Question

Au(s) ⇌ Au(l)

Above equilibrium is favoured at

• A. High pressure low temperature
• B. High pressure high temperature
• C. Low pressure, high temperature
• D. Low pressure, low temperature

Answer 1

Answer: (C)

Low pressure, high temperature

Answer 2

The equilibrium given is the melting of gold:

$Au(s) \rightleftharpoons Au(l)$

Let's analyze the effect of temperature and pressure on this equilibrium using Le Chatelier's Principle.

1. Effect of Temperature:

Melting is an endothermic process, meaning it absorbs heat.

$Au(s) + Heat \rightleftharpoons Au(l)$

According to Le Chatelier's Principle, increasing the temperature will shift the equilibrium in the direction that absorbs heat, which is towards the formation of liquid gold. Therefore, high temperature favors the melting of gold.

2. Effect of Pressure:

We need to consider the change in volume during the melting of gold. For most substances, including gold, the solid phase is denser than the liquid phase. This means that when gold melts, its volume increases.

Density of solid gold ($Au(s)$) is approximately 19.3 g/cm³.

Density of liquid gold ($Au(l)$) at its melting point is approximately 17.3 g/cm³.

Since density = mass/volume, for a given mass, a lower density corresponds to a larger volume. Thus, $V_{liquid} > V_{solid}$.

So, the process $Au(s) \rightleftharpoons Au(l)$ involves an increase in volume ($\Delta V > 0$).

According to Le Chatelier's Principle, increasing the pressure on a system at equilibrium will shift the equilibrium in the direction that reduces the volume. Conversely, decreasing the pressure will shift the equilibrium in the direction that increases the volume. Since melting gold leads to an increase in volume, low pressure will favor the formation of liquid gold.

Combining both factors:

To favor the equilibrium $Au(s) \rightleftharpoons Au(l)$ (i.e., melting of gold), we need:

• High temperature (because melting is endothermic)
• Low pressure (because melting of gold leads to an increase in volume)

Therefore, the equilibrium is favored at low pressure and high temperature.