Thermochemistry

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Thermochemistry

• Brown, LeMay Ch 5
• AP Chemistry
• Adapted from http//matador.mvhs.fuhsd.org/kavita
  _gupta/ch5ppt.ppt
• Monta Vista High School

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5.1 Thermochemistry

• From Greek therme (heat) study of energy changes
  in chemical reactions
• Energy capacity do work or transfer heat
• Joules (J) or calories (cal) 1 cal 4.184 J
• Kinetic energy of motion dependent on mass
  velocity
• Applies to motion of large objects molecules
• Linked to thermal energy (objects T above 0 K)

James Prescott Joule(1818-1889)
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• Potential stored in fields (gravitational and
  electrical/magnetic) dependant on position
  relative to another object
• Applies to large objects where gravity is
  overriding force, but not significantly to
  molecules where gravity is negligible and
  electrostatic forces dominate
• Associated with chemical energy stored in
  arrangement of atoms or subatomic particles
  (electrostatic nuclear forces, bonding between
  atoms)

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Vocabulary

• System isolated portion of study (typically
  just the chemicals in a reaction)
• Surroundings everything else (container, room,
  Earth, etc.)
• Closed system easiest to study because exchanges
  energy with surroundings but matter is not
  exchanged.
• Force a push or pull on an object
• Work energy transferred to move an object a
  certain distance against a force W (F)(d)
• Heat energy transferred from a hotter object to
  a colder one

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5.2 Laws of Thermodynamics

• 0th Law 2 systems are in thermal equilibrium
  when they are at the same T.
• Thermal equilibrium is achieved when the random
  molecular motion of two substances has the same
  intensity (and therefore the same T.)
• 1st Law Energy can be neither created nor
  destroyed, or, energy is conserved.
• 2nd and 3rd Laws discussed in Ch. 19

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Internal energy, E

• Includes
• Translational motion
• Rotational motion of particles through space
• Internal vibrations of particles.
• It is difficult to measure all E, so the change
  in internal energy (DE) is typically measured
• DE Efinal - Einitial
• DE gt 0 Increase in energy of system
  (gained from surroundings)
• DE lt 0 Decrease in energy of system (lost
  to surroundings)

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• When a system undergoes a chemical or physical
  change, the change in internal energy (E) is
  equal to the heat (q) added or liberated from the
  system plus the work (w) done on or by the
  system
• DE q w

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Sign Conventions (Table 5.1)

• q gt 0 Heat is added to system
• q lt 0 Heat is removed from system (into
  surroundings)
• w gt 0 Work done to system
• w lt 0 System does work on surroundings

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Example

• Octane and oxygen gases combust within a closed
  cylinder in an engine. The cylinder absorbs 1150
  J of heat and a piston is pushed down by 480 J
  during the reaction. What is the change in
  internal energy of the system?
• q is (-) since heat leaves system w is (-) since
  work is done by system. Therefore,
• DE q w (-1150 J) (-480 J) - 1630 J
• 1630 J has been liberated from the system (C8H18
  and O2) added to the surroundings (engine,
  atmosphere, etc.)

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Heat reactions

• Endothermic energy added to system, DE
• Exothermic energy exits system, - DE

E
E
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State functions

• Property of a system that is determined by
  specifying its condition or its state
• The value of a state function depends only on its
  present state and not on the history of the
  sample.
• T E are state functions.
• Consider 50 g of water at 25C EH2O does not
  depend on how the water got to be 25C (whether
  it was ice that melted or steam that condensed
  or)
• Work (w) and heat (q) are not state functions
  because the ratio of q and w are dependent on the
  scenario.
• Consider the combustion of gasoline in a car
  engine vs. burning in the open.

