Slide 22-1

Unless otherwise stated, all images in this file have been reproduced from:

Blackman, Bottle, Schmid, Mocerino and Wille,Chemistry, 2007 (John Wiley)

ISBN: 9 78047081 0866

Slide 22-2

Chem 1101

A/Prof Sébastien PerrierRoom: 351

Phone: 9351-3366

Email: s.perrier@chem.usyd.edu.au

Prof Scott KableRoom: 311

Phone: 9351-2756

Email: s.kable@chem.usyd.edu.au

A/Prof Adam BridgemanRoom: 222

Phone: 9351-2731

Email: a.bridgeman@chem.usyd.edu.au

Slide 22-3

Highlights of last lecture

•Concepts:•Exothermic and endothermic processes•Energy level diagrams•Heat•Bomb calorimeters•Heat capacity

•Calculations:

•Heat capacity

•Bomb calorimeter

Slide 22-4

Chemical reactions and energy

From last lecture: Difference in Eint = heat

∆Eint = q

Kindergarten version of

“First Law of Thermodynamics”

Slide 22-5

But!... There are other types of energy!

• electrical

• light

• spring

• piston

We call these “work” (w)

First Law of Thermodynamics:

∆Eint = q + w

(⇒ today)

Slide 22-6

Other types of energy…

In each case, which has higher internal energy?

Recognition that there are other types of energy

Slide 22-7

Other types of energy

Other types of energy:Type chemical example field equation

electrical battery electrochemistry ∆E = V I t

light “glow stick” photochemistry ∆E = h ν

spring mechanical engineering ∆E = ½ k x2

piston engine thermochemistry ∆E = -P ∆V

Exercise: Convince yourself in each case that the equation above has units of energy (J). You will need to look up unknown symbols and their units in any First Year Chem text.

[ Joule = kg m2 s-2 ]

Slide 22-8

Other types of energy…

What’s more… the energy changes in a chemical reaction are not confined to one type of energy:

Take a fully charged battery:1. Discharge through incandescent light bulb

2. Discharge through motor

3. Discharge through wire resistor

What kind of energy is produced in each case?

Slide 22-9

Other types of energy…

What’s more… the energy changes in a chemical reaction are not confined to one type of energy:

C8H18 (l)+ 12.5 O2 (g) → 8 CO2 (g) + 9 H2O (g)

∆Eint = q ∆Eint = q + w

Slide 22-10

Types of energy

Other types of energy:Type chemical example field equation

electrical battery electrochemistry E = V I t

light “glow stick” photochemistry E = h ν

spring mechanical engineering E = ½ k x2

piston engine thermochemistry E = -P ∆V

We will focus on –P∆V in this topic

Slide 22-11

Thermochemistry

So, only consider, for now, w = -P ∆Vleave until later…

Electrochem. (later this semester)

Photochemistry (2nd & 3rd year chem)

Mechanical work (engineering and physics)

So… ∆Eint = q + w

= q – P ∆V

In words… when energy changes in a reaction it produces (uses) heat and pressure/volume changes between reactants and products.

Slide 22-12

Enthalpy

Rearrange… ∆Eint + P ∆V = q ≡ ∆H

ENTHALPY or

HEAT OF REACTION∆H =

It is the experimentally observed heat change for a chemical reaction under conditions of constant pressure.Notice: under conditions of constant volume, ∆H =q =∆Eint

Slide 22-13

Enthalpy ∆Eint + P ∆V = q ≡ ∆H

Why is this so important?Many practical chemical reactions are performed under constant pressure, rather than constant volume conditions, for example:

• laboratory experiments in open containers

• biological reactions in living systems

• atmospheric reactions

• combustion reactions (except in closed system)

So, if you were to measure the heat change in the reaction, you would be measuring ∆H, not ∆Eint

Therefore 1. At constant P, calorimetry gives ∆H

2. Tables of ∆H predict heat change under exp’tal cond’ns

Additional Note: Even for gases, ∆H and ∆E are very close (-> problems)

Slide 22-14

More Calorimetry…

At constant pressure, the “coffee-cup”calorimeter measures the heat of reaction, ∆H.

• thermally insulated

• usually used for liquids

• esp. good for

• heat of dissolution

• heat capacity of solids

• aqueous reactions

Slide 22-15

Calorimetry (from last lecture)

At constant volume, the “bomb” calorimeter measures the internal energy change, ∆E

(∆V = 0 therefore

q = ∆Eint + P ∆V = ∆Eint )

• thermally insulated

• usually used for combustion reactions

• must know the heat capacity of the calorimeter.

