Thermochemistry

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Thermochemistry
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• Energy is the capacity to do work.
• Radiant energy comes from the sun and is earths
  primary energy source
• Thermal energy is the energy associated with the
  random motion of atoms and molecules
• Chemical energy is the energy stored within the
  bonds of chemical substances
• Nuclear energy is the energy stored within the
  collection of neutrons and protons in the atom
• Potential energy is the energy available by
  virtue of an objects position
• Kinetic energy is the energy of motion

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Energy Changes in Chemical Reactions
Heat is the transfer of thermal energy between
two bodies that are at different temperatures.
Temperature is a measure of the thermal energy.
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Thermochemistry is the study of heat change in
chemical reactions.
The system is the specific part of the universe
that is of interest in the study.
Surrounding the rest of the universe.
open
isolated
closed
energy
nothing
mass energy
Exchange
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Collision Theory

• To explain factors affecting rxn rate
• For two molecules to react, they must come in
  contact or collide.
• When molecules collide,
• Ineffective collision occurs molecules may not
  orient in right position. No Rxn.
• Effective collision occurs molecules possess
  enough kinetic energy to position.

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What Happen Just Before Rxn

• Molecules approaches each other.
• Repulsive force increases as they approach each
  other.
• There is a minimum energy to push the molecules
  to collide ? Activation Energy (Ea)
• Supplying enough kinetic energy will kick-start
  the reaction.

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Graphing Rxn Spontaneous
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Gibbs Free Energy (?G)

• If ?G is positive, or ?G gt 0Indication Rxn ?
  Nonspontaneous
• If ?G is negative, or ?G lt 0Indication System ?
  Spontaneous
• The previous example is spontaneous.

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Rate Influencing Factor

1. Surface Area
2. Temperature
3. Concentration
4. Presence of Catalysts

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Factor 1 Surface Area

• Similar to solubility
• More surface area enhances collision frequency
• Homogeneous Rxn
• The reactants are in the same physical state.For
  example, gas reacting with gas
• Heterogeneous Rxn
• The reactants are in DIFFERENT physical stateFor
  example Solid reacting with liquid/aq. soln

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Factor 2 Temperature

• Increase in temperature provide more kinetic
  energy for reaction to proceed
• Fast-moving molecules have better chance of
  effective collision.
• It also enhances frequency of collision.
• Refrigerator is designed to SLOW reaction rate.

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Factor 3 Concentration

• Higher chemical concentration generally promotes
  reaction rate
• Concentration is directly proportional to the
  frequency of collision, in general.
• Just like medicine, we need higher dosage for
  severely-ill patient.

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Factor 4 Catalyst

• In human body, we have enzymes acting as
  catalyst.
• It does not undergo permanent change.
• It provides an alternative reaction pathway of
  Lower Activation Energy.
• Inhibitor reduces a reaction rate by preventing
  the reaction occurring in the usual way.
• Example Food preservatives

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Effect of Catalyst on Rxn
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The First Law of Thermodynamics

• Relating DE to Heat(q) and Work(w)
• Energy cannot be created or destroyed.
• Energy of (system surroundings) is constant.
• Any energy transferred from a system must be
  transferred to the surroundings (and vice versa).
• From the first law of thermodynamics

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First Law of Thermodynamics
Energy can be converted from one form to another
but energy cannot be created or destroyed.
Second Law of Thermodynamics
The entropy of the universe increases in a
spontaneous process and remains unchanged in an
equilibrium process.
DSuniv DSsys DSsurr gt 0
Spontaneous process
DSuniv DSsys DSsurr 0
Equilibrium process
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Enthalpy Changes

• The difference between the potential energy of
  the reactants and the products during a physical
  or chemical change is the Enthalpy change or ?H.
• AKA Heat of Reaction at constant pressure

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Thermochemical Equations
Is DH negative or positive?
System absorbs heat
Endothermic
DH gt 0
6.01 kJ are absorbed for every 1 mole of ice that
melts at 00C and 1 atm.
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Thermochemical Equations
Is DH negative or positive?
System gives off heat
Exothermic
DH lt 0
890.4 kJ are released for every 1 mole of methane
that is combusted at 250C and 1 atm.
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Graphing Rxn Endothermic
The temperature goes down
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Graphing Rxn Exothermic
The temperature goes up
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Cold and hot packs

• How do instant hot and cold packs work?

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Hot pack

• Pressing the bottom , the diaphragm breaks.
• Calcium chloride dissolves in water and warms it.
• The beverage gets warm.

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Exothermic process

• Heat flows into the surroundings from the system
  in an exothermic process.

