Definitions [1]
Define molar heat capacity.
The heat capacity for 1 mole of a substance, is called molar heat capacity (cm). It is defined as “The amount of heat absorbed by one mole of the substance to raise its temperature by 1 kelvin”.
Theorems and Laws [1]
State and explain Hess’s law of constant heat summation.
Hess’s law of constant heat summation states that, “The change in enthalpy for a reaction is the same whether the reaction takes place in one or a series of steps.”
The Hess’s law is a direct consequence of the fact that the enthalpy is a state function, and so the enthalpy change depends only on the initial and final states of the system and not on the path by which the reaction takes place.
Example: The conversion of A to C can take place directly
in a single step.
A → C, ΔH° = ΔH1
The reaction can also proceed in two steps, for which the ΔH° values are known.
Step (1): A → B, ΔH° = ΔH2
Step (2): B → C, ΔH° = ΔH3
______________________________
Overall: A → C, ΔH° = ΔH2 + ΔH3
According to Hess’s law, ΔH1 = ΔH2 + ΔH3. The sequence of steps is represented in the figure.
Concepts [28]
- Thermodynamic Terms
- The State of the System
- The Internal Energy as a State Function - Work
- The Internal Energy as a State Function - Heat
- The Internal Energy as a State Function - the General Case
- Work
- Enthalpy, H - a Useful New State Function
- Enthalpy, H - Extensive and Intensive Properties
- Enthalpy, H - Heat Capacity
- Enthalpy, H - The Relationship Between Cp and Cv for an Ideal Gas
- Measurement of ∆U and ∆H Calorimetry - ∆U Measurements
- Measurement of ∆U and ∆H Calorimetry - ∆H Measurements
- Standard Enthalpy of Reactions
- Enthalpy Changes During Phase Transformations
- Standard Enthalpy of Formation
- Thermochemical Equations
- Hess’ Law of Constant Heat Summation
- Standard Enthalpy of Combustion
- Enthalpy of Atomization
- Bond Enthalpy
- Enthalpy of Solution
- Lattice Enthalpy
- Is Decrease in Enthalpy a Criterion for Spontaneity
- Entropy and Spontaneity
- Gibbs Energy and Spontaneity
- Entropy and Second Law of Thermodynamics
- Absolute Entropy and Third Law of Thermodynamics
- Gibbs Energy Change and Equilibrium
