Question

When pressure is applied to the equilibrium system: Ice \(\rightleftharpoons\) water, which of the following phenomenon will happen? (a) more ice will be formed (b) ice will sublime (c) more water will be formed (d) equilibrium will not disturb

Short answer

The phenomenon that will occur is (c) more water will be formed.

Step-by-step solution

Identify the Principle Involved

The response of an equilibrium system to an external pressure change is predicted by Le Chatelier's Principle. This principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.

Analyze the Equilibrium System

The given equilibrium involves the phase change between ice and water. This equilibrium can be represented as solid ice in equilibrium with its liquid form, water. When the pressure is increased, the system tends to adjust itself to reduce that change.

Predict the Effect of Pressure

Increasing the pressure on the system favors the phase with the higher density. Since ice has a lower density than water, applying pressure will cause the equilibrium to shift to the phase with higher density, which is water.

Determine the Outcome

Given the shift in equilibrium due to the increased pressure, the outcome will be the formation of more water from ice to reduce the change in external pressure.

Key concepts

Chemical Equilibrium

Chemical equilibrium refers to the state of a chemical reaction in which the rates of the forward and reverse reactions occur at a constant rate, leading to no overall change in the concentration of reactants and products over time. Although the compounds may still be reacting, they do so in such a way that the amount of each substance remains steady. For instance, water and ice can coexist at 0°C (32°F), at which they are in equilibrium; the melting of ice occurs at the same rate as the freezing of water, maintaining a balance. Understanding how disturbances such as temperature, concentration, and pressure affect equilibrium is crucial, and this is where Le Chatelier's Principle comes into play.

Le Chatelier's Principle asserts that if an external condition is changed, the equilibrium will shift to counteract the imposed change. Application of this principle aids in predicting how equilibrium will adjust when subjected to stress. It is an invaluable tool for scientists and engineers, who use it to control reactions to obtain desired products more efficiently.

Phase Change

Phase change refers to the transition of a substance from one state of matter—solid, liquid, or gas—to another. These changes occur due to variations in energy, temperature, or pressure surrounding the substance. Common phase changes include melting (solid to liquid), freezing (liquid to solid), vaporization (liquid to gas), and condensation (gas to liquid). Particularly relevant to our discussion is the melting/freezing equilibrium.

As seen in the equilibrium of ice and water, phase changes are reversible and can reach a point of dynamic equilibrium. In a closed system, water molecules escape into the air (evaporation) as well as condense from the air (condensation) at the same rate, representing a vapor-liquid equilibrium. Similarly, ice and water reach a balance between freezing and melting rates at equilibrium. This balance can be influenced by alterations in external conditions such as pressure, often evaluated using Le Chatelier's Principle.

Equilibrium Response to Pressure

When considering phase equilibria, pressure is an important factor that can significantly influence the point of equilibrium. According to Le Chatelier's Principle, increasing the pressure on a system in equilibrium will favor the phase that occupies less volume since this would reduce the imposed pressure change.

For example, gases show a clear response to pressure changes due to their high compressibility. However, solid-liquid equilibria, such as that of ice and water, also respond to pressure variations although to a lesser degree due to their relatively incompressible nature. The principle still holds: a phase with higher density — implying it has a smaller volume per unit of mass — will be favored under increased pressure.

Density and Pressure Relationship in Equilibrium

The density of a substance is defined as its mass per unit volume. In the context of equilibrium reactions involving phases of different densities, pressure has a noteworthy effect. High pressure intuitively favors the formation of the phase with the higher density because it is more 'packed' and occupies less space.

Linking this concept to our ice-water equilibrium, ice has a lower density than water, meaning that for the same mass, ice occupies a greater volume compared to water. Therefore, when the external pressure increases, equilibrium shifts toward the formation of more water since it has a higher density (lower volume) and this response is in line with minimizing the increased pressure. This provides a clear example of how the relationship between density and pressure plays a crucial role in determining the direction in which an equilibrium will shift in response to external pressure changes.