1 energy & chemistry energy is the capacity to do work or transfer heat. heat is the form of...

1

Energy & ChemistryEnergy & Chemistry

ENERGY is the capacity to do work or transfer heat.

HEAT is the form of energy that flows between 2 objects because of their difference in temperature.

Other forms of energy —• light• electrical• kinetic and potential

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Energy & Chemistry

• Burning peanuts supply sufficient energy to boil a cup of water.

• Burning sugar (sugar reacts with KClO3, a strong oxidizing agent)

http://xbeams.chem.yale.edu/~batista/113/movies/06z01vd1.mov

http://xbeams.chem.yale.edu/~batista/113/movies/06m02vd1.mov

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Energy & Chemistry

• These reactions are PRODUCT FAVORED

• They proceed almost completely from reactants to products, perhaps with some outside assistance.

http://xbeams.chem.yale.edu/~batista/113/movies/06z01vd1.mov

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ThermodynamicsThermodynamics• Thermodynamics is the science of heat

(energy) transfer.

Heat energy is associated with molecular motions.

Heat transfers until thermal equilibrium is established.

http://xbeams.chem.yale.edu/~batista/113/movies/06s05vd1.mov

http://xbeams.chem.yale.edu/~batista/113/movies/06s10an.mov

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Directionality of Heat Transfer• Heat always transfer from hotter object to

cooler one.

• EXOthermic: heat transfers from SYSTEM to SURROUNDINGS.

T(system) goes downT(surr) goes up

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Directionality of Heat Transfer• Heat always transfer from hotter object to

cooler one.

• ENDOthermic: heat transfers from SURROUNDINGS to the SYSTEM.

T(system) goes upT (surr) goes down

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Consider the formation of water

H2(g) + 1/2 O2(g) --> H2O(g) + 241.8 kJ

USING ENTHALPYUSING ENTHALPY

Exothermic reaction — heat is a “product” and ∆H = – 241.8 kJ

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Making liquid H2O from H2 + O2 involves two exothermic steps.

USING ENTHALPYUSING ENTHALPY

H2 + O2 gas

Liquid H2OH2O vapor

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The Concept of Equilibrium

Chemical equilibrium occurs when a reaction and its reverse reaction proceed at the same rate.

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The Concept of Equilibrium

• As a system approaches equilibrium, both the forward and reverse reactions are occurring.

• At equilibrium, the forward and reverse reactions are proceeding at the same rate.

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A System at Equilibrium

Once equilibrium is achieved, the amount of each reactant and product remains constant.

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A System at EquilibriumRates become equal Concentrations become constant

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Depicting Equilibrium

In a system at equilibrium, both the forward and reverse reactions are running simultaneously. We write the chemical equation with a double arrow:

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What Does the Value of K Mean?

• If K >> 1, the reaction is product-favored; product predominates at equilibrium.

• If K << 1, the reaction is reactant-favored; reactant predominates at equilibrium.

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The Reaction Quotient (Q)

• To calculate Q, one substitutes the initial concentrations on reactants and products into the equilibrium expression.

• Q gives the same ratio the equilibrium expression gives, but for a system that is not at equilibrium.

17If Q = K,

the system is at equilibrium.

18If Q > K,there is too much product and the

equilibrium shifts to the left.

19If Q < K,there is too much reactant, and the

equilibrium shifts to the right.

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Le Châtelier’s Principle“If a system at equilibrium is disturbed by a change in temperature, pressure, or the concentration of one of the components, the system will shift its equilibrium position so as to counteract the effect of the disturbance.”

Systems shift from “Q” towards “K”.Stresses include:

temperature, pressure, volume, or concentration of a system will result in predictable and opposing changes in the

system in order to achieve a new equilibrium state.

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Equilibrium and

Le Chatelier’s Principle

22Chemical Equilibrium

Reversible Reactions:

A chemical reaction in which the productscan react to re-form the reactants

Chemical Equilibrium:

When the rate of the forward reaction equals the rate of the reverse reaction and the concentration of products and reactants remains unchanged

2HgO(s) 2Hg(l) + O2(g)

Arrows going both directions ( ) indicates equilibrium in a chemical equation

23LeChatelier’s Principle

When a system atequilibrium is placed

understress, the system willundergo a change in sucha way as to relieve thatstress.

Henry Le Chatelier

http://upload.wikimedia.org/wikipedia/commons/a/a6/Lechatelier.jpg

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When you take something away from a system at equilibrium, the system shifts in such a way as to replace what you’ve taken away.

Le Chatelier Translated:

When you add something to a system at equilibrium, the system shifts in such a way as to use up what you’ve added.

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LeChatelier Example #1

A closed container of ice and water at equilibrium. The temperature is raised.

