Question

Explain what is meant by a state function. Give two examples of quantities that are state functions and two that are not state functions.

Short answer

A state function depends only on the system's state, not on the path. Examples: enthalpy and entropy. Non-state functions: work and heat.

Step-by-step solution

Understanding State Functions

A state function is a property of a system that only depends on the current state of the system, not on the path taken to reach that state. This means that any change in a state function is path-independent.

Examples of State Functions

Examples of state functions include enthalpy (H) and entropy (S). Both these quantities only depend on the initial and final states of a system, irrespective of the process the system underwent to evolve from one state to another.

Non-State Functions Explanation

Non-state functions are properties that depend on the path taken during a process. This means their values can change based on how a process occurs, as opposed to just the initial and final states.

Examples of Non-State Functions

Examples of non-state functions include work (W) and heat (Q). These quantities depend on the specific pathway taken by the system to transition between states and not just on the states themselves.

Key concepts

Understanding Enthalpy

Enthalpy, denoted as $H$, is a state function widely used in thermodynamics. It represents the total heat content of a system and is particularly helpful in processes occurring at constant pressure. The formula for enthalpy change is $\mathrm{\Delta }H=H_{final}-H_{initial}$. This change provides us with the heat absorbed or released during a process. Since enthalpy is a state function, its value relies only on the current state of the system and not the path taken to reach that state.

• Commonly used in chemistry to examine reactions conducted at constant pressure.
• Helps to determine if a reaction is endothermic (absorbs heat) or exothermic (releases heat).

Exploring Entropy

Entropy, represented by $S$, is another crucial state function that measures the disorder or randomness of a system. Entropy provides insight into the energy dispersed in a system at a given temperature. It inherently increases over time; this is a natural tendency toward disorder in isolated systems.
Unlike non-state functions, entropy depends solely on the initial and final states. However, calculating absolute entropy often involves complex pathways because temperature and volume changes can affect entropy significantly.

• Useful in determining the feasibility of a process or the direction it will naturally proceed.
• Helps to explore the second law of thermodynamics stating that entropy of an isolated system never decreases.

Defining Non-State Functions

Non-state functions, unlike enthalpy and entropy, are path-dependent properties. This means that the values of non-state functions can vary based on the process undertaken to reach the final state. Some common examples include work and heat.

• Their values depend heavily on the means by which a state is reached.
• Critically important in fields like engineering where process specifications matter.

Understanding Work

Work, represented as $W$, is a non-state function that reflects the energy transferred when an object is moved by a force. In thermodynamics, work depends on the path or process following which the state change occurs. For instance, the work done in a gas expansion depends on how it is performed, such as isothermally or adiabatically.

• Calculated as $W=\int PdV$ for gases, indicating it is path-dependent.
• An essential concept in understanding energy transfer in mechanical processes.

Exploring Heat

Heat, denoted by $Q$, is another non-state function similar to work in that it is path-dependent. It quantifies the transfer of energy due to temperature differences and is sensitive to the process specifics. The amount of heat exchanged depends on how a process flows between initial and final states and not solely on the states themselves.

• Given the formula $Q=\int CdT$ for small changes, linking it to temperature change.
• Essential in understanding thermal energy exchange in chemical and physical processes.
• In practical applications, tracking how heat moves through materials can dictate efficiency and safety.