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5.3 Enthalpy, H

• Since most reactions occur in containers open to
  the air, w is often negligible. If a reaction
  produces a gas, the gas must do work to expand
  against the atmosphere. This mechanical work of
  expansion is called PV (pressure-volume) work.
• Enthalpy (H) change in the heat content (qp) of
  a reaction at constant pressure
• H E PV
• ?H ?E P?V (at constant P)
• ?H (qp w) (-w)
• ?H qp

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• Sign conventions
• ?H gt 0 Heat is gained from surroundings ?H in
  endothermic reaction
• ?H lt 0 Heat is released to surroundings - ?H in
  exothermic reaction

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5.4 Enthalpy of Reaction (?Hrxn)

• Also called heat of reaction
• Enthalpy is an extensive property (depends on
  amounts of reactants involved).
• Ex CH4 (g) 2 O2 (g) ? CO2 (g) 2 H2O (l)
• ?Hrxn - 890. kJ
• Combustion of 1 mol CH4 produces 890. kJ
• of 2 mol CH4 ? (2)(-890. kJ) -1780 kJ
• What is the ?H of the combustion of 100. g CH4?

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• ?Hreaction - ?Hreverse reaction
• CH4 (g) 2 O2 (g) ? CO2 (g) 2 H2O (l)
• ?H - 890. kJ
• CO2 (g) 2 H2O (l) ? CH4 (g) 2 O2 (g)
• ?H 890. kJ
• 9 minute review of thermochemistry concepts on
  You Tube

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• ?Hrxn depends on states of reactants and
  products.
• CO2 (g) 2 H2O (g) ?CH4 (g) 2 O2 (g) ?H 802
  kJ
• 2 H2O (l) ? 2 H2O (g) ?H
  88 kJ
• So
• CO2 (g) 2 H2O (l) ? CH4 (g) 2 O2 (g) ?H
  890. kJ

890. kJ
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5.5 Calorimetry

• Measurement of heat flow
• Heat capacity, C amount of heat required to
  raise T of an object by 1 K
• q C DT
• Specific heat (or specific heat capacity, c)
  heat capacity of 1 g of a substance
• q m c DT
• Ex How much energy is required to heat 40.0 g
  of iron (c 0.45 J/(g K) from 0.0ºC to 100.0ºC?
• q m c DT (40.0 g)(0.45 J/(g K))(100.0 0.0
  ºC)
• 1800 J

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•  
• Calorimetry is an experimental technique used to
  measure the heat transferred in a physical or
  chemical process.
• The apparatus used in this procedure is called as
  a Calorimeter. There are two types of
  calorimeters
• Constant Pressure Calorimeter The coffee cup
  calorimeter is an example of this type of
  calorimeter. The system in this case is the
  contents of the calorimeter and the
  surroundings are cup and the immediate
  surroundings.

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• During the rxn
• qrxn q solution 0, where qrxn is the heat
  gained or lost in the chemical reaction and
  qsolutionis the heat gained or lost by solution.
  Heat exchange in this system (qrxn), is equal to
  enthalpy change. Assuming no heat transfer takes
  place between the system and surroundings, qrxn
  q solution 0 chemlab.truman.edu
  

