Can anyone pls explain me the sign conventions that we use in chemistry thermodynamics for heat and work...also kindly explain how to identify what will happen to heat in positive or negative work and vice-verse...
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1https://chemistry.stackexchange.com/questions/66088/why-do-chemistry-and-physics-have-different-sign-convention-in-thermodynamics?rq=1 – Mithoron Dec 25 '22 at 20:52
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According to the international standard ISO 80000 Quantities and units – Part 5: Thermodynamics,
for a closed thermodynamic system
$$\Delta U=Q+W$$
where $Q$ is amount of heat transferred to the system and $W$ is work done on the system provided that no chemical reactions occur
The same sign convention is used in the IUPAC Green Book Quantities, Units, and Symbols in Physical Chemistry.
The given equation in integrated form is $\Delta U=Q+W$. $Q\gt0$ and $W\gt0$ indicate an increase in the energy of the system.
Loong
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There exist two ways for defining the sign of the work, depending who was defining it.
The definition proposed by the chemists implies that all energies that are added to a system are positive. $\Delta U = Q + W$
The approach of physics teachers is different. It is a mechanical approach. It implies that the system is like a steam machine which is heated to produce useful work. The heat is positive when entering the system. But the work is positive when it produces some useful effect (with this heat) outside of the machine. For physicists, the internal energy $U$ is this fraction of the heat that remains inside the system and does not produce useful work. For physicists, $\Delta U = Q - W$. So as the work produced by the system is positive, it means that if some work is entering the system, this work must be negative.
Edit : More details for AtharvZope. Imagine a syringe half filled with a volume $V_o$ of a gas (air or water vapor). If the syringe is heated by introducing a heat quantity $Q$ under constant atmospheric pressure $p$, its gas volume increases to $V_1 > V_o$. So $\Delta V$ is equal to $\Delta V = V_{fin} - V_{in} = V_1 - V_o > 0$. This sign is the same for both definitions of work. As a consequence, in such a phenomena, $p\Delta V $ is positive all over the world, for chemists or for physicists. But, if $p\Delta V $ is always positive, the work $W =|p\Delta V|$ has not the same sign everywhere. $p\Delta V$ is always positive, but $W$ is negative for chemists as the corresponding energy is lost by the system ($W_{chem} = - p\Delta V$), and the same work is positive for physicists, because the syringe has produced a work which could be used for moving a car or producing electricity, etc. ($W_{phys} = + p\Delta V$)
Whatever the choice of the sign of $W$, the change in internal energy $\Delta U$ is the same for physics and for chemistry : $\Delta U = Q + W_{chem} = Q - W_{phys} = Q - p\Delta V$
Maurice
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Interesting. I have not heard about it as American versus Europian way( in contrary to redox potentials). I have heard about originally the way of chemistry - work done on a system (Q + W) and the way of physics - work done by a system ( Q - W ), later rather as way of science and way of engineering. But it has many exceptions and deviations, with good habit to explicitly say what convention is used. – Poutnik Dec 25 '22 at 20:47
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Can you pls explain the american system in a more simpler way...if you don't mind...pls – Atharv Zope Dec 26 '22 at 02:35
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Seconding Poutnik - I do not think there is a distinction between America and Europe, but rather between scientific disciplines. I learned q-w in American physics classes and q+w in American chemistry classes. – Andrew Dec 26 '22 at 17:29
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@Andrew. These "geographic" definitions were given to me by my chemistry teachers during my studies once upon a time. It may have been wrong. So I have edited my answer to follow your informations. I have replaced the names of the continents by the names of the scientific disciplines. – Maurice Dec 26 '22 at 20:29
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