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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: [email protected]
Prof Scott KableRoom: 311
Phone: 9351-2756
Email: [email protected]
A/Prof Adam BridgemanRoom: 222
Phone: 9351-2731
Email: [email protected]
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