Thermochemistry

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Thermochemistry

Thermochemistry is a subdiscipline of physical chemistry that studies heat changes in chemical change processes, such as chemical reactions.

Chemical reactions can be considered to occur at constant pressure, or they can be considered to occur at constant volume; that of the receptacle where they are being carried out.

In the case of heat exchanges at constant pressure, the characteristic magnitude of thermochemistry is the increase in enthalpy, that is, the heat change that will occur in the transition from reactants to products.

Constant pressure process: The heat exchanged in the process is equivalent to the enthalpy change of the reaction.

Constant volume process: The heat exchanged under these conditions is equivalent to the change in internal energy of the reaction.

Exothermic and Endothermic Processes

Depending on the heat exchanged with the environment, processes can be classified as exothermic and endothermic.

In this case, the processes are chemical reactions, and the enthalpy of the reaction tells us whether the reaction has lost or gained energy.

Exothermic: They release heat into the environment, that is, they lose heat: qp < 0

Endothermic: They absorb heat from the environment, that is, they gain heat: qp > 0

In relation to the Gibbs free energy, the enthalpy change of reaction, along with entropy and temperature, also governs the spontaneity or nonspontaneity of a reaction.

Hess's Law

Germain Henry Hess was a Russian physical chemist of Swiss origin who laid the foundations of modern thermodynamics. He primarily worked on gas chemistry and stated the following law:

"In a chemical reaction expressed as the algebraic sum (or difference) of other chemical reactions, since it is a state function, the overall reaction enthalpy is also the algebraic sum (or difference) of the enthalpies of the other reactions."

Consider the reaction A→B. And let's assume the existence of the following intermediate reactions, with known Δ r HT^o:

We see that we can set up a thermodynamic cycle such that, instead of going from A to B directly, we go through all the intermediate reactions described above:

Since enthalpy is a state function, the process does not depend on the path, and therefore, it is irrelevant whether we do it directly or taking the other reactions into account.

Note that the reaction D→C goes in the opposite direction to the one we are interested in to complete the cycle. Therefore, we must reverse the direction of the energy flow to obtain the reaction we want, and this is achieved by changing the sign of the enthalpy change. That is,

Sometimes, we must multiply the reaction enthalpy of one of the intermediate reactions by a stoichiometric coefficient so that the linear relationship between the different enthalpy changes is fulfilled.

We know that the enthalpy of a reaction depends on temperature, since it depends on internal energy. Sometimes, we'll be interested in knowing the standard enthalpy change at a temperature different from the one we have data for.

Activity: Answer the following questions

Remember to review the answers to the open questions at the bottom of this page.

1. In an exothermic reaction, the enthalpy change is:

2. What does the equation qv = ΔrU represent?

3. If we reverse a chemical reaction, what happens to its enthalpy?

4. What additional aspect determines the spontaneity of a reaction along with the enthalpy?

5. Hess's Law states that:

Once you click this button, the reactants will close and you will not be able to change your answer.

6. What does it mean for a reaction to be exothermic?

7. What happens if you need to adjust an intermediate reaction in Hess's Law?

8. How does the enthalpy of a reaction vary with temperature?

9. Why is standard temperature important in thermochemical calculations?

10. Why is it sometimes necessary to multiply the enthalpy of a reaction by a stoichiometric coefficient when applying Hess's Law?

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Answers to open questions

6. This means that the reaction releases heat into the surroundings, which implies that the enthalpy of the reaction is negative. Δ r HT^o < 0

7. The enthalpy is multiplied by the corresponding stoichiometric coefficient to maintain proportionality.

8. Enthalpy depends on temperature, since it is related to the internal energy of the system, which also varies with temperature.

10. So that the linear relationship between the different enthalpy changes is fulfilled.

5. Professor Dave Explains. (2015, 23 noviembre). Thermochemistry: Heat and Enthalpy [Vídeo]. YouTube. https://www.youtube.com/watch?v=ZVhJ4TO8a-M https://www.youtube.com/watch?v=ZVhJ4TO8a-M

6. The Organic Chemistry Tutor. (2016b, julio 17). Thermochemistry Equations & Formulas - Lecture review & Practice Problems [Vídeo]. YouTube. https://www.youtube.com/watch?v=LsqKL3pBVMA https://www.youtube.com/watch?v=LsqKL3pBVMA

7. CrashCourse. (2013d, junio 18). Enthalpy: Crash course Chemistry #18 [Vídeo]. YouTube. https://www.youtube.com/watch?v=SV7U4yAXL5I https://www.youtube.com/watch?v=SV7U4yAXL5I

Thermochemistry

Exothermic and Endothermic Processes

Hess's Law

Activity: Answer the following questions

1. In an exothermic reaction, the enthalpy change is:

2. What does the equation qv = ΔrU represent?

3. If we reverse a chemical reaction, what happens to its enthalpy?

4. What additional aspect determines the spontaneity of a reaction along with the enthalpy?

5. Hess's Law states that:

6. What does it mean for a reaction to be exothermic?

7. What happens if you need to adjust an intermediate reaction in Hess's Law?

8. How does the enthalpy of a reaction vary with temperature?

9. Why is standard temperature important in thermochemical calculations?

10. Why is it sometimes necessary to multiply the enthalpy of a reaction by a stoichiometric coefficient when applying Hess's Law?

Still have questions?

Answers to open questions