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chemlab.truman.edu
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Sample Problems 1. 0.500g of magnesium chips
are placed in a coffee-cup calorimeter and 100.0
ml of 1.00 M HCl is added to it. The reaction
that occurs is Mg (s) 2HCl (aq) ? H2 (g)
MgCl2 (aq) The temperature of the solution
increases from 22.2oC (295.4 K) to 44.8 oC (318.0
K). Whats the enthalpy change for the reaction,
per mole of Mg? (Assume specific heat capacity of
solution is 4.20 J/(g K) and the density of the
HCl solution is 1.00 g/ml.) Ans. -4.64 x 105
J/mol Mg
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Sample Problem 2 . 200. ml of 0.400 M HCl is
mixed with the same amount and molarity of NaOH
solution, inside a coffee-cup calorimeter. The
temperature of the solutions before mixing was
25.10 oC, and 27.78 oC after mixing and letting
the reaction occur. Find the molar enthalpy of
the neutralization of the acid, assuming the
densities of all solutions are 1.00 g/ml and
their specific heat capacities are 4.20 J/(g
K).
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Constant Volume Calorimeter
Constant Volume Calorimeter The bomb calorimeter
is an example of this type of calorimeter. The
bomb, its contents and water are defined as the
system. The heat generated by the combustion
reaction warms the bomb and the water around
it.
The heat measured at a constant volume (qv) is
equal to internal energy change (d E). For a bomb
calorimeter Qrxn qbomb qwater 0
Bomb Calorimeter 2 minute video YT 
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Constant Volume Calorimeter Bomb Calorimeter
www.chem.ufl.edu/itl/2045/matter/FG05_016.GIF
www.chem.ufl.edu/itl/2045/matter/FG05_016.GIF
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Sample Problem
1. Octane (C8H18) is a primary constituent of
gasoline, and it burns in air as follows C8H18
(l) 25/2 O2 (g) ? 8CO2 (g) 9H2O (g) A sample
of octane weighing 1.00g is burned in a constant
volume calorimeter, which is an insulated
container with 1.20 kg of water inside. The
temperature of the water and the bomb rises from
25.00 oC (298.15 K) to 33.20 oC (306.35 K). The
bombs heat capacity is 837 J/K. What is the heat
of combustion per gram of octane? Per mole of
octane? Ans. -48.1 kJ/g -5.49 x 103 kJ/mol
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Sample Problem
2. A sample of table sugar (sucrose, C12H22O11)
weighing 1.00g is burned in a bomb calorimeter.
The temperature of the 1.50 x 103g of water in
the calorimeter rises from 25.00 oC to 27.32 oC.
The heat capacity of the bomb is 837 J/K and 4.20
J/(g K) for water. Calculate the heat evolved
per gram of sucrose and per mole of sucrose
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Sample Problem
When 1.000g of olive oil (glyceryl trioleate,
C57H104O6) is burned in pure oxygen in a bomb
calorimeter, the temperature of the water bath
increases from 22.000 oC to 22.241 oC. The
calorimeters heat capacity is 9.032 kJ/ oC and
glyceryl trioleate combusts with oxygen as
follows C57H104O6 80O2 ? 57CO2 52H2O. (a)
How many dietary Calories are in olive oil, per
gram? (b) What is the change in internal energy,
?E, in kJ for the combustion of 1 mol of glyceryl
trioleate? Ans. 0.521 Cal/g oil, -1.93 x 103
kJ/mol
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5.6 Hess Law

• If a rxn is carried out in a series of steps,
• ?Hrxn ? (?Hsteps) ?H1 ?H2 ?H3

Germain Hess(1802-1850)
Ex. What is DHrxn of the combustion of
propane? C3H8 (g) 5 O2 (g) ? 3 CO2 (g) 4 H2O
(l) 3 C (s) 4 H2 (g) ? C3 H8 (g) ?H1
-103.85 kJ C (s) O2 (g) ? CO2 (g) ?H2
-393.5 kJ H2 (g) ½ O2 (g) ? H2O (l) ?H3
-285.8 kJ
C3H8 (g) ? 3 C (s) 4 H2 (g) ?H1 103.85
kJ
3 3(
)
4 4(
)
?H?rxn 103.85 3(- 393.5) 4(- 285.8) -
2219.8 kJ
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5.7 Enthalpy of Formation (?Hf)

• Formation a reaction that describes a substance
  formed from its elements
• NH4NO3 (s)
• Standard enthalpy of formation (?Hf?) forms 1
  mole of compound from its elements in their
  standard state (at 298 K)
• C2H5OH (l)
• ?Hf? - 277.7 kJ
• ?Hf? of the most stable form of any element
  equals zero.H2, N2 , O2 , F2 , Cl2 (g)
• Br2 (l), Hg (l)
• C (graphite), P4 (s, white), S8 (s), I2 (s)

Ex 2 N2 (g) 4 H2 (g) 3 O2 (g) ? 2
2 C (graphite) 3 H2 (g) ½ O2 (g) ?
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Hess Law (again)
Ex. Combustion of propane C3H8 (g) 5 O2 (g) ?
3 CO2 (g) 4 H2O (l) Given Compound ?H?rxn
(kJ/mol) C3H8 (g) -103.85 CO2
(g) -393.5 H2O (l) -285.8 H2O (g) -241.82
?H?rxn 3(- 393.5) 4(- 285.8) 1(-103.85)
5(0) - 2219.8 kJ