Solutions Solution – Solution, A homogeneous mixture in which all of the material is in the same...

SolutionsSolutions

– SolutionSolution, A homogeneous mixture in which all of the material is in the same state.

– Substances present in lesser amounts, called solutessolutes, are dispersed uniformly throughout the substance in the greater amount, the solventsolvent

– Aqueous solution Aqueous solution — a solution in which the solvent is water

– Nonaqueous solution Nonaqueous solution — any substance other than water is the solvent

– Many of the chemical reactions that are essential for life depend on the interaction of water molecules with dissolved compounds.

Polar SubstancesPolar Substances An individual water molecule consists of two hydrogen atoms bonded to

an oxygen atom in a bent (V-shaped) structure.

The oxygen atom in each O–H covalent bond attracts the electrons more strongly than the hydrogen atom.

O and H nuclei do not share the electrons equally.

Aqueous SolutionsAqueous Solutions

– Hydrogen atoms are electron-poor compared with a neutral hydrogen atom and have a partial positive charge, indicated by the symbol δ+.– The oxygen atom is more electron-rich than a neutral oxygen atom and has a partial negative charge, indicated by the symbol 2δ-.

Unequal distribution of charge creates a polar bond, which makes them good solvents for ionic compounds.Individual cations and anions are called hydrated ions.

Aqueous SolutionsAqueous Solutions Electrolyte — any compound that can form ions

when it dissolves in water – When strong electrolytes dissolve,

constituent ions dissociate completely, producing aqueous solutions that conduct electricity very well.

– When weak electrolytes dissolve, they produce relatively few ions in solution. Aqueous solutions, of weak electrolytes do not conduct electricity as well as solutions of strong electrolytes.

– Nonelectrolytes dissolve in water as neutral molecules and have no effect on conductivity.

CHCH33COCO22HH(aq)(aq) → CH → CH33COCO22--(aq)(aq) +H +H++

(aq)(aq)

MolarityMolarity Most common unit of concentration Most useful for calculations involving the stoichiometry

of reactions in solution Molarity of a solution is the number of moles of solute

present in exactly 1 L of solution: Units of molarity — moles per liter of solution (mol/L),

abbreviated as M Relationship among volume, molarity, and moles is

expressed as:

mL

mmolL

molessolution of liters ofnumber

solute of moles ofnumber molarity

M

M

Aqueous SolutionsAqueous Solutions

Preparation of SolutionsPreparation of Solutions

Calculating Moles from VolumeCalculating Moles from Volume

Calculating Volume from Mass Calculating Volume from Mass

Problem: What mass of oxalic acid, HProblem: What mass of oxalic acid, H22CC22OO44, is required , is required

to make 250. mL of a 0.0500 M solution?to make 250. mL of a 0.0500 M solution?


0.250 L of water was used to make 0.250 L of solution. 0.250 L of water was used to make 0.250 L of solution. Notice the water left over.Notice the water left over.



(Vs) (Ms) = moles of solute = (Vd) (Md).

PROBLEM: You have 50.0 mL of 3.0 M NaOH and you PROBLEM: You have 50.0 mL of 3.0 M NaOH and you want 0.50 M NaOH. What do you do?want 0.50 M NaOH. What do you do?


Ion Concentrations in SolutionIon Concentrations in Solution Classify each compound as either a strong electrolyte or a

nonelectrolyte.

If a compound is a nonelectrolyte, the concentration is the same as the molarity of the solution.

If a compound is a strong electrolyte, determine the number of each ion contained in one formula unit and find the concentration of each species by multiplying the number of each ion by the molarity of the solution.

PROBLEM: Dissolve 5.00 g of NiClPROBLEM: Dissolve 5.00 g of NiCl22•6 H•6 H22O in enough water to O in enough water to

make 250 mL of solution. Calculate molarity of the solution make 250 mL of solution. Calculate molarity of the solution and the concentration of each of the ions.and the concentration of each of the ions.


• Zinc reacts with acids Zinc reacts with acids to produce Hto produce H22 gas. gas.

