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

€

ΔHrxn,T0 = ΔHrxn,298K

0 + ΔCP T '( )dT '298K

T

∫

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ΔCP T '( ) = ν iCP ,i T '( ) −i

prod

∑ ν jCP , j T '( )j

react

∑

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