) ion and copper metal.

)s(Cu)aq(Fe)aq(Cu)s(Fe 22

Loss of 2 e-1 oxidation

Gain of 2 e-1 reduction

RedOx ReactionsRedOx Reactions

• The species that is reduced itself The species that is reduced itself causes another species to be causes another species to be oxidized and is therefore known as oxidized and is therefore known as the the oxidizing agent.oxidizing agent.

• Similarly, the species that are Similarly, the species that are oxidized causes another to be oxidized causes another to be reduced and is therefore known as reduced and is therefore known as the the reducing agent.reducing agent.– Note: An element is reduced/oxidized thNote: An element is reduced/oxidized th

e compound containing that element is te compound containing that element is the oxidizing/reducing agent.he oxidizing/reducing agent.


• Oxidation NumbersOxidation NumbersThe concept of oxidation numbers is a simple way of keeping track of electrons in a reaction.

The oxidation number (or oxidation state) of an atom in a substance is the actual charge of the atom if it exists as a monatomic ion.

Alternatively, it is hypothetical charge assigned to the atom in the substance by simple rules.

Oxidation-Reduction Reactions

Oxidation Number RulesOxidation Number Rules

Oxidation NumbersOxidation Numbers

• It is possible to predict the upper and It is possible to predict the upper and lower limits of main group lower limits of main group elements…elements…– The upper limit is equal to the group The upper limit is equal to the group

numbernumber– The lower limit is the group number-8The lower limit is the group number-8

•Keep in mind couple things like… Oxygen Keep in mind couple things like… Oxygen will never have an ON=+6 and Fluorine will will never have an ON=+6 and Fluorine will never have an ON=+7never have an ON=+7

Types of Chemical Types of Chemical ReactionsReactions• Describing Oxidation-Reduction ReactionsDescribing Oxidation-Reduction Reactions


Look again at the reaction of iron with copper(II) sulfate.

)s(Cu)aq(Fe)aq(Cu)s(Fe 22

The Iron losses 2 electrons so it is oxidized and at the same time it is the reducing agent.

The Copper gains 2 electrons and so it is reduced and at the same time it is the oxidizing agent.

Writing RedOx reactionsWriting RedOx reactions

• There are two ways to deal with RedOx There are two ways to deal with RedOx reactions (balancing purposes):reactions (balancing purposes):– Treat them as any other reactionTreat them as any other reaction– We can write this reaction in terms of two We can write this reaction in terms of two

half-reactionshalf-reactions..

• Driving force for these type of reactions Driving force for these type of reactions is the exchange of electrons. is the exchange of electrons.

Types of Chemical Types of Chemical ReactionsReactions• Describing Oxidation-Reduction ReactionsDescribing Oxidation-Reduction Reactions


A half-reaction is one of the two parts of an oxidation-reduction reaction. One involves the loss of electrons (oxidation) and the other involves the gain of electrons (reduction).

e2)aq(Fe)s(Fe 2

)s(Cue2)aq(Cu2

oxidation half-reaction

reduction half-reaction

Types of Chemical Types of Chemical ReactionsReactions• Types of Oxidation-Reduction ReactionsTypes of Oxidation-Reduction Reactions


Most of the oxidation-reduction reactions fall into one of the following simple categories:

Combination Reactions

Decomposition Reactions

Displacement Reactions

Combustion Reactions

Types of Chemical Types of Chemical ReactionsReactions• Combination ReactionsCombination Reactions


A combination reaction is a reaction in which two substances, usually two elements, combine to form a third substance..

)(2)()(2 2 sNaClgClsNa Sodium and chlorine combine in a fiery reaction. (see Figure)

Types of Chemical Types of Chemical ReactionsReactions• Combination ReactionsCombination Reactions


Other combination reactions involve compounds as reactants.

)s(CaSO)g(SO)s(CaO 32

Types of Chemical Types of Chemical ReactionsReactions• Decomposition ReactionsDecomposition Reactions


A decomposition reaction is a reaction in which a single compound reacts to give two or more substances.

)g(N)g(OH4)s(OCr)s(OCr)NH( 22327224

Types of Chemical Types of Chemical ReactionsReactions• Displacement ReactionsDisplacement Reactions


)g(H)aq(ZnCl)aq(HCl2)s(Zn 22

A displacement reaction (also called a single- replacement reaction) is a reaction in which an element reacts with a compound, displacing an element from it.

Types of Chemical Types of Chemical ReactionsReactions• Combustion ReactionsCombustion Reactions


A combustion reaction is a reaction in which a substance reacts with oxygen, usually with the rapid release of heat to produce a flame.

)g(OH 10)g(CO 8)g(O 13)g(HC 2 222104

Types of Chemical Types of Chemical ReactionsReactions• Balancing Simple Oxidation-Reduction ReactionsBalancing Simple Oxidation-Reduction Reactions


At first glance, the equation representing the reaction of zinc metal with silver(I) ions might appear to be balanced.

)s(Ag)aq(Zn)aq(Ag)s(Zn 2

However, a balanced equation must have a charge balance as well as a mass balance.