• Have 10.0 g of ZnHave 10.0 g of Zn• What volume of 2.50 What volume of 2.50

M HCl is needed to M HCl is needed to convert the Zn convert the Zn completely?completely?

SOLUTION STOICHIOMETRYSOLUTION STOICHIOMETRY

Zinc reacts with acids to produce HZinc reacts with acids to produce H22 gas. If you have 10.0 g of Zn, gas. If you have 10.0 g of Zn,

what volume of 2.50 M HCl is needed to convert the Zn completely?what volume of 2.50 M HCl is needed to convert the Zn completely?

SOLUTION STOICHIOMETRYSOLUTION STOICHIOMETRY

Limiting Reactants in SolutionsLimiting Reactants in Solutions The concept of limiting reactants applies to reactions that are

carried out in solution and reactions that involve pure substances.

If all the reactants but one are present in excess, then the amount of the limiting reactant can be calculated.

When the limiting reactant is not known, one can determine which reactant is limiting by comparing the molar amounts of the reactants with their coefficients in the balanced chemical equation.

Use volumes and concentrations of solutions of reactants to calculate the number of moles of reactants.

Ionic EquationsIonic Equations• Chemical equation for a reaction in solution can be written

in three ways:

1. Overall equation — shows all of the substances present in their undissociated form

2. Complete ionic equation — shows all of the substances present in the form in which they actually exist in solution

3. Net ionic equation – Derived from the complete ionic equation by omitting all

spectator ions, ions that occur on both sides of the equation with the same coefficients

– Demonstrate that many different combinations of reactants can give the same net chemical reaction

There are three ways to write reactions in aqueous solutions.

Molecular equation:Molecular equation: Show all reactants & products in molecular or ionic form

(s)(aq)4(aq)4(s) Cu + ZnSO CuSO + Zn Total ionic equation: Total ionic equation:

Show the ions and molecules as they exist in solution

Net ionic equation:Net ionic equation: Shows ions that participate in reaction and removes spectator ions. Spectator ions do not participate in the reaction.

(s)-2aq4

2aq

-2aq4

2aq(s) Cu +SO+ ZnSO+ Cu+Zn

(s)2aq

2aq(s) Cu + ZnCu + Zn

Ionic EquationsIonic Equations

MgMg(s)(s)+ 2HCl+ 2HCl(aq)(aq)→ H→ H2(g) 2(g) + MgCl+ MgCl2(aq)2(aq)

The molecular formula above can be written as the total ionic The molecular formula above can be written as the total ionic formulaformula

MgMg(s)(s)+ 2H+ 2H++(aq)(aq)+ 2Cl+ 2Cl--

(aq)(aq)→ H→ H2(g)2(g)+ Mg+ Mg2+2+(aq)(aq)+ 2Cl+ 2Cl--

(aq)(aq)

The two ClThe two Cl-- ions are ions are SPECTATOR IONS SPECTATOR IONS — they do not — they do not participate. Could have used NOparticipate. Could have used NO3-3- for the spectator ion as salts of for the spectator ion as salts of nitrates are all soluble.nitrates are all soluble.

By leaving out the spectator ions out you get the net ionic reactionBy leaving out the spectator ions out you get the net ionic reaction

MgMg(s)(s) + 2 H + 2 H++(aq)(aq) ---> H ---> H2(g)2(g) + Mg + Mg2+2+

(aq)(aq)

Net Ionic EquationsNet Ionic Equations

Classifying Chemical ReactionsClassifying Chemical Reactions

Aqueous Chemical Reactions: MetathesisAqueous Chemical Reactions: Metathesis• EXCHANGE REACTIONSEXCHANGE REACTIONS

AX + BY AY + BX

PrecipitationPrecipitationPb(NOPb(NO33))2(aq)2(aq) + 2 KI + 2 KI(aq) (aq) → PbI→ PbI2(s)2(s) + 2KNO + 2KNO3(aq)3(aq)

Pb2+(aq) + 2 I-(aq) → PbI2(s)Pb2+(aq) + 2 I-(aq) → PbI2(s)Neutralization:Neutralization:

NaOHNaOH(aq)(aq) + HCl + HCl(aq)(aq)→ NaCl→ NaCl(aq)(aq)+ H+ H22OO(l)(l)

OHOH--(aq)(aq) + H + H++

(aq)(aq) → H→ H22OO(lq)(lq)

Gas FormationGas FormationMgCOMgCO3(s)3(s)+ 2+ 2HCl(aq)(aq) → 2Mg(Cl) → 2Mg(Cl)2(aq)2(aq)+ H+ H22OO(l)(l) + CO2(g)

The anions exchange places between cations.The anions exchange places between cations.

