1. Chapter 17 Energy Heat, Energy, and Temperature changes
Pages
Thermochemistry:
Causes of Change in Systems
Thermodynamics:
Movement of energy in/out of a system
Heat, Energy, and Temperature changes
Standard unit of heat is the Joule, J
Standard unit of temperature is Kelvin, K

2. Heat vs Temperature Heat Temperature
measure of energy change in a system.
Temperature
measure of the kinetic energy (movement) of the particles in a system.
Gaining or losing heat energy in a substance can change its temperature.
Exothermic
System loses energy to surroundings
Endothermic
System gains energy from surroundings

3. Specific Heat Specific Heat The symbol we use is cp. Specific Heat…
measure of how a substance reacts to heat energy changes.
The symbol we use is cp.
The “p” stands for constant pressure while heat is added or lost.
Specific Heat…
is the heat energy required to raise one gram of a pure substance one degree Celsius.
is a property of matter, and different species have different Specific Heat.

4. Specific Heat Capacity
Substance
J/g/oC or J/g/K
cal/g/oC or cal/g/K
Water (0 oC to 100 oC)
4.186
1.000
Zinc
.387
0.093
Ice (-10 oC to 0 oC)
2.093
0.500
Steam (100 oC)
2.009
0.480
Brass
.380
0.092
Wood (typical)
1.674
0.400
Soil (typical)
1.046
0.250
Air (50 oC)
Aluminum
.900
0.215
Tin
.227
0.205
Glass (typical)
.837
0.200
Iron/Steel
.452
0.108
Copper
0.0924
Silver
.236
0.0564
Mercury
.138
0.0330
Gold
.130
0.0310
Lead
.128
0.0305
Metals have very low cp,
which is why metals often feel cold to the touch.
Table on page 513
Water has a very high cp,
4.184 J/g·0C
Substances with lower cp will rise in temperature faster and require less energy to do so than do substances with high cp.
1 calorie=4.184 Joules

5. Enthalpy, ΔH
Enthalpy
heat energy transferred for a specific change to take place.
We specify enthalpy with ΔH. “Δ” means “change in”.
Exothermic reaction
negative enthalpy (-ΔH )
Endothermic reaction
Positive enthalpy (+ΔH).
Elements in their standard (unbonded) state have a ΔH of zero.
O2  ΔH=0 kJ/mol, Fe(s)  ΔH=0 kJ/mol

6. Enthalpy, ΔH ΔHfus = heat of fusion ΔHvap = heat of vaporization
Some common changes involving ΔH:
ΔHfus = heat of fusion
ΔHvap = heat of vaporization
ΔHcond = heat of condensation
ΔHsub = heat of sublimation
ΔHrxn = heat of reaction
ΔHf = heat of formation
ΔHsol = heat of solution
ΔHcomb = heat of combustion
State change to/from?
Sign of ΔH?

7. Devices to Measure ΔHcomb
The Calorimeter (shown)
Heat energy is transferred from a reaction inside the calorimeter to the water in the calorimeter. The temperature change of the water is observed.
A Bomb Calorimeter uses a chamber of pure oxygen to measure heats of combustion to the 1/1000 of a joule.
When two objects are in contact, they eventually obtain Thermal Equilibrium; their temperatures become equal.
Text page 519

8. Reaction Enthalpy If ΔH is negative, the reaction is exothermic.
C6H12O6 + 6O2 6CO2 + 6H kJ
ΔHrxn = kJ/mol
If ΔH is positive, the reaction is endothermic.
2H2O kJ  2H2 + O2
ΔHrxn = kJ/mol
energy

9. Spontaneity Spontaneous
A reaction that will proceed on its own once started.
Sometimes, all the reaction needs to get going is the kinetic energy of nearby atoms.
Spontaneous combustion
occurs when the kinetic energy of oxygen molecules striking a fuel have enough energy on their own to start the combustion reaction.

10. Spontaneity
In the diagram, the hump is called a reaction energy barrier - the amount of energy required for the reaction to begin.
We can reduce this barrier with an appropriate catalyst.
All reactions have some sort of reaction barrier (sometimes very small)
The energy needed to overcome this barrier is the activation energy

11. DHrxn
Exothermic Reaction: products have a lower energy state than do the reactants.
How are things different in an endothermic reaction?

12. Hess’s Law
Hess’s Law:
If two reactions begin with the same reactants in the same condition and end with the same products in the same condition, they must have the same enthalpy change.
Consider Species A, B, C, D :
A + B + energy  C then C + energy D
Must be the same as A + B + 2energy  D
It doesn’t matter if you perform a reaction in several steps or produce your final product in one step, the enthalpy change will be the same.

