Question

Define these terms: enthalpy and enthalpy of reaction. Under what condition is the heat of a reaction equal to the enthalpy change of the same reaction?

Short answer

The heat of reaction equals the enthalpy change at constant pressure.

Step-by-step solution

Define Enthalpy

Enthalpy, denoted as \( H \), is a thermodynamic quantity that represents the total heat content of a system. It is the sum of the internal energy of the system plus the product of its pressure and volume, i.e., \( H = U + PV \), where \( U \) is the internal energy, \( P \) is pressure, and \( V \) is volume. It is often used to calculate the heat changes in chemical reactions conducted at constant pressure.

Define Enthalpy of Reaction

The enthalpy of reaction, \( \Delta H_{reaction} \), is the change in enthalpy observed in a chemical reaction, under standard conditions. It is the difference between the enthalpy of the products and the enthalpy of the reactants. It provides information about whether a reaction is exothermic (releases heat) or endothermic (absorbs heat).

Condition for Heat of Reaction = Enthalpy Change

The heat of reaction is equal to the enthalpy change of the same reaction when the reaction occurs at constant pressure and without doing non-PV work. In this case, any heat exchanged with the surroundings is equivalent to the change in enthalpy, as no other forms of work affect the system's energy.

Key concepts

Thermodynamic Quantity

In the world of thermodynamics, a 'thermodynamic quantity' is a property that helps us understand the state and behavior of a system. Enthalpy, denoted by the symbol \( H \), is one such quantity. It provides vital insight into how heat is involved in various processes, particularly chemical reactions. This quantity is a combination of the internal energy, \( U \), plus the product of pressure (\( P \)) and volume (\( V \)). Thus, \( H = U + PV \).

This equation suggests that enthalpy can be thought of as the total heat content of a system under constant pressure. It helps predict whether a process will release or absorb heat, guiding us in analyzing the energetics of reactions. By understanding enthalpy, we can better gauge how energy is exchanged in everyday processes, from heating our homes to running engines.

Enthalpy of Reaction

When we discuss the 'enthalpy of reaction,' we refer specifically to the change in enthalpy during a chemical reaction. This term is written as \( \Delta H_{reaction} \). It represents the difference in heat content between the products and reactants when a reaction is conducted under standard conditions.

The value of \( \Delta H_{reaction} \) helps us determine the thermal nature of the reaction:

• If \( \Delta H_{reaction} \) is negative, the reaction is exothermic, meaning it releases heat to the surroundings.
• If \( \Delta H_{reaction} \) is positive, the reaction is endothermic, absorbing heat from the surroundings.

This knowledge is useful for predicting how temperature and heat will affect the reaction environment, influencing everything from industrial processes to biological systems.

Constant Pressure

In thermodynamics, many reactions are analyzed under 'constant pressure,' a condition where the pressure in the system does not change as the reaction proceeds. This scenario is common in open systems exposed to the atmosphere, like laboratory reactions or natural processes occurring outdoors.

Under constant pressure, the heat absorbed or released by a chemical reaction corresponds directly to the change in enthalpy, \( \Delta H \). Thus, the heat of reaction equals the enthalpy change. The reaction can then be simplified to focus solely on changes in heat, without needing to account for work done by the system involving changes in volume.

Operating under constant pressure allows for straightforward thermodynamic calculations as it aligns closely with many real-world scenarios. It simplifies the analysis and is foundational in studying reactions, providing accurate predictions about the energy changes involved.