Precipitation ReactionsPrecipitation Reactions• A reaction that yields an insoluble product, a

precipitate, when two solutions are mixed

• Are a subclass of exchange reactions that occur between ionic compounds when one of the products is insoluble

• Used to isolate metals that have been extracted from their ores and to recover precious metals for recycling

Water Solubility of Ionic CompoundsWater Solubility of Ionic CompoundsIf one ion from the “Soluble If one ion from the “Soluble Compound” list is present in a Compound” list is present in a compound, the compound is compound, the compound is water soluble.water soluble.

Precipitation ReactionsPrecipitation ReactionsIn addition to understanding solubility. It is equally important to know if a reaction will occur.

KBr(aq) + NaCl(aq) → KCl(aq) + NaBr(aq)

K2Cr2O7(aq) + AgNO3(aq) → 2KNO3(aq) + Ag2Cr2O7(s)

A reaction in which an acid and a base react in stoichiometric amounts to produce water and a salt

Strengths of the acid and base determine whether the reaction goes to completion 1. Reactions that go to completion

a.Reaction of any strong acid with any strong baseb.Reaction of a strong acid with a weak basec.Reaction of weak acid with a weak base

2. Reaction that does not go to completion is a reaction of a weak acid or a weak base with water

Neutralization ReactionsNeutralization Reactions

Neutralization ReactionsNeutralization ReactionsA brief history of A brief history of

Acid-Base Acid-Base Identification SystemsIdentification Systems

System Acid (HCl) Base (NaOH)

Arrhenius

Brönsted-LowryLewis

StrongStrong Brönsted-Lowry acids are strong electrolytes Brönsted-Lowry acids are strong electrolytesHClHCl hydrochlorichydrochloric

HH22SOSO44 sulfuricsulfuric

HClOHClO44 perchloricperchloric

HNOHNO33 nitricnitricHNOHNO33

A Brönsted-Lowry Acid → HA Brönsted-Lowry Acid → H++ in water in waterACIDSACIDS

Weak Weak Brönsted-Lowry Brönsted-Lowry acids are weak electrolytesacids are weak electrolytes

CHCH33COCO22H acetic acid (CHH acetic acid (CH33COOH) COOH)

HH22COCO3 3 carbonic acidcarbonic acid

HH33POPO4 4 phosphoric acidphosphoric acid

HF HF hydrofluoric acidhydrofluoric acid

Acetic acid

Carbonic Acid

• Acids differ in the number of hydrogen ions they can donate. – Monoprotic acids are compounds capable of

donating a single proton per molecule.

– Polyprotic acids can donate more than one hydrogen ion per molecule.

Polyprotic AcidsPolyprotic Acids

Ammonia, NHAmmonia, NH33 a an Important weak Base

Brönsted-LowryBrönsted-Lowry Base → OHBase → OH-- in water in water

BASESBASES

NaOH(aq) → Na+(aq) + OH-(aq)NaOH(aq) → Na+(aq) + OH-(aq)

NaOH is a strong baseNaOH is a strong base

ACIDSACIDSNonmetal oxides can be acidsNonmetal oxides can be acids

COCO2(aq)2(aq) + H + H22OO(l)(l) → H → H22COCO3(aq)3(aq)

SOSO3(aq)3(aq) + H + H22OO(l)(l) → H → H22SOSO4(aq)4(aq)

NONO2(aq)2(aq) + H + H22OO(l)(l) → HNO → HNO3(aq)3(aq)

Acid Rain is an example of nonmetal oxides behaving Acid Rain is an example of nonmetal oxides behaving as acids. This process can result from burning coal and as acids. This process can result from burning coal and oil.oil.

