## Phases of Matter and Thermochemistry Review Sheet Answer Key

Chapter 9

1. All gases are composed of particles.
2. All gas particles are in constant, random motion.
3. All collisions are elastic. (i.e. kinetic energy is conserved)

1. Gas pressure is caused by gas particles bouncing off each other and other objects.

2. a)
b)
c) K = °C + 273 = -125 + 273 = 148 K

3. Evaporation is a cooling process because the particles with higher kenetic energy have enough to overcome the attractive intermolecular forces, therefore leaving behind lower kenetic energy particles, which are cooler. Essentially the hot molecules vaporize leaving behind the cooler ones.

4. Allotropes are two or more forms of the same element in the same phsyical state. (e.g. Graphite and diamonds, or, Oxygen(O2) and Ozone(O3))

5. See the diagram for the labeling.

6. Approximately 65 °C.

7. The substance is a liquid.

8. The diagram shows that molecules have a range of energies and that a measured temperature is the average kenetic energy of a substance. As an object becomes warmer, the range becomes wider.

9. Intermolecular forces are the attractions between molecules. The stronger the forces the harder it is to boil a substance since the forces must be overcome. Substances with weak inter molecular forces boil very easily.

10. The boiling point of a liquid is when the vapor pressure of a liquid equals the outside pressure. At higher altitude the outside pressure is lower, so particles leaving the liquid are less likely to collide with air particles, therefore the particles need less energy to escape the liquid.

Chapter 10
You will have to use pp. 266 and 276 for constants for various problems.

1. a)
b) Water would require a bit less than 2X more heat because its specific heat is almost 2X that of ethanol.

2. 3 steps:
1. heat ice from -11.0 °C to 0 °C.
2. melt ice at 0 °C
3. heat water from 0 °C to 21.0 °C

3. C2H4 + 3 O2 --> 2 CO2 + 2 H2O + 1390 kJ

4. a) DHrxn= [2 mol(33.85 kJ/mol)]-[2 mol(90.37 kJ/mol) + 0] = -113.04 kJ
b) DHrxn= [2 mol(0) + 3 mol(-393.5 kJ/mol)] - [1 mol(-822.1 kJ/mol) + 3 mol(-110.5 kJ/mol)] = -26.9 kJ

5. Equation to get: Mn(s) + O2 (g) --> MnO2 (s)
Reverse the 1st equation on the review sheet, change the sign on DHrxn
MnO(s) + 1/2 O2 (g) --> MnO2 (s)   DH = -32.5 kcal
1 O2 is needed, so multiply this equation by 2, including the DHrxn
2 MnO(s) + O2 (g) --> 2 MnO2 (s)   DH = -65.0 kcal
The 2nd equation on the review sheet is okay as written:
MnO2(s) + Mn(s) --> 2 MnO(s)   DH = -59.5 kcal
Add these last two equations together, including the DHrxn
Mn(s) + O2 (g) --> MnO2 (s)   DHrxn=-124.5 kcal

6. 2 Na2O2(s) + 4 HCl(g) --> 4 NaCl(s) + 2 H2O(1) + O2(g)   DH = ?
Leave the first equation on the review sheet as it is:
2 Na2O2(s) + 2 H2O(l) --> 4 NaOH(s) + O2(g)   DH = -30.2 kcal
Multiple the second equation on the review sheet by 4:
4 NaOH(s) + 4 HCl(g) --> 4 NaCl(s) + 4 H2O(l)   DH = -171.2 kcal