1. Thermochem Quick review

2. General Thermochemistry Terms
Thermodynamics -Study of NRG changes & flow of NRG.
Tells whether a reaction is possible.
system - That portion of the universe on which
we focus.
surroundings - Everything outside the system.
Isothermal- A change that occurs at constant
temperature.
state functions Properties that depend on the initial and
final state, not on how the change was made. ex: DH, temp, but not work

3. Energy changes can be measured using calorimetry. Often
this involves heating water under controlled conditions (a
bomb calorimeter).
Three closely related terms are:
1. heat capacity is the amount of heat needed to change a
system by 1°C.
2. molar heat capacity is the amount of heat needed to change
a mole of a substance by 1°C.
3. specific heat is the amount of heat needed to change 1 gram
of a substance by 1°C. (Water: 1 cal/g°C = J/g°C)
Note that heat capacity is an extensive property whereas the
other two are intensive.

5. Coffee cup calorimeter q=mcpDT

6. Enthalpy Heat content of a substance Symbol H Unit kilojoules/mole
D H : Hproducts - Hreactants

7. Hess’ Law If several reactions add up to give an overall reaction, the D H’s of the reactions will add up to the overall DH.
Standard Heats of Formation, DHf ° , are useful for this
purpose. This is the energy involved in making one mole of a
substance from its elements at 25°C and 1 atm pressure.
This law is often written as:
DH° reaction = DHf ° products - DHf ° reactants
Note: DHf° for elements is 0.
If you are NOT using heats of formation, you need to write
out the equations to see how they combine.

8. BOND ENERGY Bond energy is the amount of energy needed to BREAK a
certain bond.
Breaking a bond is always endothermic
You can determine the approximate energy change in a
chemical reaction by adding the bond energies of the
reactants and subtracting from this the bond energies of the products. Reactants-products
Note: this is opposite to the Hess’s Law (products -
reactants) because bond energy involves breaking bonds
whereas DH°f involve forming bonds.

9. Entropy Larger and more complex molecules have greater entropies
Symbol is S (DS)
Unit J/K mol or J.mol-1K-1
The degree of disorder (randomness) in a system
Spontaneity tends toward + DS
The entropy of a pure crystalline substance at absolute zero is 0.
Larger and more complex molecules have greater entropies

10. FREE ENERGY There is an upper limit to how much work can be derived
from any system. That maximum work is called the Gibb’s
Free Energy, DG.
The theoretical maximum work can be achieved if the steps
are so small that they can go either way, if they are
reversible.

11. DG = DH – TDS
This represents the theoretical maximum work that can be done by a system.
A reaction is spontaneous when DG is negative.
When DG = 0, the reaction is at equilibrium.
MAKE SURE THAT YOUR DH AND DS HAVE THE SAME UNITS (CONVERT J IN DS INTO kJ
DH D S Spontaneous…
… at all temperatures
… at LOW temperatures
… at HIGH temperatures
reverse rxn spontaneous

12. MC 1
CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(g) For the reaction of methane represented above, H is -1,523 kJ mol-1. What is the value of H if the combustion produced liquid water H2O(l), rather than water vapor, H2O(g)? (H for the phase change H2O(g)  H2O(l) is -44 kJ mol-1). a. -1,479 kJ/mol b. -1,567 kJ/mol c. -1,323 kJ/mol d. -1,611 kJ/mol e.-1,411 kJ/mol

13. MC 2
Which gives the thermodynamic parameters of the phase change in a system that begins as an open container of liquid water placed in a constant temperature environment of 230 K?
G < 0
H < 0
S < 0
I only
III only
I and III only
II and III only
I, II and III

14. MC 3
Which gives the thermodynamic parameters of the phase change in a system that begins as an open container of liquid water placed in a constant temperature environment of 230 K?
G < 0
H < 0
S < 0
I only
III only
I and III only
II and III only
I, II and III

15. Bond (kilojoules/mol)
MC 4
I2(g) + 3 Cl2(g)  2 ICl3(g)
According to the data in the table below, what is the value of Hof for the reaction represented above?
Average Bond Energy
Bond (kilojoules/mol)
I-I
Cl-Cl
I-Cl
+450 kJ/mol
+1,260 kJ/mo
-870 kJ/mol
-390 kJ/mol
+180 kJ/mol

16. Na2O(s) + H2O(l)  2 NaOH(s)
MC 5
H2(g) + O2(g)  H2O(l) Ho = x
2 Na(s) + O2(g)  Na2O(s) Ho= y
Na(s) + O2(g) + H2(g)  NaOH(s) Ho= z
Based on the information above, what is the standard enthalpy change for the following reaction?
Na2O(s) + H2O(l)  2 NaOH(s)
x + y + z
x + y - z
x + y - 2z
2z - x - y
z - x - y