chemistry 130 dr. john f. c. turner 409 buehler hall [email protected]

Chemistry 130

Chemistry 130

Dr. John F. C. Turner

409 Buehler Hall

[email protected]

Chemistry 130

Ch. 12: Properties of Solutions Ch. 13: Chemical EquilibriumCh. 15: Acid-base equilibria Ch. 16: Solution Equilibria Ch. 17: Thermodynamics

Spring Break

Ch. 18: Electrochemistry Ch. 19: Nuclear Chemistry Ch. 20 – 22: Descriptive Chemistry of the elementsCh. 23 – 24: Organic, biological, medicinal and materials chemistry

Course coverage

Chemistry 130

Grading Scale Lecture course: 80%6 quizzes: ~25%3 exams: ~25%Final Examination: 30%

Laboratory course: 20%

Grade Cut-offs:A: 80%-100% B+: 78%-79.9% B: 70%-77.9%

C+: 68%-69.9% C: 60%-67.9% D: 50%-59.9%F: 0%-49.9%

There is no curve; grading is on an absolute scale.

There are no dropped quizzes or examinations.

Course mechanics

Chemistry 130

Additional Help

The tutorial center, in Buehler Hall, opposite the General Chemistry office on the 5th floor, is staffed from 9.00 a.m. until 5 p.m. daily during the semester.

All Chem 130 students are invited to the three review sessions on

Monday, Tuesday and Wednesday 7 p.m. – 10 p.m. Buehler 511.

Office hours are held on Monday from 2 – 5 p.m. at Buehler 409.

Course mechanics

Chemistry 130

The first half of the course will cover:

Properties of Solutions Solution EquilibriaChemical Equilibrium ThermodynamicsAcid-base equilibria Electrochemistry

Course overview and context

The key to understanding all of these areas is the study of thermodynamics

Chemistry 130

Properties of Solutions Solution EquilibriaChemical Equilibrium ThermodynamicsAcid-base equilibria Electrochemistry

All of these topics are very important:

On a practical level:

“Life takes place in solution” - the human body is ~70% water and fluids in the body are solutions

Many technologically important materials are solution:

Steel Palladium-hydrogen


Chemistry 130

Steel is a complicated system, the majority components of which are iron and carbon.

Phase diagram for Fe-Fe3C

Steel

There are many phases and transitions between phases, with different compositions.

Transformation temperatures are also different between different phases.

Chemistry 130

Palladium will absorb very large quantities of H2 to form a gas-

solid solution.

This is a very important process for the storage of hydrogen and the so-called 'hydrogen economy'

(nothing to do with cold fusion........)

Palladium-hydrogen

H2

Chemistry 130

Science is also a powerful description of the physical world. It is a fundamentally important cultural construction.

It is new and it is always wrong and it is always right simultaneously.

The Scientific Method

A natural law is a sum of human experience in light of experiment, theory and hypothesis

It is predictive and testable in all aspects

The material in Chem 130 addresses one fundamental property of the description of the physical world – change and the nature of change


Chemistry 130

Chemistry is the unique science that contains all the elements of a mathematically rigorous science.

It also contain elements of art, in terms of creativity

Chemists change the local structure of the universe, in principle at will.

The definition of chemistry:

“That branch of physical science and research, which deals with the several elementary substances, or forms of matter, of which all bodies are composed, the laws that regulate the combination of these elements in the formation of compound bodies, and the various phenomena that accompany their exposure to diverse physical conditions.”


Chemistry 130

Change is an essential aspect of chemistry and chemical change takes place within those natural laws that are well understood.

Chemical change is one of the fundamental mechanisms that limit or enable the compass of our lives.

Chemistry 130 is partially about the restrictions and allowances that govern chemical change.


Chemistry 130

Many changes are spontaneous; many are not. Some hypothetical changes have never been observed.

Why?

Chemical change involves a change in the number of particles that we observe:

In both of these reaction, we see the reactants disappear and the products appear.

What are the rules that govern the destruction and creation, i.e. the chemical interchange of forms of matter?


