Study Guide for Free-Response Questions on Gases and Chemical Reactions

FREE-RESPONSE QUESTIONS ON GASES AND REACTIONS

General Setup

• A demonstration involving two cylinders containing gases at 27°C (room temperature).
• Cylinders:
    - Cylinder on the left: Contains hydrogen gas (H2) and has two electrodes for an electrical spark, providing activation energy for the reaction.
    - Cylinder on the right: Contains oxygen gas (O2).
• Reaction Equation:
  $2 ext{ H}_2(g) + ext{ O}_2(g) \rightarrow 2 ext{ H}_2 ext{O}(g)$
• Final temperature of the reaction chamber is 117°C after the reaction completes.

Question Breakdown

(a) Moles of Hydrogen Gas Before Reaction

• To find the moles of hydrogen gas present before the reaction:
    - Utilize the Ideal Gas Law:
  $PV = nRT$
    - Where:
      - P is pressure (in atm)
      - V is volume (in liters)
      - n is the number of moles
      - R is the gas constant, 0.0821 L·atm/(K·mol)
      - T is temperature in Kelvin (K = °C + 273.15)
    - Given data for hydrogen:
      - Volume: 72.0 L
      - Pressure: 14.7 atm
      - Temperature: 27°C = 300.15 K
• Calculation:
    - Substitute values into the equation:
  $n = \frac{PV}{RT} = \frac{(14.7 ext{ atm})(72.0 ext{ L})}{(0.0821 ext{ L·atm/(K·mol)})(300.15 ext{ K})}$
• Final result provides the moles of hydrogen gas.

(b) Remaining Moles of Oxygen Gas

• Assuming 58% of oxygen gas is injected into the hydrogen cylinder:
    - Let the initial moles of oxygen be calculated first. If the total moles of oxygen is $n_O$, then:
  $0.58 imes n_O$
    - Remaining oxygen after injection will be:
  $ext{Remaining } O_2 = n_O - 0.58 imes n_O = 0.42 imes n_O$
• Calculate moles of oxygen gas that remain in the cylinder on the right.

(c) Total Pressure After Reaction Completion

• Given that:
    - 4.4 moles of hydrogen remains after the reaction
    - 38.6 moles of water vapor produced
    - Use the Ideal Gas Law to find the total pressure in the cylinder after the reaction:
    - Total moles in the reaction chamber:
  $n_{total} = ext{moles H}<em>2 + ext{moles O}_2 + ext{moles H}_2O$   - For the remaining pressure calculation, appropriate substitutions from the results derived in (a) and (b) should be applied:   - $P</em>{total} = \frac{n_{total}RT}{V}$

(d) Behavior as Ideal Gases

• Ideal Gas Behavior: Determine which gaseous reactant behaves most like an ideal gas and justify:
    - Discuss ideal gas conditions: low intermolecular forces, minimal volume compared to the container volume, non-polar nature.
    - Compare molecular sizes and interactions of H2 and O2
    - Conclude based on the properties of gases under high pressure and low temperature conditions.

(e) Bond Angle in Water Molecule

• The bond angle in water (H2O) is observed to be 104.5°
    - Discussion on the geometry of water:
      - VSEPR theory predicts molecular geometry based on electron pair repulsion.
      - Lone pairs on the oxygen atom cause the bending of the hydrogen atoms closer together than in a perfect tetrahedral bond angle.

(f) Hybridization of Oxygen Atom in Water

• The hybridization of the oxygen atom in a water molecule is:
    - $sp^3$ hybridization results from the mixing of one s orbital and three p orbitals, leading to tetrahedral electron pair geometry.

(g) Observed vs Calculated Pressure

• Discuss expectations for the observed pressure (P_obs) compared to calculated pressure from part (c):
• Consider the conditions impacting observed pressure:
    - Possible deviations from ideal behavior (real gas effects).
    - Potential effects of temperature change and the presence of liquid water (condensation) on total pressure measurements.
• Reasoning should reference the definitions of ideal versus real gases and observed behavior under varying conditions.