What is the difference between ΔH and ΔU?

ΔU is the change in internal energy of a system (heat at constant volume). ΔH is the enthalpy change (heat at constant pressure). ΔH = ΔU + ΔnₘRT, where Δnₘ = change in moles of gas. For reactions with no change in moles of gas (Δnₘ = 0), ΔH = ΔU.

State Hess's Law and give its application.

Hess's Law: The total enthalpy change for a reaction is independent of the path and depends only on the initial and final states. Application: Calculate ΔH for reactions that cannot be directly measured (e.g., formation of CO from C and O₂) by combining known reaction enthalpies algebraically.

What is meant by standard enthalpy of formation?

Standard enthalpy of formation (ΔfH°) is the enthalpy change when one mole of a compound is formed from its elements in their most stable standard states at 298 K and 1 bar. ΔfH° of all elements in standard state = 0 by definition.

First Law of Thermodynamics and Enthalpy

Thermodynamics • Class 11 Chemistry • NCERT • CBSE

First Law of Thermodynamics: Energy can neither be created nor destroyed — ΔU = q + w. For a system, q = heat absorbed and w = work done on the system. Enthalpy H = U + PV; at constant pressure, ΔH = qₚ (heat of reaction at constant pressure).

Key Formulas

ΔU = q + w
ΔH = ΔU + ΔnₘRT
ΔH = qₚ (at constant pressure)
w = −PΔV

Frequently Asked Questions

What is the difference between ΔH and ΔU?: ΔU is the change in internal energy of a system (heat at constant volume). ΔH is the enthalpy change (heat at constant pressure). ΔH = ΔU + ΔnₘRT, where Δnₘ = change in moles of gas. For reactions with no change in moles of gas (Δnₘ = 0), ΔH = ΔU.
State Hess's Law and give its application.: Hess's Law: The total enthalpy change for a reaction is independent of the path and depends only on the initial and final states. Application: Calculate ΔH for reactions that cannot be directly measured (e.g., formation of CO from C and O₂) by combining known reaction enthalpies algebraically.
What is meant by standard enthalpy of formation?: Standard enthalpy of formation (ΔfH°) is the enthalpy change when one mole of a compound is formed from its elements in their most stable standard states at 298 K and 1 bar. ΔfH° of all elements in standard state = 0 by definition.

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