H2C=CH2 Br2 BrCH2CH2Br

NaOHaq HClaq NaClaq H2Ol

Chemistry 130

In Chem 120, we focused on heat and heat flows that are observed in the course of a chemical reaction.

We described these heat flows as either

The internal energy, U, at constant volume

or

The enthalpy, H, at constant pressure

Both of these extensive properties are descriptions of the system only


Chemistry 130

However, even though these chemical changes are occurring locally, after we have performed these reactions, there is a little less bromine or sodium hydroxide and a little more 1,2,dibromoethane and sodium chloride in the universe than before.

We must consider the universe, from a local perspective (in order not to be too grandiose.....)


H2C=CH2 Br2 BrCH2CH2Br

NaOHaq HClaq NaClaq H2Ol

Chemistry 130

Chemistry 130

Chemistry 130

Physical Propertiesof Solutions


409 Buehler Hall

[email protected]

Chemistry 130

Solutions are extremely important chemical systems in which many reactions and physical phenomena take place.

Atmospheric chemistry often, but not always, takes place in solution

Solutions, solvents and solutes

FormationO2g h 2Og

O2g Og O3g

DestructionO3g Og 2O2g

Chemistry 130

Solutions, solvents and solutes

A solution is defined as a homogenous mixture of two or more components

The major component is called the solvent

The other, minor components are called the solutes

Solutions can occur in the solid state, the liquid state or the gaseous state

The pure components can be solid, liquid or gas prior to forming the solution

Chemistry 130

Example 1: Sodium in liquid ammoniaSolute SolventSolution

lowhigh

concentrationconcentration

low conductivity metallic conductivity

Nas NH3l

Chemistry 130

Example 2: Potassium permanganate in waterSolute SolventSolution

Very powerful oxidant

KMnO4 s H2O l

Chemistry 130

Energetics of solution

The dissolution of Na in liquid ammonia and KMnO4 in water both

involve solids that are strongly bonded.

For Na, m.p. /°C: ~97b.p. /°C: ~883

resulting in a liquid range of 785 K

KMnO4 decomposes before it melts

The forces between particles in both Na and KmnO4 are strong

So why do they dissolve?

Chemistry 130

Energetics of solution: Intermolecular forces

When considering the solubility or insolubility of a potential solute in a solvent, the first thing to consider are the intermolecular forces

There are forces between all atoms in a condensed state i.e. in the liquid or solid state.

A perfect gas has no forces between particles and cannot be liquified or solidified; however, all gases can be condensed at some temperature

He Ne Ar Kr Xe Rn

m.p./ °C [-272.2] -248.59 -189.3 -157.36 -111.7 -71 b.p. / °C -268.93 -246.08 -185.8 -153.22 -108 -61.7

Increasing polarizability

Chemistry 130

Energetics of solution: Intermolecular forcesEnergetics of solution: Intermolecular forcesEnergetics of solution: Intermolecular forces

All molecules exert weak attractions on one another due to the mutual attraction of nuclei and electrons. These attractive forces are only significant at very short distances.

At such small distances the intermolecular repulsion of the electrons on different atoms is also significant.

The electrons contained in all atoms and molecules can be perturbed by an electric field, with greater or lesser ease. This property is called polarizability.

The electron cloud around an atom or molecule can give an instantaneous dipole any time that the electrons are not distributed perfectly symmetrically.

Such a dipole can induce dipoles in other species nearby.

Chemistry 130


The polarizability of a molecule depends on the size of the atoms and how strongly the electrons are bound within the atom. At any instant, the electron density can fluctuate and the distribution can become instantaneously asymmetric.

This instantaneous and transient dipole can induce a similar dipole in a neighboring atom or molecule, resulting in a weak attraction

Chemistry 130


The attractive forces between an instantaneous dipole and an induced dipole are called London dispersion forces after the physicist Fritz London.

These forces are stronger for more polarizable species.

Some molecules have a permanent dipole because of differences in electronegativities among the atoms.

Such molecules experience dipole-dipole forces.

All molecules experience dispersion forces and induced dipoles, and polar molecules also experience dipole-dipole forces.

