lect w10 abbrev_ thermochemistry_alg

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The orbitals of 2p electrons are often

represented as being?

1. elliptical2. tetrahedral3. dumbbell shaped4. spherical

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General Chemistry IIGeneral Chemistry IICHEM 152 Unit 3CHEM 152 Unit 3

Week 10

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Week 10 Reading Assignment

Chapter 6 – Sections 6.2 – 6.8 (thermochemistry)

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HYDROCHLORICACID

SODIUMHYDROXIDE

Make a Prediction

.

Make a Prediction

Will a reactiontake place?

.

HCl + NaOH NaCl + H2O

???????????

??????????

YES!!!!!

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What are the chances of the car just by itselfTurning into the car on the right? Will this reaction occur by itself?

2 Fe2O3 4 Fe + 3O2

CHEMICAL REACTIONS have a natural DIRECTION

How can we predict that direction???

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Which way will it go?• We’ve studied:• KINETICS or the SPEED of reactions• EQUILIBRIUM or the position of a reaction

when the forward and reverse rates are equal

• Now we’d like to PREDICT DIRECTIONALITY– Based on the stability of products versus

reactants (Stored energy within the bonds)

Energy is a key element in directionality

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During a chemical reaction chemical bonds are broken and new bonds are formed.

The chemical nature of the substances present in the system changes.

ENERGY IS ABSORBED OR RELEASED IN THE PROCESS

Thermochemistry

How can we determine

whether the process

requires or releases energy?

Macro

Micro

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SurroundingsSurroundings

SystemSystem

q > 0q > 0

heatheat

ENDOTHERMICENDOTHERMIC

The surroundings get coolerThe surroundings get cooler The surroundings The surroundings get warmerget warmer

Heat is transferred from surroundings to the

system

EXOTHERMICEXOTHERMIC

SurroundingsSurroundings

q < 0q < 0

heatheat SystemSystem

Heat is transferred from the system to the

surroundings

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1 1 ccalorie =alorie = energy required to raise the energy required to raise the

temperature of 1.00 g of H temperature of 1.00 g of H22O by 1.0 O by 1.0 ooC.C.

Units of Energy

The energy transferred between two systems with different

temperatures (heat) is measured using the same units

1000 cal 1000 cal == 1 1 kilocaloriekilocalorie == 1 kcal 1 kcal1 kcal 1 kcal == 1 1 CCaloriealorie (a food (a food

“calorie”)“calorie”)But we use the unit called the But we use the unit called the

JOULEJOULE1 cal 1 cal == 4.184 Joules (exactly) 4.184 Joules (exactly)

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Most chemical reactions occur at constant Most chemical reactions occur at constant pressure P, so heat is transferred at constant pressure P, so heat is transferred at constant

PP

qqpp = = HH where where H = EnthalpyH = Enthalpy

H H = enthalpy change= = enthalpy change= heat transferred at constant Pheat transferred at constant P

HEAT VS HEAT VS ENTHALPYENTHALPY

If If HH is positive ( is positive (H>0H>0))Process is Process is ENDOTHERMICENDOTHERMIC

If If H is negative (H is negative (H<0)H<0)Process is Process is EXOTHERMICEXOTHERMIC

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ExothermicH2(g) + ½ O2(g) H2O(l) H = H = - 285.8 - 285.8

kJ/molkJ/mol

H2 + ½ O2

PE

H2O(l)

Reactants

Products

ENERGYRELEASE

DH

In this case, products have a lower potential energy than reactants.

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Endothermic NH4NO3(s) + H2O(l) NH4

+(aq) + NO3-

(aq)

NH4NO3 + H2O

PE NH4

+ + NO3

-

Reactants

Products

ENERGYABSORBE

DH

In this case, products have a higher potential energy than reactants.

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Q (Calorimeter) = − Q (reaction)

How could we MEASURE the energy transferred?

Heat coming out of the reaction all goes into the water.

CALORIMETER

What happensto the temperatureof the water?

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0.0150 mol of a substance was combusted in a bomb

calorimeter.500.0 g of water in a

calorimeter had an increase of 4.90 ºC. What was the

energy change?Cp of water is 4.184 J/(g ºC)

What was the H of the combustion reaction in units of

kJ/mol?

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Energy is never lost so….

Estimating ΔH using lab experiments

H

Sn + Cl2

SnCl2 + Cl2

SnCl4

-325 kJ

-186 kJ

-511 kJ

Sn + Cl2 SnCl2 ΔH = -325 kJ

SnCl2 + Cl2 SnCl4 ΔH= -186 kJ

Sn + 2Cl2 SnCl4 ΔH = -511 kJ

Hess’ Law – If a reaction can be written as a series of steps, theThe ΔH of the overall reaction equals the sum of the the ΔH’sOf the steps.

