Sample Final Exam Part 2 Notes

Accuracy vs. Precision

Definition:
- Accurate: Data is close to the true or accepted value.
- Precise: Data is consistent and repeatable.
- Student D's Data: Accurate but not precise.
  - Average: 122 °C (matches the known value).
  - Range: Large compared to other students.

Strong Electrolytes:
- Completely dissociate into ions in water.
- Ions move freely, conducting electricity well.
- HCl: A strong electrolyte.
  - Separates into H^+ and Cl^- ions in water.

Claim:
- The atom has a very small and very dense nucleus.
Evidence:
- Most alpha particles directed at gold foil went through.
- Occasionally, alpha particles deflected at large angles.
Reasoning:
- Most particles going through indicates the nucleus is very small.
- Deflection of alpha particles at large angles indicates the nucleus is very dense.

Claim: Atoms are divisible with subatomic particles that we now know as electrons.
Evidence:
- When applying a potential difference between two electrodes, green rays were observed, which deflected by an electric field. These green rays were observed regardless of the material of the electrodes.
Reasoning:
- Deflection of green ways towards + of the electric field indicates the particle carries - charge. The independence of material indicates it is present in all types of atoms.

Comparison of CH4 and CCl4:
- CCl_4 has a higher boiling point.
Reasoning:
- CCl_4 has more electrons and is more polarizable.
- Stronger dispersion forces require more energy to overcome, leading to a higher boiling point.

First Ionization Energy (Li to Ne):
- Trend: Increases from Li to Ne.
- Cause: Effective nuclear charge increases, pulling valence electrons closer to the nucleus and increasing their attraction.
Large Increase in Ionization Energy:
- Observation: Very large increase between the third and fourth ionization energy of B.
- Explanation: After 3 ionizations, B has a noble gas electron configuration, requiring a large amount of energy to remove an electron.

Pressure as Evidence:
- Claim: Gas molecules are in constant, random motion.
- Evidence: Pressure is created when gas particles randomly collide with the walls of their container.
- Reasoning: Collisions generate a force. Summing all of these forces over a given area accounts for the pressure observed.

Glycine in Aqueous Solution:
- Includes drawing the most stable Lewis structure with lone pairs and bonding pairs.
- Assigning formal charges to the structure.
O-C-O Bond Angle:
- Angle: 120°
- Reasoning: In O-C-O, the central C atom has 3 electron domains, and all 3 are bonding pairs. This indicates a trigonal planar molecular geometry, and the bond angle is 120°.

Hydrogen Bonding Strength:
- Hydrogen bonding intermolecular forces are stronger than ordinary dipole-dipole intermolecular forces.
Justification using Coulomb's Law:
- The covalent bond between H and N, O, or F is very polar, resulting in large partial charges (\delta^+/\delta^-).
- The small size of H allows the molecules to get closer together.

Balanced Chemical Equation:
- CH4(g) + 2O2(g) \rightarrow CO2(g) + 2H2O(l)
Enthalpy of Combustion (\Delta H_{comb}):
- \Delta H{comb} = \Sigma n \cdot \Delta H{f,prod} - \Sigma m \cdot \Delta H_{f,react}
- Given:
  - \Delta Hf(H2O, l) = -285.82 \frac{kJ}{mol}
  - \Delta Hf(CO2, g) = -393.5 \frac{kJ}{mol}
  - \Delta Hf(CH4, g) = -74.85 \frac{kJ}{mol}
- \Delta H_{comb} = [1 \cdot (-393.5) + 2 \cdot (-285.82)] - [1 \cdot (-74.85) + 2 \cdot 0]
- \Delta H_{comb} = -890.3 \frac{kJ}{mol}
Pressure Calculation:
- Given:
  - Volume (V) = 5.0 L
  - Temperature (T) = 25 °C = 298.15 K
- Ideal Gas Law: PV = nRT
- n = \frac{PV}{RT} = \frac{P \cdot 5.0}{0.08206 \cdot 298.15}
- P = \frac{nRT}{V}
- P = \frac{0.0478 \cdot 0.08206 \cdot 298.15}{5.0}
- P = 0.23 atm