Chemistry 130


The intermolecular forces that we have seen

London dispersion forces

induced dipole interactions

dipole-dipole interactions

have a strong effect on the boiling points of liquids.

Chemistry 130

Energetics of solution: Intermolecular forces

Another type of intermolecular force is the hydrogen bonding interaction.

Hydrogen bonding is an additional type of bonding interaction that requires a hydrogen atom on one molecule and a source of electron density on another molecule, usually a lone pair.

Hydrogen bonding can be intramolecular as well as intermolecular.

It can have a profound effect on the properties of a material when it is present.

Chemistry 130

Boiling points of covalent hydrides

Energetics of solution: Hydrogen bonding

Chemistry 130

The second-row hydrides NH3, H2O, and HF exhibit much higher boiling points that would be expected based on their molecular weights. Strong hydrogen bonding between the molecules is responsible for the large liquid phase range of these compounds.

.CH4 has a low boiling point because it has no lone pairs to form strong hydrogen bonds.


Chemistry 130


Chemistry 130

(a) CS2

(b) CH3OH

(c) CH3CH2OH

(d) H2O

(d) C6H5NH2


Chemistry 130

PV=nRT

In the gas phase, we can adjust the perfect gas equation

to account for these intermolecular forces in a partially empirical manner; the resulting equation is termed the Van der Waals equation:

which describes the behavior of a non-ideal gas and includes both attractive and repulsive components.

We cannot write a similarly simple equation for the liquid state

[Pa nV

2] V−bn=nRT


Chemistry 130

If we imagine two materials, A and B, which are non-ideal, then in the pure material, there will be intermolecular forces between both sets of particles

In A In B

but until they are mixed, there are clearly no forces between A and B

When they mix, the forces present in the mixture will be

and

i.e. some of the A and B particles interact


FAA FBB

FAA FBBFBA and F AB

Chemistry 130

It is the balance of forces that, in part, controls the solubility properties of solvents and solutes.

If the forces between particles are exactly and precisely equal, i.e.

then there is no change in the enthalpy or the volume

and this is termed an ideal solution.

An ideal solution is one where the intermolecular forces between solute particles, solvent particles and solute-solvent particles are identical. In this case,


FBA = FAB = FAA = FBB

Hmix = 0 Vmix = 0

Hmix = 0 Vmix = 0

Chemistry 130

Ideal solutions are extremely rare, if they exist at all. For non-ideal solutions, which are those where the forces between particles are different in the pure materials and the solution, there are three regimes.

A. The forces in the pure materials are very much stronger than those is the solution.

In this case, the enthalpy of mixing is prohibitively high and endothermic and the solution does not form spontaneously

B. The forces in the pure materials are very much weaker than in the solution

In this case, the enthalpy of mixing is large and negative and therefore exothermic. The solution forms spontaneously


Chemistry 130

The third regime is where the forces in the pure materials are only slightly stronger than in the solution.

In this case, the solution forms but the reaction in endothermic but still spontaneous.

This is an example that shows us that consideration of enthalpy is not sufficient to explain the formation of the solution.

It is analogous to the expansion of a perfect gas; as there are no forces between particles in a perfect gas, then the energy of the system is independent of the separation of the particles.

The energy is then independent of the volume and there is no reason, based on the internal energy of the system, for an ideal gas to expand


Chemistry 130

Chemistry 130

Chemistry 130

Physical Propertiesof Solutions


409 Buehler Hall

[email protected]

Chemistry 130

Prior to the formation of a solution containing A and B, one of which is the solute and one of which is the solvent, intermolecular forces exist in A and B

Forces in A:

Forces in B:

After mixing and the formation of the solution, the forces that are present are:

Forces in solution:


FAA

FBB

FBA , FAB , FAA and FBB

Chemistry 130

When a solution is formed, the relative magnitudes of the respective forces is important. Using a Hess cycle,

is negative, whereas , the enthalpy of formation of solution from the gas phase, is positive. It is the balance between these two factors that is important


A l B lAB

Ag B l Ag Bg

HvapA

HvapB

H f , solution

Hmix

Hmix = H vapA H vap

B H f , solution

HvapA or B H f , solution

Chemistry 130

is the enthalpy of vaporization for either A or B and is the energy required to form a gas at infinite separation from liquid or solid A or B..

