Chem 1101
A/Prof Sébastien Perrier
Room: 351
Phone: 9351-3366
Email: [email protected]
Prof Scott Kable
Unless otherwise stated, all images in this file have been reproduced from:
Room: 311
Blackman, Bottle, Schmid, Mocerino and Wille,
Chemistry, 2007 (John Wiley)
ISBN: 9 78047081 0866
Phone: 9351-2756
Email: [email protected]
A/Prof Adam Bridgeman
Room: 222
Phone: 9351-2731
Slide 21-2
Email: [email protected]
Slide 21-1
Highlights of last lecture
Thermochemistry
Themes:
Gas Laws:
Fuels (and alternatives)
Environmental effects of fuel
combustion
• Boyle’s Law: P ∝ 1/V
• Charles’ Law: P ∝ T
Key thematic concepts:
• Avogadro’s Law: P ∝ n
• Chemical basis of combustion;
• IDEAL GAS LAW: PV = nRT
Units and R
atm or kPa
• What makes a good fuel - a quantitative comparison;
K
L
• Strengths and weaknesses of existing and alternative fuels;
mol
• Effects of combustion on the greenhouse effect and smog.
R = 8.314 J K-1 mol-1, or
R = 0.0821 L atm K-1 mol-1
Slide 21-3
References: Blackman, Chap 8.1-8.3
or any general chemistry text will have a satisfactory
chapter on thermochemistry.
Slide 21-4
Energy Units = Joule (kg m2s-2)
Thermochemistry
Key chemical concepts:
Asian tsunami
•
Energy,
•
Enthalpy,
•
Entropy,
•
Hess’s Law,
Annual Aust.
household energy use
•
Heat capacity,
(360 x 1015 J / year)
•
Energy level diagrams.
(2 x 1021 J)
Daily energy
requirement for typical
human.
Calculations:
•
Heats of various reactions
•
•
Thermodynamic cycles
Calorimetry
(10-15 x 106 J / day)
Slide 21-5
Slide 21-6
1
“Calories” vs. calories
Heat of combustion
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
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 21-7
Slide 21-8
Weblink
Chemical reactions and Energy
Some reactions require heat to be supplied
Calculate YOUR daily needs!
e.g.
Simple dissociation reaction:
(important reaction in photochemical smog)
http://firstyear.chem.usyd.edu.au/calculators/food_energy.shtml
530Cal
590Cal
N2O4 (g) + heat → 2 NO2 (g)
110Cal
108Cal
Eint
2 NO2
heat
171Cal
100Cal
But note that the
absolute Eint can’t be
measured, only ∆Eint
N2O4
Slide 21-9
Chemical Reactions and Energy
Slide 21-10
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
Eint
O3 + NO
heat
N2O4 (g) + heat → 2 NO2 (g)
Slide 21-11
O2 + NO2
Slide 21-12
2
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 21-13
Chemical reactions and energy
products
Eint
reactants
(gives out heat
to surroundings)
endothermic
(takes in heat
from surroundings)
CaCl2 + water → Ca2+(aq) + 2 Cl−(aq) + heat
NH4NO3 + water + heat → NH4+(aq) + NO3−(aq)
Slide 21-14
Chemical reactions and energy
Kindergarten version of
“First Law of Thermodynamics”
products
∆Eint < 0
∆Eint > 0
exothermic
∆Eint = q
Exothermic
Endothermic
N2O4 + heat → 2 NO2
reactants
heat
heat
O3 + NO → O2 + NO2 + heat
Note: difference in Eint = heat
The main change in internal energy in a chemical
reaction is associated with bond energies.
Eint
Chemical Reactions and Energy
Therefore
- measuring heat ⇒ ∆Eint (bomb calorimetry)
- tabulating Eint ⇒ estimating heat
Slide 21-15
Measuring Heat…
Slide 21-16
Measuring Heat…
Heat versus Temperature
Heat versus Temperature
A “thought” experiment:
A “thought” experiment:
What is the
Difference between
20ºC
20ºC
Heat and
temperature?
Full pan
Almost emptySlide
pan
21-17
20ºC
Full pan
20ºC
Almost emptySlide
pan
21-18
3
Heat Capacity (C and c)
Measuring Heat…
What is the
Difference
between
Heat versus
Temperature
Heat and
A “thought” experiment:
temperature?
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”
60ºC
100ºC
Similarly, different substances will change temperature
by a different amount (e.g. copper vs ceramic)…
Full pan
Almost emptySlide
pan
21-19
Heat Capacity (C and c)
Specific Heat Capacities
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
Slide 21-20
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 21-21
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 = 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)
Q: Why is Desert hot during
daytime, and cold during nighttime?
Q: Why might radiator antifreeze
(ethylene glycol) be a bad idea in
summer?
Calorimetry
Slide 21-22
Bomb Calorimetry
At constant volume, the “bomb” calorimeter
measures the internal energy change, ∆E
• thermally insulated from
rest of universe
At constant volume, the “bomb” calorimeter
measures the internal energy change, ∆E
• System
• usually used for
combustion reactions
•Surroundings
• must know the heat
capacity of the calorimeter
(surroundings)
Figure 8.8 Blackman
Slide 21-23
Figure 8.8 Blackman
Slide 21-24
4
Example problem
But!... There are other types of energy!
Q: A manufacturer claims that their dessert has
“fewer than 50 kJ per serving”. The Consumer Affairs
Department commissioned you to test the claim.
• light
• spring
We call these
“work” (w)
• piston
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
• electrical
First Law of Thermodynamics:
∆Eint = q + w
(⇒ next lecture)
=> the claim is justified
Slide 21-25
Slide 21-26
Summary
CONCEPTS
Exothermic and endothermic processes
System, surroundings
Energy level diagrams
Heat and internal energy
CALCULATIONS
Bomb calorimetry
Heat capacity
Slide 21-27
5