Ice + Energy Water

The equilibrium of the system shifts to the _____ to use up the added energy.right

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A closed container of N2O4 and NO2 at equilibrium. NO2 is added to the container.

N2O4 (g) + Energy 2 NO2 (g)

The equilibrium of the system shifts to the ______ to use up the added NO2.left

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A closed container of water and its vapor at equilibrium. Vapor is removed from the system.

water + Energy vapor

The equilibrium of the system shifts to the _______ to replace the vapor.right

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A closed container of N2O4 and NO2 at equilibrium. The pressure is increased.

N2O4 (g) + Energy 2 NO2 (g)

The equilibrium of the system shifts to the _______ to lower the pressure, because there are fewer moles of gas on that side of the equation.

left

29What Happens When More of a Reactant

Is Added to a System?

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The Effect of Changes in Temperature

Co(H2O)62+

(aq) + 4 Cl(aq) CoCl4 (aq) + 6 H2O (l)

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Catalysts increase the rate of both the forward and reverse reactions.

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Equilibrium is achieved faster, but the equilibrium composition remains

unaltered.

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EnthalpyChemical energy is most commonly converted to heat, we use the symbol, ΔH to symbolize a change in energy available as heat. The symbol is sometimes read as “delta H” or an enthalpy change.

Representing Combustion Reactions

1.4 Potential Energy Diagrams

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1.4 Potential Energy Diagrams

Energy of Chemical BondsBond Energy

Endothermic

Exothermic

Representing the ΔH value associated with a chemical reaction.

Text Page 43

Review of Exothermic

► Reactants Ep is higher than Products Ep.

► Now, we must consider the activation energy (the energy needed so that the reactants bonds will break and reform to make product)

Review of Endothermic

► Reactants Ep is lower than Products Ep.

► Need to add more energy to the system for the forward reaction to take place.

► Still need to consider activation energy

Activated Complex

►Is the short-lived, unstable structure formed during a successful collision between reactant particles.

►Old bonds of the reactants are in the process of breaking, and new products are forming

►Ea is the minimum energy required for the activation complex to form and for a successful reaction to occur.

Fast and slow reactions

►The smaller the activation energy, the faster the reaction will occur regardless if exothermic or endothermic.

►If there is a large activation energy needed, that means that more energy (and therefore, time) is being used up for the successful collisions to take place.

Practice:

1. The following hypothetical reaction has an Ea of 120kJ and a ΔH of 80kJ

2a + B 2C + D Draw and label a potential energy diagram for this

reaction. What type of reaction is this? Calculate the activation energy for the reverse reaction. Calculate the ΔH for the reverse reaction.

2. Analyze the activation energy diagram below.

► What is the Ea for the forward reaction? For the reverse reaction?

► What is the ΔH for the forward reaction? For the reverse reaction?

► What is the energy of the activated complex?

Answer to #2► The activation energy (Ea) for the forward reaction is shown by (a):

Ea (forward) = H (activated complex) - H (reactants) = 400 - 100 = 300 kJ mol-1

The activation energy (Ea) for the reverse reaction is shown by (b):

Ea (reverse) = H (activated complex) - H (products) = 400 - 300 = 100 kJ mol-1

The enthalpy change for the reaction is shown by (c):

H = H (products) - H (reactants) = 300 - 100 = +200 kJ mol-1 for the forward and reverse reaction.

Concentration of Solutions► Molarity: number of moles of solute in one liter of

solution Symbol is “M” If you have 1 mole of NaOH (40g) dissolved in

enough solvent (water) to make 1 liter, you will have 1 M NaOH or one molar NaOH

Molarity (M) = amount of solute (moles) volume of solution (L)

► Molality: number of moles of solute per kilogram of solvent Symbol is “m” If you have one-half a mole of NaOH (20g)

dissolved in 1 Kg of water gives 0.5 m NaOH or one-half molal of NaOH

Molality (m) = moles solute mass of solvent (Kg)

Used in freezing point depression

Electrolyte: A solution or substance in solution consisting of various chemicals that can carry electric charges.

► Strong Electrolyte: Solutions conduct electricity well. Ionizes completely: contains many ions

because of complete dissociation. (HCl) NaCl (s) Na+

(aq) + Cl- (aq)

Na+

Na+

Na+

Cl-

Cl-

Cl-

► Non-Electrolytes: Solutions do not conduct electricity

► Does not ionize: substance exists as dissolved molecules in solution. Like sugar.

► Weak Electrolytes: Solutions conducts poorly Partially ionized: solution contains only a

few ions. Appears that only some of the

substance has dissociated or ionized. Equilibrium!!

