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Transcript of Chemistry 30 – Unit 1 Thermochemical Changes To accompany Inquiry into Chemistry PowerPoint...
Chemistry 30 – Unit 1Thermochemical Changes
To accompany Inquiry into Chemistry
PowerPoint Presentation prepared by Robert [email protected]
Preparation Info
• Systems: Open, closed, and isolated - definitions
• First Law of Thermodynamics – Total energy of the universe is constant (energy can’t be created or destroyed)
• Second Law of Thermodynamics – In the absence of energy input, a system becomes more disordered (its entropy increases)
Preparation
• Meaning?
• A system at lower temperature will be more ordered as the particles have less average kinetic energy
• Two systems in thermal contact will transfer energy such that the more ordered (cooler) one gains energy and becomes more disordered
• Consequence: heat always flows from hotter systems to cooler ones
Preparation
Important Definitions:• Thermal Energy: the total kinetic
energy of all particles of a system
• Temperature: a measure of the average kinetic energy of the particles of a system
• Heat: a transfer of thermal energy between 2 systems
Chapter 9, Section 9.1Questions:
• Which has more thermal energy, a hot cup of coffee or an iceberg?
• Which has a larger average thermal energy, a hot cup of coffee or an iceberg?
• If an iceberg and a hot cup of coffee come into contact, in which direction will heat flow?
Preparation
• Heat energy transferred will be related to the temperature change of the system
• It takes different amounts of heat energy to change the temperature of
1 g of a substance by 1°C
• This number is called the specific heat capacity, c, and is measured in units of:
Jg C
• Water has a c value of
• This means that it takes 4.19 J of heat to raise the temperature of 1 g of water by 1°C
• Water has a very large c compared to most other common substances
4.19 Jg C
Preparation
• To determine the amount of heat transferred the formula used is
• Despite what your text says on page 337, I would always take ∆t as positive
• If heat is absorbed, temperature of surroundings will decrease; if heat is released temperature of surroundings will increase
• Examples: Practice Problems 1 and 4, page 337
Q mc t
Preparation
• Practice Problem 1, page 337
• Since 1 J is such a small amount of heat energy I start my questions in kJ as shown above
• If necessary I move into MJ or GJ
0.100 2.44 25 6.1JQ mc t kg C kJgk
k C
Preparation
• Practice Problem 4, page 337
• Putting kilo top and bottom cancels out and c stays the same
• The substance is granite
• Worksheet: WS 43 (Nelson) then BLM 9.1.1 (back only)
4.9370.790 0.790
0.25000 25.0
Q mc t
kJQ J Jc g Ck
kk g Cm t g C
Preparation
Chapter 9, Section 9.1
• Energy changes in chemical reactions crucial to life
• Not just in photosynthesis, fuels, and batteries, but in the very way that your body metabolizes food and makes the energy available for life processes
• Thermodynamics: the study of energy and energy changes
Chapter 9, Section 9.1
• Recall the first law of thermodynamics: ∆Euniverse= 0
• If a system loses energy, the surroundings gain energy (get warmer)
• If a system gains energy, the surroundings lose energy (get cooler)
∆Esystem = - ∆Esurroundings
Chapter 9, Section 9.1• Energy types:
• Kinetic energy, Ek, energy of motion of particles of a system
• Temperature is a measure of the average Ek of the particles of a system
• Potential energy, Ep, stored energy, usually in chemical bonds
Chapter 9, Section 9.1• Transfer of Ek: heat flows from hotter objects
to cooler ones (Preparation section of notes)
• Breaking bonds always requires energy (endothermic); forming bonds always releases energy (exothermic)
• Chemical reaction:breaking bonds + energy1 forming bonds + energy2
• If energy1 > energy2, reaction is endothermic
• If reverse is true, it is exothermic
• Worksheet BLM 9.1.3
input output
Chapter 9, Section 9.1• New term: enthalpy (not entropy)
• Enthalpy (change), ∆H: the difference in potential energy between reactants and products, measured at constant pressure – measured in kJ (or MJ, etc)
• Molar Enthalpy (change), ∆rH: the enthalpy change for 1 mole of a specified substance – measured inkJ/mol (or MJ/mol etc)
• In common usage the word change gets left out
Chapter 9, Section 9.1• Negative ∆H’s are exothermic
(think lose heat) and temperature of surroundings increases
• Positive ∆H’s are endothermic (think gain heat) and temperature of the surroundings decreases
• Note: this increase → negative, and decrease → positive is a stumbling block for many students
Chapter 9, Section 9.1• Chemical reactions can be written using
∆H notation:
C6H12O6(s) + 6 O2(g) 6 CO2(g) + 6 H2O(l) ∆H=-2802.5 kJ
4 NO(g) + 6 H2O(g) 4 NH3(g) + 5 O2(g) ∆H=+906 kJ
• They can also be written with the heat as a term in the equation:
C6H12O6(s) + 6 O2(g) 6 CO2(g) + 6 H2O(l) + 2802.5 kJ
4 NO(g) + 6 H2O(g) + 906 kJ 4 NH3(g) + 5 O2(g)
Do ∆H Worksheet!
