Why does ITC measure Enthelpy, not Gibbs Energy?

In summary: The experiment operates under nearly constant temperature and pressure, but this does not mean that ΔG equals the heat generated/released by binding. Instead, the change in enthalpy, ΔH, is the actual heat flow, and ΔG is the difference between ΔH and the reversible heat flow, Qrev. This can give insight into the direction and reversibility of the reaction.
  • #1
jjoonathan
1
0
Isothermal Titration Calorimetry measures the enthalpy of (e.g.) protein binding by measuring the amount of heat that must be added/removed to a solution in order to perfectly balance the heat generated/released by the gradual introduction of a protein's binding partner. My understanding is that the experiment operates under constant (well, *almost* constant) temperature and pressure. But don't those conditions (constant T, P) mean that ΔG equals the heat generated/released by binding, as opposed to ΔH? How do we know we should use ΔH instead?

Some references:
[1] http://antisensescienceblog.wordpre...plained-isothermal-titration-calorimetry-itc/
[2] http://www.uic.edu/orgs/ctrstbio/manuals/leavitt.pdf
 
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  • #2
jjoonathan said:
Isothermal Titration Calorimetry measures the enthalpy of (e.g.) protein binding by measuring the amount of heat that must be added/removed to a solution in order to perfectly balance the heat generated/released by the gradual introduction of a protein's binding partner. My understanding is that the experiment operates under constant (well, *almost* constant) temperature and pressure. But don't those conditions (constant T, P) mean that ΔG equals the heat generated/released by binding, as opposed to ΔH? How do we know we should use ΔH instead?

Some references:
[1] http://antisensescienceblog.wordpre...plained-isothermal-titration-calorimetry-itc/
[2] http://www.uic.edu/orgs/ctrstbio/manuals/leavitt.pdf
The actual heat flow is the change in enthalpy, ΔH (P being constant). The change in Gibbs free energy, ΔG, is the difference between ΔH and Qrev (the reversible heat flow). The ΔG tells you how close to reversible the reaction is and what direction it will go spontaneously. It is not a measure of the heat flow at constant pressure.

AM
 

1. Why is enthalpy measured in ITC instead of Gibbs energy?

Enthalpy is measured in ITC because it is a more direct and accurate measurement of the heat energy involved in a chemical reaction. Gibbs energy takes into account both the heat energy and the change in entropy, which can be more difficult to accurately measure.

2. What is the difference between enthalpy and Gibbs energy?

Enthalpy is a measure of the total heat energy in a system, while Gibbs energy takes into account both the heat energy and the change in entropy. This means that enthalpy is a more direct measure of the energy involved in a reaction, while Gibbs energy takes into account the tendency of a reaction to occur or proceed spontaneously.

3. Why is enthalpy important in chemistry?

Enthalpy is important in chemistry because it helps us understand the energy changes that occur during a chemical reaction. It also helps us predict if a reaction will release or absorb heat, and how much energy is involved in the reaction. Enthalpy is also used to calculate other important thermodynamic quantities, such as Gibbs energy and entropy.

4. Can enthalpy and Gibbs energy be used interchangeably?

No, enthalpy and Gibbs energy cannot be used interchangeably. While they are both measures of energy, they take into account different factors and have different units. Enthalpy is measured in joules (J) or kilojoules (kJ), while Gibbs energy is measured in joules per mole (J/mol) or kilojoules per mole (kJ/mol).

5. How does temperature affect enthalpy and Gibbs energy?

As temperature increases, the enthalpy and Gibbs energy of a reaction also increase. This is because a higher temperature means there is more energy available for the reaction to proceed. However, the effect of temperature on Gibbs energy is greater, as it also takes into account the change in entropy. This means that a reaction with a positive enthalpy change (endothermic) can still be spontaneous at higher temperatures if the change in entropy is also positive.

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