Enthalpy and Thermochemistry
Enthalpy H = U + PV changes by the heat exchanged at constant pressure, and Hess's law allows enthalpy changes to be added along any path.
What is Enthalpy and Thermochemistry?
Enthalpy H = U + PV changes by the heat exchanged at constant pressure, and Hess's law allows enthalpy changes to be added along any path.
Key formula / rule: dH = dU + dn(gas) RT for reactions involving gases.
Key points
- Compute reaction enthalpy using Hess's law.
- Derive the relation between enthalpy and internal energy change.
Common exam trap
Forgetting the dn(gas) term when converting between dU and dH.
Definitions
- Term
Enthalpy and Thermochemistry
- Meaning
Enthalpy H = U + PV changes by the heat exchanged at constant pressure, and Hess's law allows enthalpy changes to be added along any path.
- Term
Enthalpy and Thermochemistry — explanation
- Meaning
Because enthalpy is a state function, reaction enthalpies can be constructed from tabulated formation enthalpies. This is what makes calorimetry data transferable between reactions.
Learning objectives
Compute reaction enthalpy using Hess's law.
Derive the relation between enthalpy and internal energy change.
Formulae
- Key point
dH = dU + dn(gas) RT for reactions involving gases.
- Key point
Hess's law follows from enthalpy being a state function.
- Key point
Standard formation enthalpy of an element in its standard state is zero.
Prerequisites
CHE-U1-TH-T2-S1-C1
Common mistakes
Forgetting the dn(gas) term when converting between dU and dH.
Reversing a reaction without reversing the sign of its enthalpy change.
Keywords
Enthalpy
Thermochemistry
Practice preview
For a reaction involving ideal gases, the relationship between enthalpy change (Delta H) and internal energy change (Delta U) is given by Delta H = Delta U + Delta n_g RT. What does Delta n_g represent?…
easy
Calculate the standard enthalpy of combustion of methane (CH4) if the standard enthalpies of formation of CH4(g), CO2(g), and H2O(l) are -74.8 kJ/mol, -393.5 kJ/mol, and -285.8 kJ/mol, respectively. The combustion reacti…
medium
Given the following reactions: 1. N2(g) + 2O2(g) -> 2NO2(g); Delta H1 = +66.4 kJ 2. 2NO(g) + O2(g) -> 2NO2(g); Delta H2 = -114.1 kJ Calculate the enthalpy of formation of NO(g).…
medium