Metal oxides can be basesMetal oxides can be basesCaOCaO(s)(s)+H+H22OO(l)(l) → Ca(OH) → Ca(OH)2(aq)2(aq)

CaO in water. Phenolphthalein indicator shows a of calcium oxide solution is basic.

BASESBASES

pH, a Concentration ScalepH, a Concentration ScalepH: a way to express acidity -- the concentration of HpH: a way to express acidity -- the concentration of H++ in in

solution.solution.

Low pH: high [HLow pH: high [H++]] High pH: low [HHigh pH: low [H++]]

Acidic solutionAcidic solution pH < 7pH < 7 Neutral Neutral pH = 7pH = 7 Basic solution Basic solution pH > 7pH > 7

Acidic solutionAcidic solution pH < 7pH < 7 Neutral Neutral pH = 7pH = 7 Basic solution Basic solution pH > 7pH > 7

The pH ScaleThe pH ScalepH pH = log (1/ [H= log (1/ [H++]) = - log [H]) = - log [H++]]In a In a neutralneutral solution, solution, [H[H++] = ] =

[OH[OH--] = 1.00 x 10] = 1.00 x 10-7-7 M at 25 M at 25 ooCC

pH = - log [HpH = - log [H++] = ] =

If the [HIf the [H++] of soda is 1.6 x 10] of soda is 1.6 x 10-3-3 M, the pH is ____. M, the pH is ____.

If the pH of Coke is 3.12, it is _____.If the pH of Coke is 3.12, it is _____.

You should know the You should know the strong acids & basesstrong acids & bases

Acid-Base Strength Acid-Base Strength IdentificationIdentification

Oxidation-Reduction Reactions in Oxidation-Reduction Reactions in SolutionSolution

• Oxidation-reduction reactions — electrons are transferred from one substance or atom to another.

• Oxidation-reduction reactions that occur in aqueous solution are complex, and their equations are very difficult to balance.

• Two methods for balancing oxidation-reduction reactions in aqueous solution are:

1. Oxidation states — overall reaction is separated into an oxidation equation and a reduction equation

2. Half-reaction

The term oxidation was first used to describe reactions in which metals react with oxygen in air to produce metal oxides.

– Metal acquires a positive charge by transferring electrons to the neutral oxygen atoms of an oxygen molecule.

– Oxygen atoms acquire a negative charge and form oxide ions (O2-).

– Metals lose electrons to oxygen and have been oxidized—oxidation is the loss of electrons.

– Oxygen atoms have gained electrons and have been reduced—reduction is the gain of electrons.

Oxidation-Reduction ReactionsOxidation-Reduction Reactions

• Oxidation and reduction reactions are now characterized by a change in the oxidation states of one or more elements in the reactants.

• Oxidation states of each atom in a compound is the charge that atom would have if all of its bonding electrons were transferred to the atom with the greater attraction for electrons. Atoms in their elemental form are assigned an oxidation state of zero.

• Oxidation-reduction reactions are called redox reactions, in which there is a net transfer of electrons from one reactant to another. The total number of electrons lost must equal the total number of electrons gained.

• Oxidants and reductants– Oxidants – Compounds that are capable of accepting electrons are called oxidants, or

oxidizing reagents, because they can oxidize other compounds.• An oxidant is reduced in the process of accepting electrons.

– Reductants – Compounds that are capable of donating electrons are called reductants, or reducing agents, because they can cause the reduction of another compound.• A reductant is oxidized in the process of donating electrons.