Energy
Surroundings
Hot pack
Temperature rises
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Cold pack

• Water and ammonium nitrate are kept in separate
  compartments.
• Pressing the wrapper, the ammonium nitrate
  dissolves in water and absorbs heat.
• The pack becomes cold.

It is used to treat sports injuries.
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Endothermic process

• Heat flows into the system from the surroundings
  in an endothermic process.

Surroundings
Cold pack
Energy
Temperature falls
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Explosions
This reaction is exothermic!
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Photosyntesis
This reaction is endothermic!
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Combustions
These reactions are exothermic!
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Enthalpy Changes in Reactions

• All chemical reactions require bond breaking in
  reactants followed by bond making to form
  products
• Bond breaking requires energy (endothermic) while
  bond formation releases energy (exothermic)

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Enthalpy Changes in Reactions

• endothermic reaction - the energy required to
  break bonds is greater than the energy released
  when bonds form.
• exothermic reaction - the energy required to
  break bonds is less than the energy released when
  bonds form.

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Enthalpy Changes in Reactions

• ?H can represent the enthalpy change for a
  number of processes
• Chemical reactions
• ?Hrxn enthalpy of reaction
• ?Hcomb enthalpy of combustion

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• Formation of compounds from elements
• ?Hof standard enthalpy of formation
• The standard molar enthalpy of formation is the
  energy released or absorbed when one mole of a
  compound is formed directly from the elements in
  their standard states.

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• Phase Changes
• ?Hvap enthalpy of vaporization
• ?Hfus enthalpy of melting
• ?Hcond enthalpy of condensation
• ?Hfre enthalpy of freezing
• Solution Formation
• ?Hsoln enthalpy of solution

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q mc?T
140
100
q n?H
Temp. (C )
q mc?T
q n?H
0
q mc?T
-40
Time
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• Data
• cice 2.01 J/g.C
• cwater 4.184 J/g.C
• csteam 2.01 J/g.C
• ?Hfus 6.02 kJ/mol
• ?Hvap 40.7 kJ/mol

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• warming ice
• q mc?T
• (40.0)(2.01)(0 - -40)
• 3216 J
• warming water
• q mc?T
• (40.0)(4.184)(100 0)
• 16736 J

• warming steam
• q mc?T
• (40.0)(2.01)(140 -100)
• 3216 J

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n 40.0 g 18.02
g/mol 2.22 mol
moles of water

• melting ice
• q n?H
• (2.22 mol)(6.02 kJ/mol)
• 13.364 kJ
• boiling water
• q n?H
• (2.22 mol)(40.7 kJ/mol)
• 90.354 kJ

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Total Energy

• 90.354 kJ
• 13.364 kJ
• 3216 J
• 3216 J
• 16736 J

• 127 kJ

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Sample Problem

• How much heat is required to change 36 g of H2O
  from -8 deg C to 120 deg C?

Step 1 Heat the ice Qmc?T Q 36 g x 2.06
J/g deg C x 8 deg C 593.28 J 0.59 kJ
Step 2 Convert the solid to liquid ?H fusion Q
2.0 mol x 6.01 kJ/mol 12 kJ
Step 3 Heat the liquid Qmc?T Q 36g x 4.184
J/g deg C x 100 deg C 15063 J 15 kJ
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Sample Problem

• How much heat is required to change 36 g of H2O
  from -8 deg C to 120 deg C?

Step 4 Convert the liquid to gas ?H
vaporization Q 2.0 mol x 44.01 kJ/mol 88 kJ
Step 5 Heat the gas Qmc?T Q 36 g x 2.02
J/g deg C x 20 deg C 1454.4 J 1.5 kJ
Now, add all the steps together
0.59 kJ 12 kJ 15 kJ 88 kJ 1.5 kJ 118 kJ
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Heat/Enthalpy Calculations

• specific heat capacity - the amount of energy ,
  in Joules (J,SI unit), needed to change the
  temperature of one gram (g) of a substance by one
  degree Celsius (C).
• c does not depend on mass, so intensive
  property.
• The symbol for specific heat capacity is a
  lowercase c
• The unit is J/ gC or kJ/ gC

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• A substance with a large value of c can absorb or
  release more energy than a substance with a small
  value of c.
• with the larger c will undergo a smaller
  temperature change with the same amount of heat
  applied.