Clearly, we must supply enough energy to overcome the intermolecular forces that are present in either A or B


HvapA or B

is the enthalpy of formation of the solution from A and B and therefore must be related to the energy released when the intermolecular interactions between A and B occur.

H f , solution

Al Bl AB

Ag Bl Ag Bg

HvapA

HvapB

H f ,solution

Hmix

Hmix = H vapA H vap

B H f , solution

Chemistry 130

Many ionic solids will dissolve in water to form solutions that will conduct electricity.

We write this type of solution formation as

In this reaction, solid sodium fluoride is reacting with water and both sodium and fluoride ions become solvated, denoted by '(aq)' as the state.

Both water and NaF are polar but it is the hydration of the ions that provides the energetic counterbalance to the large lattice energy of NaF.

Energetics of ionic solutions

NaF s H2O l Naaq+ Claq

-

H solNaF = 0.91 kJ mol−1

H fNaF = −576.6 kJ mol−1

UlatticeNaF = 915 kJ mol−1

Chemistry 130


NiCl2 s H2O l Ni aq2+ 2Cl aq

-

[Ni OH26]2+

The formation of bonds between the metal ion and the water molecules is sometimes so strong that the complex ion can be isolated.

Solutions of NiCl2 in water contain

the hexaquo ion

Most cations in water are solvated by between 4 and 6 water molecules; larger cation by up to 12 water molecules.

These bonds are strong and help to offset the lattice energy of an ionic compound, allowing it to dissolve.

[Ni OH26]2+

Chemistry 130


NiCl2 s H2O l Niaq2+ 2Claq

-

[Ni OH26]2+

The Hess cycle for the dissolution of an ionic compound in solution is easy to write down:

NiCl2 s H2O l

Ni g2+ 2Cl g

- H2O l

U lattice

[Ni OH26]2+

2Claq-

HhydNi2+

2 HhydCl -

Chemistry 130

Miscibility and partial solubility

Most materials are only partially soluble.

Systems that form a homogenous mixture over all compositions are termed miscible

Examples of fully miscible materials are

Water-EtOH

Ne-He

Benzene-hexane

Cu-Zn

Chemistry 130


Most materials are only partially soluble. When a system has attained the maximum possible solubility at a given temperature, we say that it is saturated.

In a saturated solution, the solution is in contact with the solid and there is a dynamic equilibrium that exists between the solid solute and the dissolved solute.

Although there is no net dissolution of the solute in a saturated system, the solute is precipitating and dissolving at the same rate.

There is chemical exchange between the dissolved solute and the solid solute, although the net change is zero – no more will dissolve.

Chemistry 130


Solubility also varies with temperature.

Almost all substances become increasingly soluble with temperature.

When a solution that is saturated at high temperature is allowed to cool, the solute will crystallize as the saturated concentration at low temperature is less than at high temperature.

This is a common method for purifying substances, as usually, only the pure material will crystallize.

Chemistry 130

Solubility of gases

The solubility of gases depends on temperature and the gas pressure of the system.

The temperature dependence of the concentration of a gas is given by

where is the concentration at temperature and is the enthalpy of solution for the gas.

The pressure dependence of the solubility is simpler:

where is the pressure of the gas and k is a constant.

lnc2

c1

= − Hsol

R 1T2

− 1T1

c1 T1 H sol

c = kPgas

Pgas

Chemistry 130

Vapor pressure

All pure liquids are in equilibrium with the vapor; the boiling point of a liquid is the temperature at which the vapor pressure is equal to the external pressure on the liquid.

The solvent in a solution also possesses a well-defined vapor pressure that is directly related to the composition of the solution by Raoult's Law.

Here, is the pressure of the solvent in the solution is the pressure of the pure solvent.

is the mole fraction, which is the ratio of the number of moles of solvent to the total number of moles of all components.