Slide 22-16

Other types of energy…

Electrochemistry

Photochemistry

“Cold light”Experiment

“Lemon Battery”Experiment

Slide 22-17

Reactions for demos

Cold Light:

Lemon Battery: Zn (s) + 2H+ (aq) →→→→ Zn2+ (aq) + H2 (g)

Slide 22-18

Enthalpy of special reactions

Enthalpy of vaporisatione.g water evaporating

H2O(l) (25oC) → H2O(g) (25oC) ∆Hvap = +44.0 kJ mol-1

Enthalpy of combustion:e.g. BBQ fuel (butane)

C4H10 (g) + 6.5 O2 (g) → 4 CO2 (g) + 5 H2O (g) ∆Hc = −2877 kJ mol-1

Enthalpy of atomisation:

e.g. butane

C4H10 (g) → 4 C (g) + 10 H (g) ∆Hatom = +5544 kJ mol-1

Slide 22-19

Bond enthalpies

From Housecroft and Constable, p. 97

Slide 22-20

Bond enthalpies and ∆Hatom

The atomisation enthalpy is the sum of the individual bond enthalpies:

Q: What is ∆Hatom for methanol?

H

H C O H

H

3 x C-H = 1248

1 x C-O = 359

1 x O-H = 464

∆Hatom = 2071 kJ mol-1

Slide 22-21Adapted from Silberburg, p.369

Ent

halpy

(kJ

/mol)

∆Hatom to estimate ∆Hc

2 x C=O: 1612

4 x OH: 1856

Total: 3468

Difference

∆Hc = -820 kJ mol-1

CH4(g) + 2 O2(g) →→→→ CO2(g) + 2 H2O (g); ∆∆∆∆Hc = ?

4 x CH: 1652

2 x O=O: 996

Total: 2648

All units kJ mol-1

Slide 22-22

∆Hatom to estimate ∆Hc

In chemical language:kJ mol-1

CH4 (g) + 2O2 (g) → C (g) + 4 H (g) + 4 O (g) ∆H = 2648

C (g) + 4 H (g) + 4 O (g) → CO2 (g) + 2 H2O (g) ∆H = -3468

This is called HESS’S LAWIn words… If you add up chemical equations to form a new (overall) equation, then the overall enthalpy is the sum of the enthalpies.

Note: Using ∆Eatom to estimate reaction enthalpy is only approximate

CH4 (g) + 2O2 (g) →→→→ CO2 (g) + 2 H2O (g) ∆∆∆∆Hc = -820

Slide 22-23

Hess’s Law

Hess’s Law is one of the most important Laws in chemistry. It allows us to estimate thermodynamic quantities for reactions we haven’t (or can’t) measure.

It doesn’t only apply to atomisation: ∆H (kJ/mol)

CO (g) + ½ O2 (g) → CO2 (g) -283.0

NO (g) → ½ N2 (g) + ½ O2 (g) -90.3

Reaction in catalytic converter to remove NO and CO

CO(g) + NO(g) →→→→ CO2(g) + ½ N2(g) -373.3

Slide 22-24

Experimental aside…

Lasers are now used to measure chemical energies with outstanding accuracy.

In a photochemistry experiment, formaldehyde (H2CO) is excited by a laser. When the laser wavelength is shorter than 329.73 nm HCO and H are detected.

1- Write a chemical expression to describe the experiment.

H2CO + hν → H + HCO

2- Draw an energy level diagram to show what is happening in this experiment.

3- What is the C-H bond energy in formaldehyde (in kJ/mol)?

λ = 329.9 nm = 329.9×10-9 m

E = hc/λ = 6.626×10-34 × 3.00×108 / 329.9×10-9 = 6.02×10-19 J

4- The ozone layer prevents light with λ<295 nm reaching the Earth’s surface. Will formaldehyde by photolysed at the Earth’s surface by absorption of solar radiation?

The shortest wavelength solar radiation is λ≈295 nm

=> higher energy photon than the threshold wavelength of 329.9 nm.

=>Therefore formaldehyde will decompose in sunlight

Slide 22-25

30150 30200 30250 30300 30350 30400

Wavenumber (cm−1

)

-150 -100 -50 0 50

Eavail

(cm−1

)

Two types of spectra

absorption

HCO appearance

Reaction Threshold

2161 2143

Slide 22-26

Example questions

CONCEPTS� What enthalpy means� Concept of calorimetry, and differences between

constant P and constant V calorimetry� Chemical basis of Hess’s Law� Heat capacityCALCULATIONS� Heat capacity calculations � Work and heat calculations (using First Law)� Calorimetry problems (working out ∆H or ∆Eint)� Hess’s Law calculations� Estimating reaction enthalpy from bond energies.