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FORMULA

• q heat (J)
• m mass (g)
• c specific heat capacity
• ?T temperature change
• T2 T1
• Tf Ti

• q mc?T

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• eg. How much heat is needed to raise the
  temperature of 500.0 g of water from 20.0 C to
  45.0 C?
• Solve q m c ?T
• for c, m, ?T, T2 T1

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• heat capacity - the quantity of energy , in
  Joules (J, SI unit), needed to change the
  temperature of a substance by one degree Celsius
  (C)
• The symbol for heat capacity is uppercase C
• C depends on mass, so extensive property.
• The unit is J/ C or kJ/ C

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FORMULA

• C heat capacity
• c specific heat capacity
• m mass
• ?T T2 T1

• C mc
• q C ?T

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Calorimetry

• Heat Capacity and Specific Heat
• Calorimetry measurement of heat flow (follow
  Law of conservation of energy).
• Heat lost by one substance Heat gained by other
  substance
• Calorimeter apparatus that measures heat flow.
• Heat capacity the amount of energy required to
  raise the temperature of an object (by one
  degree).
• Molar heat capacity heat capacity of 1 mol of a
  substance.
• Specific heat specific heat capacity heat
  capacity of 1 g of a substance.

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Calorimetry
Constant Pressure Calorimetry
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s of Fe 0.444 J/g 0C
Dt tfinal tinitial 50C 940C -890C
q msDt
869 g x 0.444 J/g 0C x 890C
-34,000 J
6.5
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More practice

• 5 g of copper was heated from 20?C to 80?C. How
  much energy was used to heat the Cu?
• 2. If a 3.1 g ring is heated using 10.0 J, its
  temp. rises by 17.9?C. Calculate the specific
  heat capacity of the ring. Is the ring pure gold?

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Thermochemical Equations

• The stoichiometric coefficients always refer to
  the number of moles of a substance

• If you reverse a reaction, the sign of DH changes

• If you multiply both sides of the equation by a
  factor n, then DH must change by the same factor
  n.

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Standard enthalpy (heat) of formation (DH0) is
the heat change that results when one mole of a
compound is formed from its elements at a
pressure of 1 atm.
f
The standard enthalpy of formation of any element
in its most stable form is zero.
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DH0 (reactants)
S
-
S

f
Hesss Law When reactants are converted to
products, the change in enthalpy is the same
whether the reaction takes place in one step or
in a series of steps.
?H for the overall rxn will equal to the sum of
the enthalpy changes for the individual steps.
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17.4 Section Quiz.

• Calculate ?H for NH3 (g) HCl (g) ? NH4Cl
  (s).Standard heats of formationNH3(g) -
  45.9 kJ/mol, HCl(g) - 92.3 kJ/mol, NH4Cl(s)
  - 314.4 kJ/mol
• 176.2 kJ
• ?360.8 kJ
• ?176.2 kJ
• ?268 kJ .

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Benzene (C6H6) burns in air to produce carbon
dioxide and liquid water. How much heat is
released per mole of benzene combusted? The
standard enthalpy of formation of benzene is
49.04 kJ/mol.
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17.4 Section Quiz.

• The heat of formation of Cl2(g) at 25C is
• the same as that of H2O at 25C.
• larger than that of Fe(s) at 25C.
• undefined.
• zero.

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Hesss Law
Example CH4(g) 2O2(g) ? CO2(g) 2H2O(g) ?H
-802 kJ 2H2O(g) ? 2H2O(l) ?H - 88
kJ CH4(g) 2O2(g) ? CO2(g) 2H2O(l) ?H -890
kJ
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Calculate the standard enthalpy of formation of
CS2 (l) given that
1. Write the enthalpy of formation reaction for
CS2
2. Add the given rxns so that the result is the
desired rxn.
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17.4 Section Quiz.

• According to Hesss law, it is possible to
  calculate an unknown heat of reaction by using
• heats of fusion for each of the compounds in the
  reaction.
• two other reactions with known heats of
  reaction.
• specific heat capacities for each compound in the
  reaction.
• density for each compound in the reaction.

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The critical temperature (Tc) is the temperature
above which the gas cannot be made to liquefy, no
matter how great the applied pressure.
The critical pressure (Pc) is the minimum
pressure that must be applied to bring about
liquefaction at the critical temperature.
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Wheres Waldo?
Can you find The Triple Point? Critical
pressure? Critical temperature? Where fusion
occurs? Where vaporization occurs? Melting point
(at 1 atm)? Boiling point(at 6 atm)?
Carbon Dioxide
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The melting point of a solid or the freezing
point of a liquid is the temperature at which the
solid and liquid phases coexist in equilibrium
Freezing
Melting
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Molar heat of sublimation (DHsub) is the energy
required to sublime 1 mole of a solid.
Sublimation
Deposition
DHsub DHfus DHvap
( Hesss Law)
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