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Calculate H for:

Ca(OH)2(aq) + 2HCl(aq) CaCl2(aq) + 2H2O(l)

using: CaO(s) + 2HCl(aq) CaCl2(aq) + H2O(l) H = ‒186 kJCaO(s) + H2O(l) Ca(OH)2(s) H = ‒62.3 kJCa(OH)2(s) Ca(OH)2(aq) H = ‒12.6 kJ

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The limiting reactant in a reaction can be recognized because it is the reagent

that?

1. Has the smallest coefficent in the balanced equation

2. Has the smallest mass in the reaction mixture

3. Is present in the smallest molar quantity

4. Would be used up first

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What if we can’t do experiments?

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Standard Enthalpy Standard Enthalpy ValuesValues

Most Most H values for a chemical reaction H values for a chemical reaction are labeled are labeled HHoo measured under measured under standard standard

conditionsconditions

P = 1 bar (close to 1 atmosphere)P = 1 bar (close to 1 atmosphere)T = usually 25 T = usually 25 ooC (298.15 K)C (298.15 K)

All species in standard statesAll species in standard statese.g., e.g., C = graphiteC = graphite and and OO22 = gas = gas

HHooff = standard molar enthalpy of = standard molar enthalpy of

formationformation

This is the enthalpy change when This is the enthalpy change when 1 mol of a compound1 mol of a compound is formed from is formed from elements under standard conditions.elements under standard conditions.

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HHooff, , Standard Enthalpy of Standard Enthalpy of

FormationFormationHH22(g) (g) + 1/2+ 1/2 O O22(g) (g) H H22O(l)O(l)

HHooff = -285.8 kJ/mol = -285.8 kJ/mol

HH22(g) (g) + + OO22(g) (g) H H22OO22(l)(l)

HHooff = -187.8 kJ/mol = -187.8 kJ/mol

H2O2(l) H2O(l) + ½ O2(g) ?

H2, O2

H2O(l)

H2O2(l)

PE

-285.8 kJ-187.8 kJ

By convention, By convention, HHoof f

= 0 = 0 for for elements elements in their in their

standard statesstandard states

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Substance Name Hof(kJ/mol)

CH4(g) Methane -74.8

CO(g) Carbon monoxide

-110.5

CO2(g) Carbon dioxide

-393.5

H2O(g) Water vapor -241.8

H2O(l) Liquid water -285.8

CH3OH(l) Methanol -238.7

C2H5OH(l) Ethanol -277.7

BaCO3(s) Barium carbonate

-1216.3

CaO(s) Calcium oxide -635.1

NH3(g) Ammonia -46.1

Molar Enthalpies of Formation

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Using Standard Enthalpy Using Standard Enthalpy ValuesValues

HH22O(g) O(g) ++ C(graphite) C(graphite) H H22(g) (g) ++ CO(g) CO(g)

From tables we findFrom tables we find

HH°ff of H of H22O vapor = - 241.8 kJ/molO vapor = - 241.8 kJ/mol

HH22(g) (g) ++ 1/21/2 O O22(g) (g) H H22O(g)O(g)

HH°ff of CO = - 110.5 kJ/mol of CO = - 110.5 kJ/mol

C(s) C(s) + 1/2+ 1/2 O O22(g) (g) CO(g) CO(g)

and we knowand we know HH°ff =0 for =0 for C(graph)C(graph) and and HH22(g)(g)

We can use We can use HHff°’s to calculate the heat of ’s to calculate the heat of reaction of different processes:reaction of different processes:

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In general, when In general, when ALLALL

enthalpies of enthalpies of formation are known, formation are known,

HHoorxnrxn = =

HHoof f (products) (products)

- - HHoof f (reactants)(reactants)

Calculate Calculate H of H of reaction?reaction?

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HHoorxnrxn = = HHoo

f f (products) (products)


HHoorxnrxn = = HHoo

ff(H(H22) + ) + HHooff(CO)(CO)

- - HHooff(H(H22O) – O) – HHoo

ff(C)(C)

HHoorxnrxn = = 0 +(–110.5 kJ) – (-241.8 kJ) – 00 +(–110.5 kJ) – (-241.8 kJ) – 0

HHoorxnrxn = = + 131.3 kJ+ 131.3 kJ (Endothermic) (Endothermic)

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HHoorxnrxn = = HHoo

f f (products) (products)


HHoorxnrxn = = + 131.3 kJ+ 131.3 kJ (Endothermic) (Endothermic)

If we find ∆H for the heats of formation

If we find ∆H from a calorimeter measuring the reaction

∆H = + 131.3 kJ

∆H is the same no matter what route we take.

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Nitroglycerin is a powerful explosive because it decomposes exothermically and four

different gases are formed:

2C3H5(NO3)3(l) 3N2(g)+½O2(g)+6CO2(g)+5H2O(g)

For liquid nitroglycerin Hof=-364.0 kJ/mol.

