Enthalpy and Thermochemistry
Cp and Cv, Hess's law and standard enthalpies of formation, combustion and neutralisation. (Chemistry › Thermodynamics, NEET UG syllabus.)
What is Enthalpy and Thermochemistry?
A thermodynamic property of a system, representing the total heat content. It is defined as H = U + PV, where U is internal energy, P is pressure, and V is volume.
Key formula / rule: Enthalpy definition
Key points
- Define enthalpy and explain its significance in thermochemistry.
- Relate enthalpy change (ΔH) to internal energy change (ΔU).
- Understand the relationship between Cp and Cv for ideal gases.
- State and apply Hess's Law of Constant Heat Summation to calculate reaction enthalpies.
Common exam trap
Incorrectly applying sign conventions for ΔH (exothermic vs. endothermic).
Definitions
- Term
Enthalpy (H)
- Meaning
A thermodynamic property of a system, representing the total heat content. It is defined as H = U + PV, where U is internal energy, P is pressure, and V is volume.
- Term
Thermochemistry
- Meaning
The branch of chemistry concerned with the heat changes that accompany chemical reactions and physical transformations.
- Term
Standard Enthalpy of Formation (ΔH°f)
- Meaning
The enthalpy change when one mole of a compound is formed from its constituent elements in their most stable physical states under standard conditions (298 K, 1 atm pressure).
- Term
Standard Enthalpy of Combustion (ΔH°c)
- Meaning
The enthalpy change when one mole of a substance undergoes complete combustion with oxygen under standard conditions.
- Term
Standard Enthalpy of Neutralisation (ΔH°neut)
- Meaning
The enthalpy change when one mole of water is formed from the reaction of a strong acid with a strong base in dilute solution under standard conditions (approximately -57.3 kJ/mol).
- Term
Heat Capacity at Constant Pressure (Cp)
- Meaning
The amount of heat required to raise the temperature of a substance by one ° Celsius (or Kelvin) at constant pressure.
- Term
Heat Capacity at Constant Volume (Cv)
- Meaning
The amount of heat required to raise the temperature of a substance by one ° Celsius (or Kelvin) at constant volume.
Learning objectives
Define enthalpy and explain its significance in thermochemistry.
Relate enthalpy change (ΔH) to internal energy change (ΔU).
Understand the relationship between Cp and Cv for ideal gases.
State and apply Hess's Law of Constant Heat Summation to calculate reaction enthalpies.
Define and calculate standard enthalpies of formation, combustion, and neutralisation.
Use standard enthalpy data to calculate the enthalpy change for any chemical reaction.
Formulae
- Name
Enthalpy definition
- Note
U is internal energy, P is pressure, V is volume.
- Expression
H = U + PV
- Name
Relation between ΔH and ΔU
- Note
Δng is the change in the number of moles of gaseous products minus gaseous reactants. R is the ideal gas constant, T is temperature in Kelvin.
- Expression
ΔH = ΔU + Δng RT
- Name
Relation between Cp and Cv (ideal gas)
- Note
R is the ideal gas constant.
- Expression
Cp - Cv = R
- Name
Hess's Law for calculating reaction enthalpy
- Note
n and m are stoichiometric coefficients. ΔH°f is standard enthalpy of formation.
- Expression
ΔH°reaction = ΣnΔH°f(products) - ΣmΔH°f(reactants)
Prerequisites
Basic understanding of thermodynamics (system, surroundings, internal energy, heat, work).
First Law of Thermodynamics (ΔU = Q + W).
Stoichiometry and balancing chemical equations.
Concept of state functions and path functions.
Common mistakes
Incorrectly applying sign conventions for ΔH (exothermic vs. endothermic).
Forgetting to balance chemical equations before applying Hess's Law or calculating ΔH°reaction.
Not considering the stoichiometric coefficients when using ΔH°f values.
Confusing Cp and Cv, or using the wrong value of R.
Assuming ΔH°f for an element is non-zero.
Not converting units correctly (e.g., J to kJ).
Keywords
Enthalpy
Thermochemistry
Hess's Law
Heat capacity
Standard enthalpy of formation
Standard enthalpy of combustion
Standard enthalpy of neutralisation
Cp
Cv
Exothermic
Endothermic
State function
Practice preview
Hess's Law of Constant Heat Summation states that:…
easy
Given the following thermochemical equations: 1. S(s) + O₂(g) → SO₂(g) ; ΔH₁ = -298.2 kJ 2. SO₂(g) + ½O₂(g) → SO₃(g) ; ΔH₂ = -98.7 kJ Calculate the enthalpy change for the reaction: S(s) + 3/2 O₂(g) → SO₃(g).…
medium
The standard enthalpies of combustion of C₂H₄(g), C₂H₆(g), and H₂(g) are -1411 kJ/mol, -1560 kJ/mol, and -286 kJ/mol respectively. Calculate the standard enthalpy of hydrogenation of ethene: C₂H₄(g) + H₂(g) → C₂H₆(g).…
medium