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Slide 21-1

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 21-2

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 21-3

Highlights of last lecture

Gas Laws:

• Boyle’s Law: P ∝ 1/V

• Charles’ Law: P ∝ T

• Avogadro’s Law: P ∝ n

• IDEAL GAS LAW: PV = nRT

Units and R atm or kPa Lmol

K

R = 8.314 J K-1 mol-1, or

R = 0.0821 L atm K-1 mol-1

Slide 21-4

Thermochemistry

Themes: Fuels (and alternatives)

Environmental effects of fuel combustion

Key thematic concepts:

• Chemical basis of combustion;

• What makes a good fuel - a quantitative comparison;

• Strengths and weaknesses of existing and alternative fuels;

• Effects of combustion on the greenhouse effect and smog.

References: Blackman, Chap 8.1-8.3

or any general chemistry text will have a satisfactory chapter on thermochemistry.

Slide 21-5

Thermochemistry

Calculations:

• Heats of various reactions

• Thermodynamic cycles

• Calorimetry

Key chemical concepts:

• Energy,

• Enthalpy,

• Entropy,

• Hess’s Law,

• Heat capacity,

• Energy level diagrams.

Slide 21-6

Energy Units = Joule (kg m2s-2)

Daily energy requirement for typical human.

(10-15 x 106 J / day)

Annual Aust. household energy use

(360 x 1015 J / year)

Asian tsunami

(2 x 1021 J)

2

Slide 21-7

“Calories” vs. calories

1 calorie = 4.184 J (reported by Nicholas Clement in 1824)

= energy to heat 1 g of H2O by 1°C

1 “Calorie” = 1 kilocalorie = 1000 calories

Slide 21-8

Heat of combustion

� 45 g Sultana bran + O2 →

CO2 + H2O + mineral oxides + HEAT

� Internal energy of molecules (Eint)� kinetic energy (~T)

� vibrational and rotational energy (~T)

� bond energy

Slide 21-9

Weblink

� Calculate YOUR daily needs!

http://firstyear.chem.usyd.edu.au/calculators/food_energy.shtml

530Cal 590Cal 110Cal

108Cal

171Cal 100Cal

Slide 21-10

Chemical reactions and Energy

Some reactions require heat to be supplied

e.g. Simple dissociation reaction:

(important reaction in photochemical smog)

N2O4 (g) + heat → 2 NO2 (g)

N2O4

2 NO2heat

Eint

But note that the absolute Eint can’t be measured, only ∆∆∆∆Eint

Slide 21-11

Chemical Reactions and Energy

N2O4 (g) + heat → 2 NO2 (g)Slide 21-12

Chemical Reactions and Energy

Some reactions evolve heat

e.g. More complex reaction

(O-transfer reaction, important in ozone depletion)

O3 + NO → O2 + NO2 + heat

O3 + NO

O2 + NO2heat

Eint

3

Slide 21-13

System, surroundings & universe

� SYSTEM = the thing (reaction) we are interested in

� SURROUNDINGS = everything else

(usually, we only have to worry about things that can be affected by the system, e.g. in thermal contact)

� UNIVERSE = SYSTEM + SURROUNDINGS

Slide 21-14

Chemical Reactions and Energy

O3 + NO → O2 + NO2 + heat

N2O4 + heat → 2 NO2 exothermic

(gives out heat to surroundings)endothermic

(takes in heatfrom surroundings)

CaCl2 + water → Ca2+(aq) + 2 Cl−(aq) + heat

NH4NO3 + water + heat → NH4+(aq) + NO3

−(aq)

Slide 21-15

Chemical reactions and energy

reactants

productsheat

EintEint

products

heatreactants

Endothermic

∆Eint > 0

Exothermic

∆Eint < 0

The main change in internal energy in a chemical reaction is associated with bond energies.

Slide 21-16

Chemical reactions and energy

Note: difference in Eint = heat

∆Eint = q

Therefore - measuring heat ⇒ ∆Eint (bomb calorimetry)

- tabulating Eint ⇒ estimating heat

Kindergarten version of

“First Law of Thermodynamics”

Slide 21-17

Measuring Heat…

Heat versus TemperatureA “thought” experiment:

20ºC 20ºC

Full pan Almost empty pan

What is theDifference between

Heat and temperature?

Slide 21-18

Measuring Heat…

Heat versus TemperatureA “thought” experiment:

20ºC 20ºC

Full pan Almost empty pan

4

Slide 21-19

Measuring Heat…

Heat versus TemperatureA “thought” experiment:

What is theDifference between

Heat and temperature?

60ºC

Full pan Almost empty pan

100ºC

Slide 21-20

Heat Capacity (C and c)

So the more you have of a substance, the less the temperature will rise for a given input of heat…

q = c x ∆T

c is called the “heat capacity”

Similarly, different substances will change temperature by a different amount (e.g. copper vs ceramic)…

Slide 21-21

Heat Capacity (C and c)

q = c x ∆T

The “heat capacity” must depend on the type and amount of substance present – usually mass or moles. For pure substances we can define:

q = m c ∆T

q = n C ∆T

m = mass of substance (g)n = amount of substance (mol)c = specific heat capacity (J K-1 g-1 )C = molar heat capacity (J K-1 mol-1)

Slide 21-22

Specific Heat Capacities

Q: How much energy (heat) does it take to heat an Al saucepan (0.2 kg) containing 1L of water from room temp. (20ºC) to boiling? Assume thermal equilibrium between the pan and water at all times.

Q: Why is Desert hot during daytime, and cold during nighttime?

Q: Why might radiator antifreeze (ethylene glycol) be a bad idea in summer?

q = m1 C1 ∆T + m2C2∆T

= (m1C1 + m2C2 ) ∆T

= (1000 x 4.184 + 200 x 0.9) x 80

= 3.49 x 105 J (349 kJ)

Slide 21-23

Calorimetry

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

• thermally insulated from rest of universe

• usually used for combustion reactions

• must know the heat capacity of the calorimeter (surroundings)

Figure 8.8 Blackman Slide 21-24

Bomb Calorimetry

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

• System

•Surroundings

Figure 8.8 Blackman

5

Slide 21-25

Example problem

Q: A manufacturer claims that their dessert has “fewer than 50 kJ per serving”. The Consumer Affairs Department commissioned you to test the claim.

Your experiment:

You put one serving of the dessert into a bomb calorimeter (c = 8.15 kJ/K) and burnt it to completion in excess O2. The temperature increased by 4.94ºC. Is the claim justified?

q = c x ∆T

= 4.94 x 8.15 = 40.2 kJ => the claim is justified

Slide 21-26

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

• electrical

• light

• spring

• piston

We call these “work” (w)

First Law of Thermodynamics:

∆Eint = q + w

(⇒ next lecture)

Slide 21-27

Summary

CONCEPTS� Exothermic and endothermic processes� System, surroundings� Energy level diagrams� Heat and internal energy

CALCULATIONS� Bomb calorimetry� Heat capacity