Thermodynamics: Laws and Processes

First Law of Thermodynamics

• The mathematical statement of the First Law of Thermodynamics is ΔE = q – w, where ΔE is the change in internal energy, q is the heat added to the system, and w is the work done by the system.

Cyclic Process (Isothermal Expansion of Ideal Gas)

• ΔE = 0
• q = w

Isochoric Process (Constant Volume)

• No work of expansion occurs, so w = 0.
• ΔE = qv, meaning the change in internal energy equals the heat added at constant volume.

Adiabatic Process

• There is no heat exchange (q = 0).
• ΔE = – w, the decrease in internal energy equals the work done by the system.

Isobaric Process (Constant Pressure)

• ΔE = q – w
• ΔE = q – PΔV

Enthalpy

• In a constant volume process, the heat content of a system is the same as its internal energy (E), since no PV work is done.
• In a constant-pressure process, the system expends energy in doing PV work.
• Enthalpy (H) is defined as H = E + PV, representing the total heat content of a system at constant pressure.

Change in Enthalpy

• ΔH = H2 – H1, where ΔH is the difference in enthalpy between the final state (H2) and the initial state (H1) of a system.
• ΔH = (E2 + P2V2) – (E1 + P1V1) = (E2 – E1) + (P2V2 – P1V1) = ΔE + ΔPV
• At constant pressure: ΔH = ΔE + PΔV = ΔE + w

Enthalpy and First Law

• ΔE = q – w
• q = heat transferred
• ΔH = q when a change in state occurs at constant pressure

Enthalpy of Reaction

• ΔH = Hproducts – Hreactants
• This is also represented as ΔH = Hp – Hr. where p = products and r = reactants

Thermochemical Equations

• Indicates the amount of heat change (evolved or absorbed) in a reaction or process.

Heat of Formation

• This is the enthalpy change when one mole of a compound is formed from its elements.
• The standard heat of formation of all elements is assumed to be zero.

Examples of Heat of Formation

• Fe(s) + S(s) → FeS(s) has ΔHf = – 24.0 kcal
• 2C(s) + H2(g) → C2H2(g) has ΔHf = + 53.14 kcal
• 1/2 H2(g) + 1/2 Cl2(g) → HCl(g) has ΔHf = −22.0 kcal

Heat of Fusion

• The heat change (or enthalpy change) when one mole of a solid substance is converted into the liquid state at its melting point.

Heat of Vaporization

• The heat change (or enthalpy change) when one mole of liquid is converted into vapour or gaseous state at its boiling point.

Heat of Sublimation

• The heat change (or enthalpy change) when one mole of a solid is directly converted into the gaseous state at a temperature below its melting point.

Heat of Transition

• The change in enthalpy which occurs when one mole of an element changes from one allotropic form to another.

Heat of Combustion

• The change in enthalpy of a system when one mole of the substance is completely burnt in excess of air or oxygen.