1. Thermochemistry
Part 2

2. Thermochemisty Study of chemical reactions in relation to energy
The capacity to do work or the transfer of heat
Work
The energy used in moving an object against a force.

3. Thermochemisty Ek = ½ mv2 Kinetic energy Ek = kinetic energy m= mass
The energy of motion
Depends on mass and velocity
Ek = kinetic energy
m= mass
v= velocity
Chemistry is interested in kinetic energy of molecules
Ek = ½ mv2

4. Thermochemisty Eel = kQ1 Q2 d Potential energy Electrostatic energy
The energy by virtue of its position in relation to other things, stored energy
Electrostatic energy
Eel = electrostatic energy
k= constant of proportionality (8.99 x109 J-m/C2)
C in k is coulomb, unit of electrical charge
d= distance (in meters)
Q1 and Q2 are the charges of the electron (1.60 x10-19 C)
Eel =
kQ1 Q2 d

6. Units of Energy joule (J): SI unit of energy 1J = 1 kg-m2/ s2
kJ: kilojoules
calorie: non SI unit of energy
1 cal = J (exact definition)
Calorie (capital C) in nutrition is:
1 Cal = 1000 cal = 1 kcal (exact definition)

7. System vs. Surroundings
What is under observation or study
Surroundings
Everything else

8. Different Systems Open System Closed System Isolated System
Matter and energy can be exchanged between system and surroundings
Closed System
ONLY energy can be exchanged between system and surroundings
Isolated System
NEITHER matter or energy is exchanged between system and surroundings

9. Work w= F x d Remember work is energy of a force moving an object Work
Product of a force over a distance
w: work
F: force, influence exerted on an object (push or pull)
d: distance
w= F x d

10. 1st Law of Thermodynamics
Energy can be neither created nor destroyed; it is conserved.
Involves energy, system and surroundings

11. Internal energy ΔE = Efinal - Einitial
Sum of all of the potential energy and kinetic energy of a system.
Looking at the energy in a system after some change to the system.
ΔE = Efinal - Einitial
Δ read as delta, always means: final - initial

12. ΔE = Efinal - Einitial If ΔE is + If ΔE is - Efinal > Einitial
Endothermic
If ΔE is -
Efinal < Einitial
Exothermic

13. Energy Diagram
Exothermic or Endothermic?
Why?

14. ΔE = q + w ΔE = q + w ΔE q w Change in internal energy of a system
Heat added or released from a system w
work done on or by a system
ΔE = q + w

15. You Try
If you have a reaction that releases 873 J of heat and is able to move a piston up by doing 283 J of work against the surroundings, what would the change in energy be?
What is the sign of the energy?
Would it be gaining or losing energy?

16. You Try ΔE = q + w ΔE = -873 J + -283 J
q = -873 J (why is it negative?)
w = -283 J (why is it negative?)
ΔE = -873 J J
ΔE = J
Sign is negative
Losing energy

17. Energy as a State Function
a property of a system that is determined by specifying the system’s condition or state (in terms of temperature, pressure, etc)
Value of a state function depends on the present state of the system, not the path it took to get there.
ΔE = Efinal - Einitial

18. Wrap your ear around this!
E is a state function
q and w are not state functions
Yet
Explain?
ΔE = q + w p

19. Enthalpy H = E + PV Abbreviated with an H Enthalpy
From enthalpein (Greek: to warm)
H = E + PV

20. Pressure-Volume Work w = -PΔV ΔV is + ΔV is -
Work involved in the expansion or compression of gases under a constant pressure
ΔV is +
Vfinal > Vinitial
Work is done on the surroundings
ΔV is -
Vfinal < Vinitial
Work is done in the system
w = -PΔV

21. Change in Enthalpy (at a constant pressure)
ΔH = Δ(E + PV)
ΔH = ΔE + PΔV a constant pressure)

22. Let’s Derive… ΔH = ΔE + PΔV constant pressure ΔE = q + w w = -PΔV
Remember:
Now substitute:
ΔH = ΔE + PΔV constant pressure
ΔE = q + w
w = -PΔV
so… -w = PΔV
ΔH = (qp + w) - w
ΔH = qp (qp denotes constant pressure)

23. ΔH = qp ΔH > 0 endothermic ΔH < 0 exothermic
More useful than internal energy since most change in volume in reactions is small
H is a state function, even though q is not. Equation is misleading in that it is true because pressure volume work is involved and pressure is constant.
ΔH > 0 endothermic
ΔH < 0 exothermic

24. Enthalpy
Enthalpy is an extensive property, depends upon how much we have
Enthalpy change is equal in magnitude, but opposite in sign to the change in H for the reverse reaction.
Enthalpy for a reaction, depends on states of reactants and products.

25. Enthalpy Enthalpy is an extensive property, depends upon how much.
Explanation: If there is more reactants, when they react, there will be more energy released if it is exothermic

26. Enthalpy
Enthalpy change is equal in magnitude, but opposite in sign to the change in H for the reverse reaction.
Explanation: Follows the First Law of Thermodynamics

27. Enthalpy
Enthalpy for a reaction, depends on states of reactants and products.
Explanation: If the substance is a solid, it would take in energy to change to a liquid. The change in state inherently involves a change in enthalpy.

28. Enthalpies of a Reaction
For a chemical reaction (rxn):
Could be called heat of a reaction
ΔH = Hfinal - Hinitial
ΔH = Hproducts - Hreactants
ΔHrxn = Hproducts - Hreactants

29. Practice
The decomposition of slaked lime, Ca(OH)2 (s), into lime, CaO (s), and water at a constant pressure requires the addition of 109 kJ of heat per mole of Ca(OH)2.
Write a balanced thermochemical equation for the reaction
Draw an enthalpy diagram for this reaction
How much heat is needed to decompose 148 grams of slaked lime?