Psolv = xsolvPsolvo

Psolv Psolvo

xsolv

xi =ni

∑j

n j

Chemistry 130

Vapor pressure

Raoult's Law applies to any volatile component in the system and applies strictly only to ideal solutions. For strongly non-ideal solutions, it can be used when the solution is very dilute.

For two liquids, A and B, that have vapor pressures and when pure, the pressure above the liquid will be given by

As long as , then one component will be present in greater amount than the other.

Repeated condensation of the vapor, followed by boiling will increase this fraction and eventually, the two components can be separated. This is the basis for distillation.

Psolv = xsolvPsolvo

PAo PB

o

P = xA PAo xB PB

o

PAo≠PB

o

Chemistry 130

Raoult's Law example

Chloroform and carbon tetrachloride are both miscible and can be considered to be ideal. What is the pressure and the composition of the vapor of a solution consisting of 1 mole of carbon tetrachloride and 2 moles of chloroform? PCCl4

o =16.5 kPa

PHCCl3

o =28.5 kPa

Chemistry 130


Chloroform and carbon tetrachloride are both miscible and can be considered to be ideal. What is the pressure and the composition of the vapor of a solution consisting of 1 mole of carbon tetrachloride and 2 moles of chloroform?

1. Calculate the mole fractions:

PCCl4

o =16.5 kPa

PHCCl3

o =28.5 kPa

xi =ni

∑j

n j∑j

nj = nHCCl3 nCCl 4

= 12 = 3

nHCCl3=

23

;nCCl 4=

13

Chemistry 130



2. Calculate the individual pressures, using Raoult's Law

PCCl4

o =16.5 kPa

PHCCl3

o =28.5 kPa

PCCl4= xCCl4

PCCl4

o =13

× 16.5 kPa = 5.5 kPa

PHCCl3= xHCCl3

PHCCl3

o = 23

× 28.5 kPa = 19 kPa

P = xA PAo xB PB

o

Chemistry 130



3. Calculate the total pressure

Answer: the pressure of the solution = 24.5 kPa

PCCl4

o =16.5 kbar

PHCCl3

o =28.5 kbar

PCCl4= xCCl4

PCCl4

o = 5.5 kPa

PHCCl3= xHCCl3

PHCCl3

o = 19 kPa

Ptotal = PHCCl3 PHCCl3

= 5.5 19 kbar = 24.5 kPa

P = xA PAo xB PB

o

Chemistry 130



4. Calculate the composition of the vapor

PCCl4

o =16.5 kPa

PHCCl3

o =28.5 kPa

In the vapor phasePtotal=24.5 kPa

xCCl4=

PCCl4

Ptotal

= 5.524.5

= 0.224

xHCCl3=

PHCCl3

Ptotal

= 1924.5

= 0.776

P = xA PAo xB PB

o

Chemistry 130


In this example, if the vapor were condensed, the resulting liquid would have the composition

and further vaporization and condensation cycles would enrich it further in the more volatile component, in this case .

Eventually, a complete separation would be achieved and we would have accomplished a fractional distillation.

xCCl4= 0.224

xHCCl3= 0.776

HCCl3

Chemistry 130

Colligative properties

Just as the vapor pressure of a solution deviates from that of the pure solvent by virtue of the presence of the solvent, so other properties of the solution also are changed when a solute is present.

In particular, the freezing point and the boiling points are depressed and elevated respectively.

Depression of the freezing point and elevation of the boiling point are known as colligative properties; they depend only on the number of particles of the solute and not their nature.

Chemistry 130

Colligative properties

In a dilute solution, the depression of the freezing point of the solution is given by

and the elevation of the boiling point by

In both cases, m is the molality of the solution – the concentration in mol kg-1

T f = T fsolution − T f

solvent = − K f m

Tb = Tbsolution − T b

solvent = K bm

K f is the cryoscopic constant for the solvent

Kb is the ebullioscopic constant for the solvent

chemistry 130 dr. john f. c. turner 409 buehler hall [email protected]

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