Types of Chemical Types of Chemical ReactionsReactions• Balancing Simple RedOx ReactionsBalancing Simple RedOx Reactions


As mentioned before we can split the reaction into two half-cells before balancing. You will learn this method in Chem 102 when you cover chapter 20.

We will balance RedOx reactions using the Oxidation Number method.

Types of Chemical Types of Chemical ReactionsReactions• Balancing Simple RedOx ReactionsBalancing Simple RedOx Reactions

– There are some steps to follow to balance There are some steps to follow to balance these reactions.these reactions.•Assign ON to ALL elements in the reactionAssign ON to ALL elements in the reaction

• Identify the species that are oxidized/reducedIdentify the species that are oxidized/reduced

•Compute the number of eCompute the number of e--s lost in OX and gained s lost in OX and gained in RED draw lines between the two pairs including in RED draw lines between the two pairs including the number of ethe number of e--s lost/gaineds lost/gained

•Multiply one or both reactions so that both Multiply one or both reactions so that both numbers match, use these factors as balancing numbers match, use these factors as balancing coefficientscoefficients

•Balance any other species that were not involved Balance any other species that were not involved in the electron exchange. in the electron exchange.


Quantitative AnalysisQuantitative Analysis

• Analytical chemistry deals with the Analytical chemistry deals with the determination of composition of determination of composition of materials-that is, the analysis of materials-that is, the analysis of materials. materials. Quantitative analysis involves the

determination of the amount of a substance or species present in a sample of material.


• Gravimetric analysisGravimetric analysis is a type of is a type of quantitative analysis in which the quantitative analysis in which the amount of a species in a material is amount of a species in a material is determined by converting the determined by converting the species into a product that can be species into a product that can be isolated and isolated and weighed.weighed.Precipitation reactions are often used in

gravimetric analysis.

The precipitate from these reactions is then filtered, dried, and weighed.

Gravimetric Analysis


• Consider the problem of determining Consider the problem of determining the amount of lead in a sample of the amount of lead in a sample of drinking water.drinking water.


Adding sodium sulfate (Na2SO4) to the sample will precipitate lead(II) sulfate.

)s(PbSO)aq(Na2)aq(Pb)aq(SONa 42

42

The PbSO4 can then be filtered, dried, and weighed. This figure shows a similar procedure.


• Suppose a 1.00 L sample of polluted water Suppose a 1.00 L sample of polluted water was analyzed for lead(II) ion, Pbwas analyzed for lead(II) ion, Pb2+2+, by adding , by adding an excess of sodium sulfate to it. The mass an excess of sodium sulfate to it. The mass of lead(II) sulfate that precipitated was of lead(II) sulfate that precipitated was 229.8 mg. 229.8 mg. What is the mass of lead in a What is the mass of lead in a liter of the water?liter of the water? Express the answer as Express the answer as mg of lead per liter of solution.mg of lead per liter of solution.


)s(PbSO)aq(Na2)aq(Pb)aq(SONa 42

42

Quantitative AnalysisQuantitative AnalysisGravimetric Analysis

%32.68100g/mol 303.3g/mol 207.2

Pb%

• First we must obtain the mass percentage of lead First we must obtain the mass percentage of lead in lead(II) sulfate, by dividing the molar mass of in lead(II) sulfate, by dividing the molar mass of lead by the molar mass of PbSOlead by the molar mass of PbSO44, then multiplying , then multiplying by 100.by 100.

Then, calculate the amount of lead in the PbSO4 precipitated.

Pb mg 157.0 0.6832 4PbSO mg 229.8 sample in PbAmount

Quantitative AnalysisQuantitative AnalysisVolumetric Analysis

• An important method for determining the An important method for determining the amount of a particular substance is based amount of a particular substance is based on measuring the on measuring the volumevolume of the reactant of the reactant solution.solution.Titration is a procedure for determining the

amount of substance A by adding a carefully measured volume of a solution with known concentration of B until the reaction of A and B is just complete.

Volumetric analysis is a method of analysis based on titration.

Chemical ReactionsChemical Reactions

• Reactions often involve ions in aqueous solution. Reactions often involve ions in aqueous solution. Many of these compounds are electrolytes.Many of these compounds are electrolytes.

We can represent these reactions as molecular equations, complete ionic equations (with strong electrolytes represented as ions), or net ionic equations (where spectator ions have been canceled).

Most reactions are either precipitation reactions, acid-base reactions, or oxidation-reduction reactions.

Acid-base reactions are proton-transfer reactions.

Oxidation-reduction reactions involve a transfer of electrons from one species to another.

Summary


• Oxidation-reduction reactions are the most important type of reactions.Oxidation-reduction reactions are the most important type of reactions.• Oxidation-reduction reactions usually fall into the following categories: Oxidation-reduction reactions usually fall into the following categories:

combination reactions, decomposition reactions, displacement reactions, combination reactions, decomposition reactions, displacement reactions, and combustion reactions.and combustion reactions.

Molarity is defined as the number of moles of solute per liter of solution. Knowing the molarity allows you to calculate the amount of solute in a given volume of solution.

Quantitative analysis involves the determination of the amount of a species in a material.

