Thermochemistry (4.1-4.3)

1
Thermochemistry (4.1-4.3)

• Enthalpy (?H) 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 (?Hrxn) 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 Hesss Law
• Heats of formation (?Hf) 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?

2
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

3
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

4
Enthalpy as a State Function
5
Bomb Calorimetry
6
Solution Calorimetry
7
Differential Scanning Calorimeter
8
Phase Change in DSC
9
Protein Denaturation in DSC