Question

As shown here, one type of computer keyboard cleaner contains liquefied 1,1 -difluorocthane \(\left(\mathrm{C}_{2} \mathrm{H}_{4} \mathrm{~F}_{2}\right)\). which is a gas at atmospheric pressure. When the nozzle is squeezed, the 1,1 -difluoroethane vaporizes out of the nozzle at high pressure, blowing dust out of objects. (a) Based on your experience, is the vaporization a spontaneous process at room temperature? (b) Defining the 1,l-difluoroethane as the system, do you expect \(\mathrm{q}_{\mathrm{y}}\) for the process to be positive or negative? (c) Predict whether \(\Delta S\) is positive or negative for this process. (d) Given your answers to (a), (b), and (c), do you think the operation of this product depends more on enthalpy or entropy? [Sections 19.1 and 19.2]

Short answer

(a) The vaporization of 1,1-difluoroethane from the keyboard cleaner is a spontaneous process at room temperature when the nozzle is squeezed. (b) The heat transferred to the system, q, is positive for this process, as the vaporization absorbs heat from the surroundings. (c) The change in entropy, ∆S, is positive for this process, as the level of disorder increases when the molecules move from a liquid to a gaseous state. (d) The operation of the product depends more on entropy than enthalpy, as the spontaneous vaporization process involves a significant increase in disorder (positive ∆S).

Step-by-step solution

(a) Spontaneity of vaporization process at room temperature

: In our experience with this kind of keyboard cleaner, the vaporization of 1,1-difluoroethane occurs spontaneously at room temperature when the nozzle is squeezed. This means that when the pressure is released from the container, the gas vaporizes and pushes the dust out of objects.

(b) Heat transfer to the system

: When the vaporization of 1,1-difluoroethane occurs, it absorbs heat from the surroundings because the process requires energy to overcome the intermolecular forces and allow the molecules to escape from the liquid state to the gaseous state. This means that the heat transferred to the system, q, would be positive for this process.

(c) Predicting the change in entropy

: Vaporization is a process where a liquid turns into a gas. In this process, the molecules move from a more ordered state (liquid) to a less ordered state (gas). This implies that the level of disorder increases during the transformation. Therefore, the change in entropy (∆S) for this process would be positive.

(d) Operation dependence: Enthalpy or Entropy

: Given that the vaporization is spontaneous, q is positive, and ∆S is positive, we can infer that the operation of this product depends more on entropy rather than enthalpy. The spontaneous vaporization process involves a significant increase in disorder (positive ∆S), which can be more influential than the changes in the system's internal energy.

Key concepts

Spontaneous Processes

Spontaneous processes are those that occur naturally without external influence once they are initiated. Consider vaporization of a substance like 1,1-difluoroethane in a keyboard cleaner. Once the nozzle is pressed, the liquid rapidly turns into gas. This transformation occurs because vaporization is spontaneous at room temperature. A spontaneous process doesn't imply any speed of occurrence. Instead, when the conditions are right, like a pressure drop in our example, the process naturally happens. This spontaneity in vaporization is primarily driven by the inherent increase in entropy, or disorder, when molecules escape from the liquid state to become a gas. In practical terms, it's the reason why your keyboard cleaner works effectively when you spray it.

Enthalpy

Enthalpy is a thermodynamic property that reflects the total heat content of a system. In the context of vaporization, such as 1,1-difluoroethane turning into gas, enthalpy comes into play as the heat absorbed or released during the process. For vaporization processes like this, heat must be absorbed from the surroundings. This is because transforming liquid into gas requires energy to overcome intermolecular forces. This absorbed heat makes the process endothermic, meaning the enthalpy change (\(q_{ ext{vaporization}}\)) is positive. Despite enthalpy’s role in supplying energy for the phase change, the spontaneity of such operations is often more influenced by changes in entropy rather than enthalpy.

Entropy

Entropy is a measure of disorder or randomness in a system. When 1,1-difluoroethane vaporizes, its molecules move from a relatively ordered liquid state to a disordered gas state. This transition results in an increase in entropy (\(\Delta S\)), meaning the system becomes more disordered. A positive change in entropy is common for vaporization because gas particles occupy more space and possess greater freedom of movement compared to their liquid counterparts. Entropy plays a crucial role in spontaneous processes, making them occur naturally. It becomes evident that processes driven by the need for increased disorder often demonstrate spontaneous behavior.

Vaporization

Vaporization is the process through which a substance transitions from a liquid to a gaseous state. For substances like 1,1-difluoroethane, this phase change is used practically in products like keyboard cleaners. When vaporization occurs, molecules need to gain sufficient energy to overcome the forces holding them in the liquid state. This gain is facilitated by heat absorbed from the surroundings, highlighting the endothermic nature of vaporization. Moreover, this phase change increases the randomness in molecular motion, contributing to a rise in entropy. Practical applications rely on this increase in entropy and the corresponding energy input to efficiently disperse gases like cleaning aerosols. The transformation is spontaneous in such scenarios due to the favorable conditions of increased molecular disorder and positive energy absorption.