TERMOQUIMICA Teoría 7 - Transferencia calor a presión constante. Concepto de entalpía H

Transfer of Heat at Constant Pressure in Chemical Reactions

Introduction to Heat Transfer in Reactions

• The discussion focuses on heat transfer during chemical reactions at constant pressure, contrasting with previous discussions on reactions occurring at constant volume.

• Examples include outdoor reactions like burning wood, where gas release leads to volume changes while maintaining atmospheric pressure.

Importance of Constant Pressure

• Most chemical reactions are analyzed under constant pressure conditions, which is typically atmospheric pressure in open systems.

• The concept of enthalpy (H), a new state function introduced when applying the first law of thermodynamics to these reactions, will be explored extensively.

First Law of Thermodynamics Application

• The mathematical expression for the first law states that the change in internal energy (ΔU) equals heat transferred plus work done.

• Work can be expressed as pressure times the change in volume (PΔV), leading to a modified equation for ΔU.

Deriving Enthalpy from Internal Energy

• For constant pressure processes, we denote heat transfer as Qp and rearrange the equation to isolate Qp.

• This results in an expression linking internal energy change and work done due to volume changes.

Analyzing Reactants and Products

• In a chemical reaction, reactants have initial volumes (Vr) and internal energies (Ur), while products have final volumes (Vp) and internal energies (Up).

• The variation in volume is defined as Vp - Vr; similarly, the change in internal energy is Up - Ur.

Final Expression for Heat Transfer

• Substituting into our earlier equations gives us an expression for Qp involving both internal energy changes and work done due to volume variations.

• After simplification, we arrive at a formula that combines terms related to products and reactants effectively.

Grouping Terms for Clarity

• By grouping terms based on their dependence on either products or reactants, we clarify how each contributes to overall heat transfer.

• This organization allows us to express Qp clearly by factoring out common elements from both sides of the equation.

Entalpía y su Relación con la Energía Interna

Definición de Entalpía

• La entalpía (H) se define como la suma de la energía interna (u) y el producto de presión por volumen (pV), lo que implica que tiene unidades de energía.

• Se establece que H = u + pV, donde H representa la entalpía, u es la energía interna, y pV es el trabajo realizado por el sistema.

Cálculo de Variaciones en Entalpía

• Para calcular la variación de entalpía entre productos y reactivos, se utiliza: ΔH = (u + pV)_productos - (u + pV)_reactivos. Esto permite determinar cómo cambia la entalpía durante una reacción química.

• La fórmula para q_p se expresa como q_p = H_productos - H_reactivos, indicando que esta cantidad representa el calor transferido a presión constante durante una reacción.

Importancia de las Funciones de Estado

• Se destaca que la entalpía es una función de estado, similar a la energía interna. Esto significa que solo depende del estado inicial y final del sistema, no del camino seguido para llegar a esos estados.