Biological Systems

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Chase Yonamine 1J
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Joined: Fri Sep 28, 2018 12:17 am

Biological Systems

Postby Chase Yonamine 1J » Sat Feb 02, 2019 9:07 pm

Why is it in biological systems, delta U and delta H are the same.

Posts: 55
Joined: Fri Sep 28, 2018 12:19 am

Re: Biological Systems

Postby juleschang16 » Sat Feb 02, 2019 9:11 pm

Delta H is the change in enthalpy and Delta U is the change in internal energy. Internal energy is the amount of energy a system has. However, we must realize that a system doesn't exist in a vacuum. Rather, there must be some sort of change in volume/pressure because of the presence of a system. If we hold pressure constant, as it often is when a system is introduced, volume must change (we know this from gas laws). This leads us to the concept of enthalpy which is, in essence, a convenient way to MEASURE a system's internal energy at a constant pressure.

The following equation relates the two quantities:


Matthew Tran 1H
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Joined: Fri Sep 28, 2018 12:16 am

Re: Biological Systems

Postby Matthew Tran 1H » Sat Feb 02, 2019 11:22 pm

In biological systems, delta H and delta U are usually the same because most reactions take place in aqueous solution and not in the gas phase. Since liquids are basically incompressible/inexpandable, we can ignore the work of expansion term (PdeltaV), leaving us with deltaH=deltaU.

Arta Kasaeian 2C
Posts: 30
Joined: Mon Jan 07, 2019 8:22 am

Re: Biological Systems

Postby Arta Kasaeian 2C » Sun Feb 03, 2019 2:20 pm

Almost all biological reactions take place in aqueous solutions, not gaseous state, therefor a change in volume is not possible. Delta H = Delta U + P (Delta V), as Delta V = 0 -> Delta H = Delta U

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