Question

The enthalpy change for a reaction is a state function and it is an extensive property. Explain.

Short answer

The enthalpy change for a reaction (\(\Delta H\)) is a state function as it depends only on the initial and final states of the reaction and not on the path taken during the reaction. It is an extensive property because its value depends on the amount of material participating in the reaction.

Step-by-step solution

Understand state functions

A state function is a property that depends only on the current state of a system and not on the path taken to reach that state. For example, energy, pressure, and temperature are state functions, as their values depend on the current state and are independent of the process taken to reach the current conditions.

Understand extensive properties

An extensive property is a property of a system that depends on the amount of material or size of the system. In other words, if you double the amount of material or change the size of the system, the value of the extensive property will also change. Examples of extensive properties include mass, volume, and enthalpy.

Define enthalpy

Enthalpy (H) is a thermodynamic property of a system and is defined as the internal energy of the system (U) plus the product of its pressure (P) and volume (V). It can be represented mathematically as: \[ H = U + PV \]

Enthalpy change as a state function

The enthalpy change for a reaction (\(\Delta H\)) is the difference between the enthalpy of the products and the enthalpy of the reactants. Since enthalpy is a state function, the change in enthalpy (\(\Delta H\)) depends only on the initial and final states of the reaction, and not on the path taken during the reaction. Therefore, the enthalpy change for a reaction is a state function.

Enthalpy change as an extensive property

As mentioned earlier, extensive properties depend on the amount of material or size of the system. When considering enthalpy change in a reaction, it is dependent on the amount of reactants undergoing the reaction. For example, if twice the amount of reactants are used in the reaction, the enthalpy change (\(\Delta H\)) will also be doubled. Thus, the enthalpy change for a reaction is an extensive property. In conclusion, the enthalpy change for a reaction is a state function because it depends only on the initial and final state of the reaction and not the path taken during the reaction. It is also an extensive property because its value depends on the amount of material participating in the reaction.

Key concepts

State Function

Enthalpy, symbolized as H, is a thermodynamic quantity that is a classic example of a state function. But what does that mean? Think of a state function as a bit like your location on a map. It doesn't matter whether you took the scenic route or the fastest highway to get there; your current location is independent of the path you took. Similarly, state functions are properties of a system—such as pressure, temperature, and yes, enthalpy—that solely depend on the current conditions, not the history.

To dive a bit deeper, consider a hiking trail up a mountain. Whether you meander slowly along a winding path or take a direct, steep climb, the elevation gain—the equivalent of a state function in this analogy—remains consistent. In chemistry, during chemical reactions, it's the change in enthalpy ()Delta H) that interests us, reflecting the difference in heat content between products and reactants. This change is determined only by the states of these reactants and products, not the 'route' the reaction took to get from one to the other.

Extensive Property

Extensive properties are like the ingredients for a recipe—the more ingredients you have, the more of the final dish you can make. In the realm of thermodynamics, these properties depend on the size or amount of material present in the system. Mass and volume are straightforward examples: double the amount of dough, and you'll get twice as many cookies. Enthalpy belongs in this category too.Why is this important?

Understanding Proportionality

Grasping the concept of an extensive property helps in predicting how system properties will change with size. If a chemical reaction releases energy (as heat), doubling the amounts of reactants will essentially double the energy released. This proportionality is crucial for scaling up experiments from a lab bench to industrial production where predicting outcomes based on quantities is essential.

Thermodynamics

The science of thermodynamics puts the spotlight on energy: its forms, its transfer, and how it relates to matter. It's the backstage manager of every chemical scene, overseeing how energy is stored, moved, and transformed, especially during chemical reactions.

Here's a simple breakdown: a reaction that takes in energy from the surroundings is endothermic, while one that releases energy is exothermic. Enthalpy change, a key player in this energetic drama, measures the heat absorbed or released under constant pressure. It allows us to quantify whether a reaction warms up its surroundings or cools them down, an insight that's invaluable in everything from creating hand warmers to designing engines.

Chemical Reactions

Chemical reactions are the essential processes that make and break chemical bonds, transforming reactants into products. Like a dance, substances come together, interact, and emerge changed. The enthalpy change is akin to the energy of the dance—it tells us how much energy is released or required for the reaction to occur.

Energy Profiles of Reactions

Imagine you're watching a dance, and you can visually map the energy of the performance from start to finish. Chemical reactions have energy profiles, too. These illustrate the energy required to break the original bonds and the energy released when new bonds form. The difference between the two gives us the enthalpy change. A drop in energy (negative )Delta H) signals an exothermic reaction, where energy is given off. Conversely, a climb (positive )Delta H) indicates an endothermic reaction, requiring an energy input. Whether creating life-saving medicine or simply cooking in your kitchen, understanding the enthalpy change in chemical reactions helps us control and utilize these processes effectively.