Transcript Chapter 5

Chapter 5.1-5.3
Thermochemistry
Thermochemistry

Thermo means heat or energy
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Energy: Capacity of doing work
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Work: Force x displacement
Types of Energy
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Kinetic Energy (KE)
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Energy due to motion
Ek = ½ mv2
Where m = mass & v = velocity
Types of Energy

Potential energy (PE)
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Energy due to position
higher PE
Gravity
lower PE
Units: Calorie (cal), Joule (J)
1 Cal = 4.184 J
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System
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Where we focus our attention
Surroundings
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Everything else
First Law of Thermodynamics
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Energy is conserved
10 J added
(q)
Piston pushed up & work (W)
done by the gas (say 6 J)
First Law of Thermodynamics
What happens to the difference (10 – 6)?
 Retained inside. (causes Internal energy
change, ΔE): 10 = 4 + 6
q (Heat applied) = ? q = ΔE + W (first law
Thermodynamics)
or ΔE = q-W (here -W, Work done by the gas)
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ΔE = q +W (here +W, Work done on the gas)
Note: Two different sign conventions exist
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Enthalpy (H)
ΔE = q-W (work done by a gas)
q = ΔE + W
q = ΔE + P ΔV
ΔH = ΔE + P ΔV (constant pressure)
H = E + PV
Enthalpy or heat Content (H)
Begin
2H2 + O2
Say
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484 kJ
End

2H2O
1 kJ
Note: 484 kJ is the Heat content of the
reactants (HR) and 1 kJ is the heat
content of the products (HP).
Enthalpy or heat Content (H)
ΔH =
=
=
Hp –HR
1 – 484
-483 kJ (exothermic)
Here, initial heat content> Final heat cont.
 What happens to the difference?
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It is given off.
Enthalpy or heat Content (H)
2H2O 
2H2 + O2
1 kJ (HR)
484 kJ (HP)
Is heat absorbed or given off?
Heat is absorbed
ΔH =
Hp –HR
=
484 – 1
=
483 kJ (endothermic)
 Here, initial heat cont < Final heat cont