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
Slide 22-1
Highlights of last lecture
Room: 222
Phone: 9351-2731
Slide 22-2
Email: [email protected]
Chemical reactions and energy
From last lecture: Difference in Eint = heat
•Concepts:
∆Eint = q
•Exothermic and endothermic processes
•Energy level diagrams
•Heat
•Bomb calorimeters
•Heat capacity
Kindergarten version of
“First Law of Thermodynamics”
•Calculations:
•Heat capacity
•Bomb calorimeter
Slide 22-3
But!... There are other types of energy!
• electrical
• light
• spring
Slide 22-4
Other types of energy…
Recognition that there are other types of energy
We call these
“work” (w)
• piston
First Law of Thermodynamics:
∆Eint = q + w
(⇒ today)
In each case, which has higher internal energy?
Slide 22-5
Slide 22-6
1
Other types of energy
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 x 2
piston
engine
thermochemistry
∆E = -P ∆V
What’s more… the energy changes in a chemical
reaction are not confined to one type of energy:
Take a fully charged battery:
1.
3. Discharge through wire resistor
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 ]
Discharge through incandescent light bulb
2. Discharge through motor
What kind of energy is produced in
each case?
Slide 22-7
Other types of energy…
Slide 22-8
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)
Other types of energy:
Type
chemical example
field
equation
electrical
battery
electrochemistry
photochemistry
E=VIt
E=hν
E = ½ k x2
E = - P ∆V
light
“glow stick”
spring
mechanical
engineering
piston
engine
thermochemistry
We will focus on –P∆V in this topic
∆Eint = q
∆Eint = q + w
Slide 22-9
Thermochemistry
Slide 22-10
Enthalpy
So, only consider, for now,
w = - P ∆V
leave until later…
Electrochem. (later this semester)
Photochemistry (2nd & 3rd year chem)
Mechanical work (engineering and physics)
Rearrange…
∆H =
So… ∆Eint = q + w
∆Eint + P ∆V = q ≡ ∆H
ENTHALPY or
HEAT OF REACTION
= q – P ∆V
In words… when energy changes in a reaction it produces
(uses) heat and pressure/volume changes between reactants
and products.
Slide 22-11
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 22-12
2
Enthalpy
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)
At constant pressure, the “coffee-cup”
calorimeter measures the heat of reaction, ∆H.
• thermally insulated
• usually used for liquids
So, if you were to measure the heat change in the reaction, you would be
measuring ∆H, not ∆Eint
Therefore
More Calorimetry…
∆Eint + P ∆V = q ≡ ∆H
• esp. good for
• heat of dissolution
• heat capacity of solids
1. At constant P, calorimetry gives ∆H
• aqueous reactions
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 22-13
Slide 22-14
Calorimetry (from last lecture)
At constant volume, the “bomb” calorimeter
measures the internal energy change, ∆E
Other types of energy…
Photochemistry
(∆V = 0 therefore
“Cold light”
Experiment
q = ∆Eint + P ∆V = ∆Eint )
• thermally insulated
Electrochemistry
• usually used for
combustion reactions
“Lemon Battery”
Experiment
• must know the heat
capacity of the calorimeter.
Slide 22-15
Slide 22-16
Reactions for demos
Enthalpy of special reactions
Cold Light:
Enthalpy of vaporisation
e.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.
Lemon Battery:
Zn (s) + 2H+ (aq) → Zn2+ (aq) + H2 (g)
Slide 22-17
butane
C4H10 (g) → 4 C (g) + 10 H (g)
∆Hatom = +5544 kJ mol-1
Slide 22-18
3
Bond enthalpies and ∆Hatom
Bond enthalpies
The atomisation enthalpy is the sum of the
individual bond enthalpies:
Q: What is ∆Hatom for methanol?
3 x C-H = 1248
H
H
C
O
H
H
From Housecroft and Constable, p. 97
Enthalpy (kJ/mol)
2 x O=O:
Total:
∆H c = ?
Slide 22-20
All units kJ
In chemical language:
kJ mol-1
mol-1
996
2648
∆Hatom = 2071 kJ mol-1
∆Hatom to estimate ∆Hc
CH4(g) + 2 O2(g) → CO2(g) + 2 H2O (g);
1652
1 x O-H = 464
Slide 22-19
∆Hatom to estimate ∆Hc
4 x CH:
1 x C-O = 359
2 x C=O:
1612
4 x OH:
1856
Total:
3468
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
CH4 (g) + 2O2 (g) → CO2 (g) + 2 H2O (g)
∆Hc = -820
This is called HESS’S LAW
In words… If you add up chemical equations to form a new (overall)
equation, then the overall enthalpy is the sum of the enthalpies.
Difference
Adapted from Silberburg, p.369
∆Hc = -820 kJ mol-1
Note: Using ∆Eatom to estimate reaction enthalpy is only approximate
Slide 22-21
Hess’s Law
Slide 22-22
Experimental aside…
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.
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.
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
CO(g) + NO(g) → CO2(g) + ½ N2(g)
-373.3
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
Reaction in catalytic converter to remove NO and CO Slide 22-23
Slide 22-24
4
Two types of spectra
Example questions
−1
Eavail (cm )
-150
-100
-50
0
50
2143
2161
absorption
HCO appearance
30150
30200
30250
30300
30350
30400
−1
Wavenumber (cm )
Reaction Threshold
Slide 22-25
CONCEPTS
What enthalpy means
Concept of calorimetry, and differences between
constant P and constant V calorimetry
Chemical basis of Hess’s Law
Heat capacity
CALCULATIONS
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.
Slide 22-26
5