CH3COOH (aq) CH3COO- (aq) + H+ (aq)

CH3COO-H+

CH3COOH

Acids and Bases► Arrhenius Acid: produces H+ ions, in aqueous

solution H+ and H3O+ are the same thing, H can’t really lose its

only e- HCl(g) + H2O(l) H3O+ (aq) + Cl- (aq)

► Arrhenius Base: produces OH- ions, in aqueous solution referred to as alkaline KOH(s) + H2O(l) K+(aq) + OH-(aq)

► B.L. Acid: molecule or ion that is a proton donor a proton is H+ HCl + NH3 NH4

+ + Cl-

► B.L. Base: molecule or ion that is a proton acceptor H2O + NH3 NH4

+ + OH-

Strength of Acids and Bases► Acids and Bases are considered strong if they

completely ionize to form strong electrolytes► Strong Acids: HCl, HNO3, H2SO4, HBr, HI, HClO4

they easily lose their hydrogen ion► Weak Acids: HF, HC2H3O2 (CH3COOH)

They want to keep their hydrogen until someone wants it more than them

► Strong Bases: those that are made up of hydroxide and group 1 or 2 metal.

► Weak Bases: substances that do not contain hydroxide, but rather generate hydroxide ions when reacting with water

►Strong acids and bases can be present in low concentrations

►To measure concentration of a solution of an acid we use Molarity, looking at the amount of H+ in a given volume.

►To measure strength of an acid we use pH, which measures the amount of H+ ions.

► pH of 4 could be a low concentration of a strong acid or a high concentration of a weak acid.

► Acid-base indicators are compounds whose colors are sensitive to pH. (like phenolphthalein)

► Indicators colors change in the presence of and acid or base depending on the indicator

► Some indicators change color at low pH and some at high pH

► Neutralization occurs when [H+] = [OH-]

Indicators

pH and pOHA neutral solution has a pH of 7. pH scale : ranges from 0 to 14, where 7 is

neutral. The lower the number, the more acidic (0 - 6) The higher the number, the more basic (8-14)

pH: the measure of aciditypOH: the measure of alkalinity or

how basic it ispH + pOH = 14pH = -log[H3O+]pOH = -log[OH-]In neutral solution [H3O+] = 1.0 x 10-7 In neutral solution [OH-] = 1.0 x 10-7

Chapter 8: Electron Configurations, Atomic Properties and the Periodic Table

52

Periodic Atomic Properties of the Elements

Periodic law states that certain sets of physical and chemical properties recur at regular intervals when the elements are arranged according to increasing atomic number

EOS

•Consider atomic radii: distance between the nuclei of two atoms

• The distance between the nucleus and the outer edge of the electron cloud


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Ionic Radii

EOS

•The ionic radius of each ion is the portion of the distance between the nuclei occupied by that ion•If the size of an atom is determined by the outermost electrons, what happens if you remove or add an electron?


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Definitions► Ionization energy is the energy

required to remove an electron from a ground state atom in the gaseous state• to remove an electron, energy must be supplied to

overcome the attraction of the nuclear charge (endothermic, always +)

► Electron affinity is the energy change that occurs when an electron is added to a gaseous atom How much an atom ‘likes’ Electrons (+ or -)

►the more negative it is the higher the EA

►Electronegativity is the ability of an atom in a molecule to attract atoms to itself.

Take note: these refer to atoms Electronegativity refers to molecules

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Zeff & Shielding

The order in which electrons are assigned to subshells in an atom, as well as other properties are because of effective nuclear charge (Zeff)

Shielding: electrons closer to the nucleus screen or shield the effect of nuclear charge on valence electrons

►the number of shielding electrons increases when you reach the end of the period and go on to the next period.

►Shielding increases in steps as you start a new period or go down a group

►Video


http://www.youtube.com/watch?v=phkcJo-94pQ

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Atomic Radii

► Atomic Radii decrease as atomic numbers increase in an given period (going across). A proton and electron are added so the

effective nuclear charge increases because each proton has more of an effect than each additional electron

► As that attraction between the nucleus and electrons increases, and the atomic radius decreases

► Atomic Radii increase going down In going from top to bottom of a group, the

valence electrons are assigned to orbitals with increasingly higher values of n (prin. Quantum number)

► The underlying electrons requires some space, so the electrons of the outer shell must be further (Your are adding energy levels)

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Atomic Radii

►Zeff effective nuclear charge: the nuclear charge experienced by a particular electron in a multi-electron atom

Adding protons Increases the attraction of the nucleus and pulls the

electron cloud closer to the nucleus resulting in a smaller atomic radius (coulomb’s Law the force between two charged particles is related to the product of their charges and the distance between them.)

►Atomic radii of transition metals trend a little differently►Exceptions in atomic radii also exist in the lanthanide and actinide series because of how the f subshells are uniquely filled by electrons

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Transition Metal Atomic Trends

► From left to right across a period, the radii initially decrease, then size remains almost the same, then slightly increases toward the end.