13. Hess’s Law Enthalpy of Reaction
ΔHrxn = Hproducts – Hreactants =
ΣHf, all the products – ΣHf, all the reactants
“ Sum of ”
Enthalpy of formation
Example on page 330
Do # 16, p332

14. ΔSrxn = ΣSproducts – ΣSreactants
Entropy, ΔS
Entropy
is a measure of relative disorder. Thermodynamics tells us that the universe tends towards disorder or entropy.
Entropy calculations are very similar to enthalpy calculations:
ΔSrxn = ΣSproducts – ΣSreactants
Entropy has the unit J/K*mol

15. Entropy, ΔS The Universe tends towards entropy
entropy plays a part in predicting whether or not a reaction will be spontaneous.
Solids have very low entropy
Gases have very high entropy
Solutions also have high entropy

16. Temperature and Entropy
Recall:
Kinetic Molecular Theory (Particle Model):
All matter is made up of particles in constant and random motion.
Temperature is a measurement of the kinetic energy (movement) of particles in a system.
Increasing temperature increases particle movement, increasing the disorder in a system, increasing its entropy.

17. Entropy Values We can make generalizations about a reaction’s entropy;
2KClO3(s)  2KCl(s) + 3O2(g)
2 solids  2 solids + 3 gases
Entropy appears to increase in this reaction.

18. 143.7 J/mol*K  82.6 J/mol*K + 205.1 J/mol*K
Entropy Values
We can assign values to a species’ entropy of formation, called standard entropy, and calculate a reaction’s entropy quantitatively.
2KClO3(s)  2KCl(s) + 3O2(g)
143.7 J/mol*K  82.6 J/mol*K J/mol*K
Using ΔSrxn = Sproducts – Sreactants, the reaction has a total entropy change of J/mol*K

19. Negative Enthalpy (-ΔH) Positive Entropy (+ΔS)
Entropy Values
A positive ΔS = increase in entropy
A negative ΔS = decrease in entropy
Do not confuse entropy and enthalpy!
Tending toward spontaneity:
Negative Enthalpy (-ΔH)
Positive Entropy (+ΔS)

20. FREE ENERGY !!!! Free Energy:
A measurement of a system’s ability to change. Or, a measurement of the force required for a system to change.
We can link both enthalpy and entropy into one value, called the Gibbs Free Energy value, ΔG, named in honor of American chemist, J. Willard Gibbs, who developed an equation to relate enthalpy and entropy to whether or not a reaction was likely to occur.

21. ΔG = ΔH -TΔS Free Energy, ΔG Free energy, ΔG:
allows us to assign a value to an entire reaction to predict whether a reaction is spontaneous, product favored.
or nonspontaneous, reactant-favored.
The basic equation for free energy is:
ΔG = ΔH -TΔS
Free Energy
Enthalpy
Entropy
temperature in Kelvin

22. Gibbs Free Energy, ΔGrxn
Negative Gibbs Energy (-ΔGrxn)
Spontaneous, Product favored
Positive Gibbs Energy (+ΔGrxn)
Nonspontaneous, Reactant favored
A ΔG of zero means that neither the products or reactants are favored-the reaction is in equilibrium.

23. DG = DH - TDS

24. Some factors will affect reaction rate:
Reaction Rates
Reaction rates
how fast a reaction proceeds.
Some factors will affect reaction rate:
Temperature of reactants: higher = faster
Concentration of reactants: greater = faster
Surface area of reactants: greater = faster
(powders react faster than chunks)
Catalyst presence: catalysts make rxns faster
Catalysts reduce activation energy!
End of C17, conclusion follows

25. In conclusion… Specific Heat Capacity, cp Enthalpy, ΔH Entropy, ΔS
the amount of heat energy required to raise 1 gram, 1 degree
Enthalpy, ΔH
the heat energy transferred in a reaction
Entropy, ΔS
the change in disorder of the species in a reaction
Gibbs Free Energy, ΔG
measure of spontaneity; how product favored or reactant favored a reaction is

26. In conclusion… Specific Heat Capacity, cp Enthalpy, ΔH Entropy, ΔS
Hey! 1 calorie=4.18 joules!
In conclusion…
Specific Heat Capacity, cp
Joules / gram * degree
Enthalpy, ΔH
kilojoules / mole
Entropy, ΔS
Joules / mole*Kelvin
Gibbs Free Energy, ΔG
Although you can use degrees Celsius or Kelvin for the first three, you MUST use Kelvin with Gibbs Free Energy
Recall that K = C
End of Chapter 17

27. CCSD Syllabus Objectives
16.1: Thermodynamics, definition
16.2: Exothermic/Endothermic
16.3: Changes in Enthalpy
16.4: Thermochemical Calculations
16.5: Energy Diagrams
16.6: Enthalpy-Entropy-Free Energy
17.1: Kinetics Definition
17.2: Factors that Affect Reaction Rate

28. Aligned Labs and Demos Lab: Flaming Peanut Calorimetry Lab
Lab: Metals Lab
Lab: NaOH-HCl Enthalpy of Reaction Lab
Lab: KI-H2O2 Kinetics Lab
Demo: Carbon Snake (Kinetics)