07/02/2016
Chem 121
G. Thermochemistry
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Energy
1st law
Enthalpy, enthalpy of combustion, fuels
Calorimetry
Enthalpy of reaction
Hess's Law, calculations
Energy
Energy: capacity to do work or transfer heat
Matter possesses energy in the form of:
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Units of energy
Joules
e.g. a 4 kg mass moving at 1 m/s has a kinetic energy
EK = ½ m v2
Calories
Nutritional calorie
(You may also come across the Kilowatt-hour, a common non-SI unit of energy.)
System and surroundings
Open system:
Closed system:
Isolated system:
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Transferring energy: work and heat
Energy measure
First Law of Thermodynamics
System
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Surroundings
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transfer
System
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Surroundings
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0
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for a chemical reaction:
C(s) + O2(g)  CO2(g)
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Reverse reaction: CO2(g)  C(s) + O2(g)
Change in internal energy depends on heat and work
q:
w:
A reminder:
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e.g. Altitude change depends only on initial
and final altitude
work done when a force moves an object
pressure = force / area, rearrange to get
substitute F into the eqn. for work
If ΔV is positive, system does work
on the surroundings
If external pressure is zero
(expansion against a vacuum)
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Enthalpy
Chemical changes typically take place at constant pressure, heat flow easy to measure
Thermodynamic quantity for the heat change at constant pressure, enthalpy change ΔH
so, when P is constant:
Enthalpy is a state function (this follows because U, P and V are all state functions)
Recall that:
ΔE = q + w
Enthalpy of reaction
ΔH = Hfinal  Hinitial
so in chemical reactions
ΔH is called the enthalpy of reaction,
Thermochemical equation is the balanced chemical reaction and the value of ΔH
2H2O2(ℓ)
2H2O2(ℓ)  2H2O(ℓ) + O2(g) ΔH = 196 kJ
ΔH = 196 kJ
ΔH = +196 kJ
2H2O2(ℓ)  2H2O(g) + O2(g) ΔH = 108 kJ
2H2O(ℓ) + O2(g)
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Calorimetry
Heat capacity and specific heat
Size of the temperature change per unit of heat depends on the
nature of the substance
Molar heat capacity, Cm –
Specific heat, Cs –
Specific heat can be determined experimentally
specific heat 
amount of heat transferred
(grams of substance) x (change in temperature)
An example: determining the specific heat of lead:
(a) Heat 150g lead to 100°C
(b) Add the hot lead to 50g water at 22°C
(c) Measure the final temperature
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Calorimetry is the measurement of heat flow
Constant-pressure calorimetry
Heat flows between reaction and solution
Constant-volume calorimetry
Heat from chemical reaction absorbed by water and all
components of calorimeter
calorimeter plus water is an object -
Hess’s Law
We can use tabulated ΔH values to calculate the enthalpy of reactions
Example: N2(g) + 3H2(g)  2 NH3(g) Hrxn = ?
 N2(g) + 3H2(g)  N2H4(g) + H2(g) H = 95.4 kJ
 N2H4(g) + H2(g)  2 NH3(g) ΔH =  187.6 kJ
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Hess’s Law : The enthalpy change of an overall process is the sum of the
enthalpy changes of its individual steps
Procedure:
e.g. what is ΔH for ½N2(g) + O2(g) → NO2(g)
• Arrange reactions so (final) reactants appear on
given:
the left and products appear on the right
N2(g) + O2(g)  2NO( g) ΔH = +180.50 kJ
NO2(g)  NO(g) + ½ O2(g) ΔH = +57.07 kJ
• Any reaction that is reversed must
have the sign of its ΔH changed
• All intermediates must occur on both
the right and the left so they cancel
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• A reaction can be multiplied by
a coefficient, but ΔH for that reaction
must be multiplied by the same
coefficient.
Hess’s Law
We can use tabulated ΔH values to calculate the enthalpy of reactions
ΔH depends on amounts and initial and final states of reactants and products
ΔH is a state function, so does not depend upon how we get from reactants to products
Example: N2(g) + 3H2(g)  2 NH3(g) Hrxn = ?
 N2(g) + 2H2(g)  N2H4(g)
H = 95.4 kJ
 NH3(g)  ½ N2H4(g) + ½ H2(g)
ΔH = + 93.8 kJ
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Enthalpies of formation
Hess’s law can be used with tabulated ΔH values
for both chemical and physical changes
Physical changes:
vapourization ΔHvap
fusion ΔHfus
Reactions:
combustion reactions, ΔHcomb
formation reactions: ΔHf
Magnitude of ΔH depends on T, P and physical state of reactants and products
Thermodynamic Standard state (provides a reference point, like sea level)
Standard enthalpy change
Standard enthalpy of formation
e.g. for ethanol: C2H5OH(ℓ)
For pure elements in their most stable form at standard conditions
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Using enthalpies of formation to calculate enthalpies of reaction
steps:
Foods and Fuels
Fuel value is the energy released when one gram of substance is combusted
Foods differ in fuel value and body’s ability to store
Fuel value:
Bio-availability:
Fuels
Combustion produces CO2 and H2O;
Both have large negative ΔHf°
fossil fuels:
coal: solid, mined, abundant
Energy use, USA data, 2008
petroleum: liquid, refined
natural gas: gas, 85% methane, 10% ethane,
3% propane
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