Thermochemistry (4.1-4.3)
• Enthalpy (ΔH) is related to the heat exchange that occurs during a
chemical or physical process under constant pressure
– Processes that generate heat are exothermic, processes that absorb heat are
endothermic
– Phase changes also have enthalpies associated with them (e.g., heat of vaporization)
– Enthalpies are state functions
• Heats of reaction (ΔHrxn) can be measured or calculated for most reactions
– They can be measured through calorimetry (e.g., bomb or solution calorimetry)
– They can be calculated using Hess’s Law
• Heats of formation (ΔHf) are the heats associated with forming one mole
of a compound from its elements in their standard states
– Heats of formation are typically reported under standard conditions: 298.15 K and a
pressure of 1 bar for each gaseous component
– Why would this be a problem in biochemistry?
Temperature Dependence of Enthalpy (4.4)
• Heat capacities (CP or CV) are related to the amount of heat needed to
raise the temperature of a substance by 1 °C (section 2.4)
– This quantity tells us how much heat a substance can store (e.g., water has a high heat
capacity thus it stores lots of heat, metals have low heat capacities)
– Over short temperature ranges, the heat capacities are typically temperature
independent (i.e., constant value)
• The amount of heat evolved or absorbed during a chemical process at a
given temperature is related to a number of factors
– The standard heats of formation are useful, but only tell part of the story
– If the reaction occurs at elevated (or depressed) temperatures, heat capacities of the
reactants and products are needed
– If phase changes occur, then enthalpies associated with the phase changes are needed
H
0
rxn,T
 H
0
rxn,298K

T
 C T'dT'
P
298K

prod
react
i
j
CP T'   iCP,i T'   j CP, j T'
Differential Scanning Calorimetry (4.6)
• Differential scanning calorimetry (DSC) is a way of measuring energy
changes associated with physical transitions
– Phase transitions in compounds, metals, polymers
– Denaturation of biopolymers can be considered a phase transition
• DSC uses differences in heating of a sample and a standard to determine
thermodynamic parameters
– Sample and standard temperatures are ramped up over time, but are kept equal
– The difference in the amount of heat needed to maintain temp. is the difference in heat
capacities (assuming no chemical or physical changes)
• When a chemical or physical change occurs, the heating of the sample
changes drastically
– During a phase change, heat goes towards changing physical state, not temp. increase
– For protein denaturation, the heat capacity of the natural protein (CN) is significantly
different than the denatured protein (CD)
– The melting temp. of the protein is the temp. at the peak maximum
– The area under the curve is the enthalpy associated with the corresponding transition
Enthalpy as a State Function
Bomb Calorimetry
Solution Calorimetry
Differential Scanning Calorimeter
Phase Change in DSC
Protein Denaturation in DSC