Thermodynamics Practice
Slick Rick is offering a “good deal” on platinum coins.
He proffers a one ounce (28.3 g) platinum coin to you
for a mere $175, a real steal. You know that he is
smart enough not to try to slip you a different weight
coin, but you suspect that perhaps the coin is just
plated with platinum. You “accidentally” drop the coin
in your precisely 50.0 mL of coffee in a Styrofoam cup
that you only drink at 60.0C. If the temperature drops
to 57.6C, is the coin likely real?
Specific Heat of Platinum: 0.13 J/gK
Specific Heat of Water: 4.184 J/gK
Hint: This will not be given on the
exam. It is the conversion factor
between J and calories though.
What is the specific heat of the coin?
Is it the value of platinum?
Finding Specific Heat
First Law of Thermo:
0kJ (coffee cup)
DE = 0 = q water + q metal + q cal
q metal = -q water
mmCmDTm= - mwCwDTw
q metal = mmCmDTm
q water = mwCwDTw
Cm= - mwCwDTw
mmDTm
Cm= - 50.0g (4.184 g/JC) (60.0C - 57.6C) = .48 g / J C
28.3 g (57.6C - 21C)
Lithium chloride dissolving in water is an exothermic
reaction. If 3.45 g of solid lithium chloride is dissolved in
85.7 mL of water in a coffee cup calorimeter, and the
temperature of the water goes from 21.0C to 29.6C,
what is the DHsol for LiCl?
LiCl(s)
LiCl(aq)
First Law of Thermo:
0kJ (coffee cup)
DE = 0 = q water + q rxn + q cal
q rxn = -q water
What amount of energy
was absorbed by the
water?
q water = mwCwDTw
q rxn = n DHrxn
Note: This looks exactly like
other phase changes!
Trinitroglycerin, C3H5N3O9, is an explosive. The enthalpy
of decomposition (below) at 1 atm pressure is -1541.4
kJ/mol. What is the DHf for nitroglycerin?
C3H5N3O9(l)
N2(g) + CO2(g) + H2O(l) + O2(g)
DHf
CO2 (g)
H2O (g)
H2O (l)
Energy (kJ/mol)
-393.5
-241.8
-285.8
Need to balance the reaction!
C3H5N3O9(l)
1.5N2(g) + 3CO2(g) + 2.5H2O(l) + .25O2(g)
C3H5N3O9(l)
1.5N2(g) + 3CO2(g) + 2.5H2O(l) + .25O2(g)
DHrxn = S(DHfprod) – S(DHf react) =
= 1.5 DHf N2 + 3DHf CO2 + 2.5DHf H2O + .25 DHf O2
– DHf Nitro
-1541.4 kJ =(0) + 3mol(-393.5 kJ/mol) + 2.5mol (-285.8kJ/mol) + 0
– DHf Nitro
DHf Nitro = -353.6 kJ/mol
The heat of combustion of fructose, C6H12O6, is
-2812 kJ/mol. If a fresh golden delicious apple
weighing 4.23 oz (120 g) contains 16.0 g of fructose,
what Caloric content does the fructose contribute to
the apple?
C6H12O6 + 6O2(g)
DHf
CO2 (g)
H2O (g)
H2O (l)
1 calorie = 4.184 J;
6CO2(g) + 6H2O(l)
Energy (kJ/mol)
-393.5
-241.8
-285.8
1 Dietary Calorie = 4.184 kJ
Hess’s Law
Using the following reactions and enthalpies:
H2(g) + F2(g)
C(s) + 2F2(g)
2C(s) + 2H2(g)
2HF(g)
CF4(g)
C2H4(g)
DH = -537kJ
DH = -680kJ
DH = +52.3kJ
What is the DHrxn for the following process (give answer in kJ):
C2H4(g)
+
6F2(g)
2CF4(g)
+ 4HF(g)