► The small increase in atomic radii is because of the d subshell is filled with electrons and thus the ele-eletron repulsions cause the size to increase

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Atomic Radii Properties

► The increased number of energy levels (n) increases the distance over which the nucleus must pull and therefore reduces the attraction for electrons

► Full energy levels provide shielding between the nucleus and valence electrons, so you see an increase in shielding as the level gets full EOS

Illustration

http://wps.prenhall.com/wps/media/objects/1054/1079855/media/AABTHAM0.html

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Ionic RadiiCations are smaller than the atoms from which they are formed

– the nucleus attracts the remaining electrons more strongly

EOS

Anions are larger than the atoms from which they are formed

– the greater number of electrons repel more stronglyThink of the proton/electron ratio, -as electrons are lost, the ratio of p+/e- increases and so the electrons are held closer vv.


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Isoelectronic Configurations

Isoelectronic species are elements that all have the same number of electronsFor isoelectronic species, the greater the nuclear charge, the smaller the species

EOS

Effective nuclear charge

http://wps.prenhall.com/wps/media/objects/4974/5093961/emedia/ch08/AABTHAC0.html


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Atomic and Ionic Radii

EOS


63

Ionization Energy

Ionization energy is the energy required to remove an electron from a ground state atom in the gaseous state

• to remove an electron, energy must be supplied to overcome the attraction of the nuclear charge (endothermic, always +)Continual removal of electrons increases

ionization energy greatlyIt gets harder with each electron removedBecause the nucleus has a stronger pull

B B+ + e– I = 801 kJ mol–1B+ B+2 + e– I = 2427 kJ mol–1B+2 B+3 + e– I = 3660 kJ mol–1B+3 B+4 + e– I = 25,025 kJ mol–1

EOS

B+4 B+5 + e– I = 32,822 kJ mol–1

Illustration

http://wps.prenhall.com/wps/media/objects/1054/1079855/media/AABTGZI0.html

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Ionization energy►First ionization energy- energy is increased

with each successive removal because the electron is being removed from an increasingly positive ion The remaining electrons are held more tightly Notice the large jump at the 3rd level for Mg. There is a large increase as you remove electrons

from lower (inner) energy subshells


65

First Ionization Energies

EOS

Illustration

http://www.lsua.us/chem1001/Chap6Movies/aabthap0.html


66

Ionization energy

►Ionization energy increases as atomic number increases in any given period Zeff increases the attraction of the nucleus and holds

the electrons more tightly►Exceptions: group II to III, IE drops because the p electrons

do not penetrate the nuclear region as well as s electrons so aren’t as tightly held

►Drop in IE also occurs between V & VI because of increased repulsion created by the first pairing of electrons, that is stronger than the increase in Zeff, lowering the energy required to remove the electron


67

Ionization energy

►Ionization energy decreases as atomic number increases down a column or group The increased number of energy levels (n) increases the

distance over which the nucleus must pull, reducing the attraction for electrons

A full energy level provides some shielding between the nucleus and valence electrons


68

Electron Affinity

Electron affinity is the energy change that occurs when an electron is added to a gaseous atom-How much an atom ‘likes’Electrons (+ or -)-the more negative it is the higher the EA

(energy is flowing out of the system)

EOS

Electron affinities are expressed as negative because the process is exothermic

Illustration

http://wps.prenhall.com/wps/media/objects/1054/1079855/media/aabthae0.html

69

Electronegativity

►A measure of the attraction of an atom for the pair of outer shell electrons in a covalent bond with another atom Pattern is same as electron affinity for same reasons Both are attraction nucleus has for electrons, one in

forming an ion (EA) and one in forming a molecule (EN) Fluorine is the most electronegative. The closer it is to

fluorine, the more electronegative it is.


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Metals, Nonmetals, and Metalloids

Metals have a small number of electrons in their valence shells and tend to form positive ions

Except for hydrogen and helium, all s-block elements are metals

EOS

All d- and f-block elements are metals


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Metals, Nonmetals, and Metalloids

Atoms of a nonmetal generally have larger numbers of electrons in their valence shell than do metals, and many tend to form negative ions

Nonmetals are all p-block elements and include hydrogen and helium

EOS

Metalloids have properties of both metals and nonmetals


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Metals

►Metals react by losing electrons A loosely held electron will result in a more reactive metal This is tied directly to ionization energy With an increased # of energy levels (n), comes increased

distance from the nuclear attraction and thus a more loosely held electron available for reactions


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Non-metals

►Non-metals tend to gain electrons, a strong nuclear attraction will result in a more reactive non-metal

►This means that an atom with the highest Zeff and the least number of energy levels should be the most reactive nonmetal (F) because its nucleus exerts the strong pull


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A Summary of Periodic Trends

EOS

1 energy & chemistry energy is the capacity to do work or transfer heat. heat is the form of...

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thermal equilibrium

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