Unit 7 Mini Quiz 2

Q1a

Define equilibrium in terms of the rates of the forward and reverse processes for:

C₂H₅OH(l) ⇌ C₂H₅OH(g)

🟠 Answer:

• Equilibrium is when the forward and reverse reactions occur at equal rates.

✅ Explanation:

Equilibrium is reached when the rate of evaporation (liquid → gas) equals the rate of condensation (gas → liquid). Although both processes still occur at the molecular level, the concentrations of liquid and gaseous ethanol remain constant.

Q1b

Sketch a graph showing how the concentration of liquid and gaseous ethanol change over time as equilibrium is established.

🟠 Answer:

Acceptable graphs:

• Liquid starts high and levels off lower
• Gas starts low and levels off higher

✅ Explanation:

As ethanol evaporates, the concentration of vapor increases while the liquid decreases. At equilibrium, both curves flatten as dynamic equilibrium is reached. The system shows no net change, although molecular changes still occur.

Q1c

The student opens the container, allowing ethanol vapor to escape. Predict the effect on the system and explain in terms of equilibrium shifts.

🟠 Answer:

• The equilibrium shifts to the right (towards the gas).
• This is to replace the lost ethanol vapor.

✅ Explanation:

When the gaseous ethanol is removed, the system is no longer at equilibrium. According to Le Châtelier’s Principle, the system shifts to the right to produce more gas and restore balance.

Q2a

Explain how the following is a reversible reaction:

NH₄Cl(s) ⇌ NH₃(g) + HCl(g)

🟠 Answer:

• Both forward and reverse reactions occur simultaneously.

✅ Explanation:

Ammonium chloride decomposes into NH₃ and HCl gases (forward). The gases can recombine to reform NH₄Cl (reverse). This back-and-forth ability is what defines a reversible reaction.

Q2b

Describe the molecular-level processes occurring in this system at dynamic equilibrium.

🟠 Answer:

• Forward: NH₄Cl → NH₃ + HCl
• Reverse: NH₃ + HCl → NH₄Cl
• Rates are equal at equilibrium

✅ Explanation:

At the molecular level, solid NH₄Cl continuously decomposes while the gases recombine. Dynamic equilibrium means these processes occur at equal rates, so the overall macroscopic concentrations remain unchanged.

Q2c

Sketch a graph of pressure vs time as the system approaches equilibrium. Explain the trend.

🟠 Answer:

• Pressure increases rapidly, then levels off.
• Graph shows asymptotic approach to constant pressure.

✅ Explanation:

Initially, the forward reaction dominates, producing NH₃ and HCl gases → pressure rises. As equilibrium is approached, the rate of decomposition equals recombination. Gas production balances out and pressure becomes constant.