Summary


In gravimetric analysis, you determine the amount of a In gravimetric analysis, you determine the amount of a species by converting it to a product you can weigh.species by converting it to a product you can weigh.

Summary

In volumetric analysis, you determine the amount of a species by titration.

Operational SkillsOperational SkillsUsing solubility rules.Using solubility rules.Writing net ionic equations.Writing net ionic equations.Deciding whether precipitation occurs.Deciding whether precipitation occurs.Classifying acids and bases as weak or strong.Classifying acids and bases as weak or strong.Writing an equation for a neutralization.Writing an equation for a neutralization.Writing an equation for a reaction with gas formation.Writing an equation for a reaction with gas formation.Assigning oxidation numbers.Assigning oxidation numbers.Balancing simple oxidation-reduction reactions.Balancing simple oxidation-reduction reactions.Calculating molarity from mass and volume.Calculating molarity from mass and volume.Using molarity as a conversion factor.Using molarity as a conversion factor.Diluting a solution.Diluting a solution.Determining the amount of a substance by Determining the amount of a substance by gravimetric analysis.gravimetric analysis.Calculating the volume of reactant solution needed.Calculating the volume of reactant solution needed.Calculating the quantity of a substance by titration.Calculating the quantity of a substance by titration.

DissociationDissociation

Return to Lecture

Electrolytic SolutionsElectrolytic Solutions

Negatively charged ions move towards

the positive electrode and

positively charged ions moves towards

the negative electrode

Return to Lecture

ConductivityConductivity

Since there are no “free” ions on non-electrolytic solutions there is no flow of energy and the bulb does not lit. The “free” ions on

electrolytic solutions completer the circuit and allow energy to flow lighting the bulb.

Return to Lecture

Weak vs Strong Weak vs Strong ElectrolytesElectrolytes

While both types of electrolytes conduct electricity the While both types of electrolytes conduct electricity the amount of “free” ions manifests itself by the brightness of amount of “free” ions manifests itself by the brightness of

the light emitted by the bulb.the light emitted by the bulb.Return to Lecture

Water Solvation of IonsWater Solvation of Ions

Return to Lecture

PrecipitationPrecipitation

Mixing KI (aq) Mixing KI (aq) and Pb(NOand Pb(NO33))2 2 (aq) (aq)

leading to leading to precipitation of precipitation of

PbIPbI22

Return to Lecture

Return to Lecture

Figure 4.7: Household acids and bases. Figure 4.7: Household acids and bases. Photo courtesy of American Color.Photo courtesy of American Color.

Return to Lecture

Figure 4.8: Figure 4.8: Preparation of red Preparation of red cabbage juice as an cabbage juice as an acid-base acid-base indicator.indicator.Photo courtesy Photo courtesy

of James Scherer.of James Scherer.

Return to Lecture

Neutralization ReactionNeutralization Reaction

Neutralization reaction between Acetic Acid (Vinegar) and Baking Soda (Sodium

Bicarbonate).

Making a SolutionMaking a Solution

a) Measure the mass of the compound to dissolve, b)make sure all solid makes it into the volumetric flask using the solvent, then dilute to the bottom of the neck, and c) add the last amount of solvent carefully making sure not to go

past the volumetric mark.

a) b) c)

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TitrationTitration

Titration of an acid with a base. a) the flask contains acid and a few drops of phenolphthalein, which is colorless in acidic conditions, b) the endpoint has

been reached (notice the faint pink color of the solution), finally in c), the solution is well beyond the end point since more

base was added.

a) b) c)

Return to Lecture

Electron ExchangeElectron Exchange

Electrons from the iron nail are transferred to the copper in solution, the solid copper plates the nail.

Notice the color change in the solution, this is due to the lower amount of copper ions in solution.

Return to Lecture

RedOx SummaryRedOx Summary

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Figure 4.13: A representation of an oxidation reduction reaction. Figure 4.13: A representation of an oxidation reduction reaction.

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Figure 4.14: Oxidation reduction reaction Figure 4.14: Oxidation reduction reaction of mercury (III) oxide into its elements. of mercury (III) oxide into its elements. Photo courtesy of James Scherer.Photo courtesy of James Scherer.

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Figure 4.15: Figure 4.15: Oxidation Oxidation reduction reaction reduction reaction of zinc metal and of zinc metal and hydrochloric acid. hydrochloric acid. Photo courtesy of American Photo courtesy of American

Color.Color.

Return to Lecture

Figure 4.16: Oxidation Figure 4.16: Oxidation reduction reaction of iron reduction reaction of iron wool and oxygen. wool and oxygen. Photo courtesy Photo courtesy

of James Schererof James Scherer. .

Return to Lecture

Calcium/Chlorine Calcium/Chlorine ReactionReaction

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Gravimetric AnalysisGravimetric Analysis

A solution of potassium chromate is poured down a stirring rod into a solution containing an unknown amount

of barium ion to precipitate Barium Chromate which is then filtered dried and weighed.

Return to Lecture

chemical reactions chapter 5 dr. victor vilchiz. types of chemical reactions acid-base reactions...

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