value for the reaction as written
Chapter 9, Section 9.1• Potential energy diagrams for the
same 2 reactions are shown below:
∆H = -2802.5 kJ
H (
kJ)
C6H12O6(s) + 6 O2(g)
6 CO2(g) + 6 H2O(l)
reactants
products
H (
kJ)
4 NO(g) + 6 H2O(g)
4 NH3(g) + 5 O2(g)reactant
s
products
∆H = +906 kJ
Chapter 9, Section 9.2• Recalling that breaking bonds always
endothermic and forming new bonds is always exothermic, more complete Ep diagrams might be shown as follows:
Endothermic Exothermic
reactants
intermediate
products
ΔH
Ep (
kJ)
reactants
intermediate
products
ΔH
Ep (
kJ)
Chapter 9, Section 9.1• Alternate forms of potential energy diagram
(from Chemistry 30 Diploma Exam Bulletin)
Chapter 9, Section 9.1• Example: Practice Problem 3, page 346a) C(s) + 2 H2(g) CH4(g) + 74.6 kJ
b) C(s) + 2 H2(g) CH4(g) ∆H = -74.6 kJ
c)
H (
kJ)
C(s) + 2 H2(g)
CH4(g)
products
reactants
∆H = -74.6 kJ
Do Ep diagrams for formation of Cr2O3(s), simple decomp* of AgI(s), and formation of SO2(g)
Chapter 9, Section 9.2
Formation of Cr2O3(s)
Ep (kJ)
reaction coordinate
2 Cr(s) + 3/2 O2(g)
Cr2O3(s)
ΔH=ˉ1139.7 kJ
Ep (kJ)
reaction coordinate
ΔH=+61.8 kJ
simple decomposition of AgI(s)
AgI(s)
Ag(s) + ½ I2(s) Ep (kJ)
reaction coordinate
ΔH=ˉ296.8 kJ
1/8 S8(s) + O2(g)
SO2(g)
formation of SO2(g)
Chapter 9, Section 9.1• Molar enthalpy of combustion: the
enthalpy change for the complete combustion of 1 mol of a substance
• Complete combustions of fossil fuels always yields CO2(g) and H2O
• Open systems – constant pressure – gases escape – H2O(g)
• Isolated systems – H2O(l)
• Human body – cellular respiration - H2O(l)
Chapter 9, Section 9.1• Table of Molar Enthalpies of Combustions
of alkanes, page 347
• Practice Problem 5b, page 347 (open system)
OR: note change in units!