Oxidation-Reduction ReactionsOxidation-Reduction Reactions

OXIDATION NUMBERSOXIDATION NUMBERS

NHNH33

ClOClO--

HH33POPO44

MnOMnO44- -

CrCr22OO772-2-

Recognizing a Redox ReactionRecognizing a Redox Reaction

2 Ag2 Ag(s)(s) + Cu + Cu2+2+(aq)(aq) → Al → Al++

(aq)(aq) + Cu + Cu(s)(s)

Thermite reactionThermite reaction

FeFe22OO3(s)3(s) + 2Al + 2Al(s)(s) → 2 Fe → 2 Fe(s)(s) + Al + Al22OO3(s)3(s)

Hydrogen Fuel CellHydrogen Fuel Cell

2 H2 H2(g)2(g) + O + O2(g)2(g) → 2H → 2H22OO(l)(l)

Balancing Redox Equations Using Balancing Redox Equations Using Oxidation StatesOxidation States

Will be covered Will be covered in Chem 102in Chem 102

Quantitative Analysis: TitrationsQuantitative Analysis: Titrations• Quantitative analysis — used to determine the amounts or

concentrations of substances present in a sample by using a combination of chemical reactions and stoichiometric calculations

• Titration – A method in which a measured volume of a solution of known concentration,

called the titrant, is added to a measured volume of a solution containing a compound whose concentration is to be determined (the unknown)

– Reaction must be fast, complete, and specific (only the compound of interest should react with the titrant)

– Equivalence point — point at which exactly enough reactant has been added for the reaction to go to completion (computed mathematically)

Acid-Base TitrationsAcid-Base Titrations Most common acids and bases are not intensely colored –

Rely on an indicator

Endpoint — point at which a color change is observed, which is close to the equivalence point in an acid-base titration

Standard SolutionsStandard Solutions A solution of a primary standard whose concentration

is known precisely A primary standard is non-hygroscopic, has a high

mass, is fairly inexpensive compound, is of known reactive ability that can be accurately weighed for use as a titrant.

A standard solution is used to determine the concentration of the titrant.

Accuracy of any titration analysis depends on accurate knowledge of the concentration of the titrant.

Most titrants are first standardized—their concentration is measured by titration with a standard solution.

TitrationTitration1. Add titrant solution from the 1. Add titrant solution from the

buret.buret.2. Reagent (base) reacts with 2. Reagent (base) reacts with

compound (acid) in solution compound (acid) in solution in the flask.in the flask.

3. Indicator shows when exact 3. Indicator shows when exact stoichiometric reaction has stoichiometric reaction has occurred.occurred.

4. Net ionic equation4. Net ionic equation HH++ + OH + OH-- → H → H22OO5. At equivalence point 5. At equivalence point moles Hmoles H++ = moles OH = moles OH--

ACID-BASE REACTIONSACID-BASE REACTIONSTitrationsTitrations

HH22CC22OO4(aq) 4(aq) + 2 NaOH + 2 NaOH(aq)(aq) → Na → Na22CC22OO4(aq)4(aq) + 2 H + 2 H22OO(l)(l)

acidacid basebase

Carry out this reaction using a Carry out this reaction using a TITRATIONTITRATION..

Oxalic acid,Oxalic acid,

HH22CC22OO44

PROBLEM: Standardize a solution of NaOH — i.e., accurately PROBLEM: Standardize a solution of NaOH — i.e., accurately determine its concentration. determine its concentration. 1.065 g of H2C2O4 (oxalic acid) requires 35.62 mL of NaOH for titration to an equivalence point. What is the concentration of the NaOH?

Quantitative Analysis: TitrationsQuantitative Analysis: Titrations

PROBLEM : Use standardized NaOH to determine the amount of an PROBLEM : Use standardized NaOH to determine the amount of an

acid in an unknown. Apples contain malic acid, Cacid in an unknown. Apples contain malic acid, C44HH66OO55. 76.80 g of . 76.80 g of

apple requires 34.56 mL of 0.663 M NaOH for titration. apple requires 34.56 mL of 0.663 M NaOH for titration.

What is weight % of malic acid? What is weight % of malic acid?

HOOCCHHOOCCH22COHCOOHCOHCOOH(aq)(aq) + 2NaOH + 2NaOH(aq)(aq) → Na → Na22CC44HH44OO5(aq)5(aq) + 2H + 2H22OO(l)(l)

Quantitative Analysis: TitrationsQuantitative Analysis: Titrations

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