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Chapter 6: Problem 148

For most biological processes, \(\Delta H \approx \Delta U\). Explain.

Short Answer

Expert verified
For most biological processes, \(\Delta H \approx \Delta U\) because these processes happen at constant pressure, and the change in the volume of the substances involved is often negligible. This means the term \(\Delta (PV)\) becomes so small that it can be neglected, making \(\Delta H\) and \(\Delta U\) nearly the same.

Step by step solution

01

Getting to know the terms

\(\Delta H\) is the heat capacity at constant pressure, also known as enthalpy, and it describes the total energy in a thermodynamic system. \(\Delta U\), on the other hand, indicates the change in the internal energy of a system. Internal energy can be calculated as \(\Delta U = \Delta H - \Delta (PV)\), where \(P\) is pressure and \(V\) is volume.
02

Approximation in biological processes

In biological systems, most processes occur at a constant pressure(\(P\)), such as 1 atmospheric pressure(\(atm\)). Secondly, the change in the volume(\(\Delta V\)) of substances involved in the reactions is often very less. Therefore, the term \(\Delta (PV)\) can be considered very small and can be neglected. This makes the \(\Delta U\) and \(\Delta H\) nearly equivalent.
03

Conclusion

The approximation \(\Delta H \approx \Delta U\) is thus justified for most biological processes due to the fact that they occur at constant pressure, and involve negligible changes in volume.

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Most popular questions from this chapter

Acetylene \(\left(\mathrm{C}_{2} \mathrm{H}_{2}\right)\) and benzene \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)\) have the same empirical formula. In fact, benzene can be made from acetylene as follows: $$ 3 \mathrm{C}_{2} \mathrm{H}_{2}(g) \longrightarrow \mathrm{C}_{6} \mathrm{H}_{6}(l) $$ The enthalpies of combustion for \(\mathrm{C}_{2} \mathrm{H}_{2}\) and \(\mathrm{C}_{6} \mathrm{H}_{6}\) are \(-1299.4 \mathrm{~kJ} / \mathrm{mol}\) and \(-3267.4 \mathrm{~kJ} / \mathrm{mol},\) respectively. Calculate the standard enthalpies of formation of \(\mathrm{C}_{2} \mathrm{H}_{2}\) and \(\mathrm{C}_{6} \mathrm{H}_{6}\) and hence the enthalpy change for the formation of \(\mathrm{C}_{6} \mathrm{H}_{6}\) from \(\mathrm{C}_{2} \mathrm{H}_{2}\).

Decomposition reactions are usually endothermic, whereas combination reactions are usually exothermic. Give a qualitative explanation for these trends.

What is the difference between specific heat and heat capacity? What are the units for these two quantities? Which is the intensive property and which is the extensive property?

State Hess's law. Explain, with one example, the usefulness of this law in thermochemistry.

A piece of silver of mass \(362 \mathrm{~g}\) has a heat capacity of \(85.7 \mathrm{~J} /{ }^{\circ} \mathrm{C}\). What is the specific heat of silver?
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