Calculate the energy transfer when 1 mole of nitroglycerin explodes

Substance Name Hof(kJ/mol)

CO2(g) Carbon dioxide

-393.5

H2O(g) Water vapor -241.8

H2O(l) Liquid water -285.8

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Using Bond Energy DataWe can estimate H by comparing the

amount of energy needed to separate atoms in the reactants (BREAKING BONDS) and the energy released in forming new compounds

(MAKING BONDS)

BOND BOND ENERGY ENERGY (kJ/mol)(kJ/mol) H—HH—H 436436 C—CC—C 346346 C=CC=C 602602 CCC C 835835 NNNN 945945

The energy required to break

a bond is a measure of

“bond strength”

The same energy is released in

making the bond.

Remember ∆H is a function of state

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Using Bond Energies

Bond energies are useful to estimate the energy required or generated by a chemical reaction:

2 H—O—O—H O=O + 2 H—O—H

How much How much

energy is energy is

released/absorbreleased/absorb

ed in this ed in this

process?process?How many bonds How many bonds

of each type are of each type are

broken/formed?broken/formed?

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Using Bond Energies 2 H—O—O—H O=O + 2 H—O—H

Energy required to break bonds:break 4 mol of O—H bonds = ??break 2 mol O—O bonds = ??

ENERGYENERGY to to break bondsbreak bonds = = 2124 2124

kJkJEnergy evolved on making bonds: make 1 mol of O=O bonds = ?? make 4 mol O—H bonds = ??

ENERGYENERGY evolved on evolved on making bondsmaking bonds = = --

2339 kJ2339 kJ

O=O O=O 499 kJ/mol499 kJ/mol

O—H O—H 460 kJ/mol460 kJ/molO—O O—O 142 kJ/mol142 kJ/mol

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Using Bond Energies

2 H—O—O—H O=O + 2 H—O—H

Net energy =+2124 kJ- 2339 kJ = - 215 kJ

More energy is More energy is evolved on making evolved on making

bonds than is used in bonds than is used in breaking bondsbreaking bonds

The reaction is exothermic!The reaction is exothermic!

or -215/2= -107.5 kJ/mol (compared to -98.0 kJ/mol)

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Using Bond EnergiesEstimate the energy needed or

generated by this chemical reaction H—H + Cl—Cl 2 H—Cl

H—H = 436 kJ/molH—H = 436 kJ/molCl—Cl = 242 kJ/molCl—Cl = 242 kJ/molH—Cl = 432 kJ/molH—Cl = 432 kJ/mol

Net energy = Net energy = HHoo= =

= energy required to break bonds = energy required to break bonds ++

energy evolved when bonds are energy evolved when bonds are mademade

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How many atoms are in 1.50 g of Al?

1. 0.05562. 18.03. 3.35 x 1022

4. 2.44 x 1025

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Using Bond EnergiesEstimate the energy needed or

generated by this chemical reaction H—H + Cl—Cl 2 H—Cl

HHo o = -186 kJ = -186 kJ

Notice that two homogeneous bonds break to form heterogeneous bonds

These tend to be exothermic

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Using Bond Energies

Which of these two molecules will generate more energy

during combustion?

Sucrose Stearic Acid

Oxygenated bonds require more energy to break than other types of

bonds (C-H)

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Does burning several logs release moreOr less energy than burning one log?

There is a relationship between the enthalpy of a reactionAnd the moles of material.

Given by the balanced equation

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Energy and Moles

What if I use up 6.00 moles of Oxygen in the following reaction?

How much energy is released?H2(g) + 1/2O2(g) H2O(l) ΔH= -286kJ

Conversion factor

6.00mol O2 ( -286kJ) = -3430 kJ

( ½ mol O2)

Proportions

6.00 mol O2 = X solving x = -3430kJ

½ mol O2 -286kJ

ΔH= -3430kJ OR +3430kJ are RELEASED

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Which Way does it go?

• If a reaction gives off heat (H = -), there is a tendency for the reaction to go forward (K > 1) – but that is not the complete story…

WHICH WAY?KCl(s) KCl(aq) H

= +And yet K > 1 – it goes

forward anyways

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Summary Activity

12C(s,graphite) + 11H2(g) + O2(g) C12H22O11(s)

11

2

No one has been able to get this reaction to run… so how could we get a value for the heat of formation of sucrose?We can combust sucrose: C12H22O11(s) + 12O2(g) 12CO2(g) + 11H2O(l)

The standard heat of combustion Hc° of sucrose is

–5640.9 kJ/mol.Can you now calculate the heat of formation of sucrose using Hess’s Law?Hfº(CO2(g)) = -393.5 kJ/mol

Hfº(H2O(l)) = -285.8 kJ/mol

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Summary Activity

Nitrogen monoxide has recently been found to be involved in a wide range of biological processes. The gas reacts with oxygen to give brown NO2 gas. 2NO(g) + O2(g) 2NO2(g)

Is the reaction endothermic or exothermic?

If 1.25 g of NO is converted completely to NO2, what quantity of heat is absorbed or released?

Hº = -114.1 kJM(NO) = 30.01 g/mol