• In thermodynamics it is acceptable to write equations with fractional coefficients – don’t do this elsewhere
• Try question 5a, page 347
C4H10(g) + 13/2 O2(g) 4 CO2(g) + 5 H2O(g) ∆H = -2657.3 kJ
2 C4H10(g) + 13 O2(g) 8 CO2(g) + 10 H2O(g) ∆H = -5314.6 kJ
Chapter 9, Section 9.1• Question 5a page 347
• Note that the value of ∆H varies directly as the number of moles of reacting substances
• This formula gets used to calculate enthalpy changes for ∆Ep like phase changes, chemical reactions, and nuclear reactions
C5H12(l) + 8 O2(g) 5 CO2(g) + 6 H2O(g) ∆H = -3244.8 kJ
rH n H
Chapter 9, Section 9.1• Example Practice Problem 3a, page
349
Find for 56.78 g of pentane
56.783244.8 2553
72.17 /
r
r
H n H
H
g kJH n H kJmolg mol
Note: from table, page 347 - comment
mol of pentane
5 123244.8 kJ
molr C HH
Chapter 9, Section 9.1• Example Practice Problem 6, page
349
• molar enthalpy change for?• a) ammonia
• b) oxygen
• c) nitrogen monoxide
• d) water
4 NH3(g) + 5 O2(g) 4 NO(g) + 6 H2O(g) ΔH = -906 kJ
3
906227
4r NH
kJ kJH molmol
2
906181
5r O
kJ kJH molmol
906227
4r NO
kJ kJH molmol
2
906151
6r H O
kJ kJH molmol
Chapter 9, Section 9.1• Do Worksheet BLM 9.1.6
Chapter 9, Section 9.2• Finding the value of energy changes
experimentally: calorimetry
• Device: calorimeter
• The following diagrams show the principle behind calorimetry – note arrow directions
Chapter 9, Section 9.2• A simple calorimeter like the one you
will use
2 nested styrofoam cups containing a measured volume of water
sitting in a beaker so that it doesn’t fall over
3rd styrofoam cup inverted on top with hole for thermometer (stirrer)
Chapter 9, Section 9.2• Assumptions in styrofoam cup
calorimetry:• Amount of energy transferred to
cups and thermometer is small and can be ignored
• The system is isolated• The solution produced has the
same density and specific heat capacity as water
• The process occurs at constant pressure
Chapter 9, Section 9.2• The enthalpy change of a chemical
reaction = energy lost or gained, and is indicated by the symbol ΔH
• Energy gained or lost by the water causes a temperature change and is indicated by the symbol Q
• In an ideal calorimeter ΔH = Q• But recall:
• Therefore
rH n H Q mc t and
rn H mc t calorimetry equation
system calorimeter “water”
Chapter 9, Section 9.2• I will redo the example on page
354 using this formula
r
mc t mc tH
n cv
limiting reagent, if not stated, or substance question asks about
• remember m c Δt is for the “water” and n (c v) for the CuSO4(aq) 2 0.05000 4.19 24.60 21.40
89.40.300 0.05000
kJkg C kJ
molr molL
kg CH
L
Since the temperature has gone up the process is exothermic
Correct answer: 89.4kJmol
Chapter 9, Section 9.2• Practice Problem 9, page 355• Note that question asks for molar
enthalpy of reaction for sodium• n will be moles of sodium (question asks)
20.175 4.19 25.70 19.302.9 10
0.3722.99
r
kJkg C kJ
molr
gmol
n H mc t
kg Cmc tH
gn
• Since temperature increases, answer is correctly expressed as
22.9 10 or 0.29kJ MJmol mol
Do Practice Problems 7, 10, 12, page 355
Chapter 9, Section 9.2• Investigation 9.A page 356 (goes
with the questions you’ve been doing)
• Molar enthalpy of combustion: Investigation 9.B, page 357
Chapter 9, Section 9.2• Bomb Calorimetry: a bomb
calorimeter is used to make accurate and precise measurements
Chapter 9, Section 9.2• Reaction takes place inside an
inner container called the “bomb” that contains pure oxygen
• Chemicals are electrically ignited and heat is released to or absorbed from calorimeter water
• Calorimeter materials: stirrer, thermometer, containers are not ignored
• With calorimeter filled to a set level with water, all of their heat capacities are combined as shown:
Chapter 9, Section 9.2
• Note that C contains the mass and specific heat capacity of each component of the calorimeter
• How do you know when to use
2 2
2 2
r H O H O ther ther stir stir contains contains
r H O H O ther ther stir stir contains contains
r
n H m c t m c t m c t m c t
n H m c m c m c m c t
n H C t
bomb calorimeter equation
versus ?r rn H C t n H mc t
Heat capacity of calorimeter
Chapter 9, Section 9.2• Look for:
- words “bomb calorimeter”- no mention of the mass or volume of water- words “heat capacity” rather than “specific heat capacity”- units J/°C rather than J/g°C
• Question 2, Worksheet 46
• Since temperature increases, answer is -286 kJ/mol
• Do rest of Worksheet 46
40.00 3.54286
1.002.02
r
kJC kJ
molr
gmol
n H C t
CC tH
gn
Chapter 9, Section 9.2• More practice with
• WS 9.1.5
Q mc t
Chapter 9, Section 9.2• Review: page 366-7 good
questions: 1, 3, 4 (no actual calculation needed), 5c (data page 347), 6a (data page 347), 8, 10, 13, 15, 16, 17, 18, 19, 21
Chapter 9, Section 9.2