# Questions tagged [enthalpy]

A thermodynamic state function describing the total energy content of a system.

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### rate of combustion of alcohols and hydrocarbons in relation to the number of carbons in the chain

I understand that the heat released when burning alcohols is greater as long as the number of carbons in the alcohol chain is increased (i.e the enthalpy of butanol is greater than propanol > ...
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### Why is the first ionization enthalpy of molecular O2 less than atomic O?

A cousin of mine asked for my help to answer one of his homework questions. It stated- Explain why- The first ionization energy of molecular oxygen $\ce{O2}$ ($\pu{1175 kJ mol-1}$), is lesser than ...
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### How does the calorimeter affect heat released by the system (q)? [closed]

Here are the 2 questions I am pondering about How would the calculated quantity of heat (q) released by a system differ if the calorimeter's heat capacity was taken into account? My thought process ...
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### Water evaporation from calorimeter giving less exothermic enthalpy value?

To find the enthalpy change of combustion for an alcohol, you use a calorimeter filled with water and heat it by burning an alcohol. I was told that usually experimental values for the enthalpy ...
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### Where does the RT term come from in the derivation for the activation enthalpy from the Eyring equation?

It is easy to show an Arrhenius-like equation from the Eyring equation, but if you do this, you get that the activation energy is about equal to the activation enthalpy. However, the real ...
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### Is enthalpy affected by isotopes?

Enthalpy is a sum of $U$ and $PV$, and I thought that for different isotopes, internal energy values may differ. Is it true? There are three stable isotopes of $\textrm{Mg}$, mass number 24, 25, and ...
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### Why is the energy needed to transform PbO+CO into Pb+O2+C equal to the sum of breaking down PbO and CO? [closed]

Since PbO to Pb+(1/2)O2 releases 217.3kJ and CO to C+(1/2)O2 releases 110.5kJ, what I think is that PbO+CO to Pb+(1/2)O2+C+(1/2)O2 releases 327.8kJ. Now, the book says PbO+CO to Pb+C+O2 releases 327....
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### What is an exothermic reaction in terms of bond breaking and bond formation? [duplicate]

Nitrogen + Hydrogen -> Ammonia This reaction has a $\Delta H$ of -93kJ This reaction is exothermic. Explain what it means in terms of bond breaking and bond formation I know an exothermic reaction ...
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### Is electrical work equal to the change in enthalpy or internal energy?

A $\pu{0.5 A}$ current from a $\pu{20 V}$ supply is passed through a water sample. $\pu{0.9 g}$ of water is vapourised at $p = \pu{1 atm}$ and $T = \pu{373 K}$. Calculate $\Delta U$ and $\Delta H$ for ...
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### What is the entalphy change with respect to pressure at Isothermal Process for *Real Gases*?

How much enthalpy of argon changes when the pressure is isothermally increased by $\pu{1 atm}$? (Gas should be considered as real gas under specified conditions) ($\pu{1 mol}$ of Argon gas is ...
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### Confusion on when to use mass of entire solution when performing q=mc∆T

I hope you are having a good day. I am studying for the AP Chemistry exam in May 2020. While studying, I encountered a doubt. In Problem 1 (the question on the top), the educator saw that the ...
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### How can temperature be constant during a reaction? [closed]

So I was reading my textbook about the topic thermodynamics. Here the second paragraph States that the above equation describes the combustion of liquid ethanol at constant temperature and pressure. ...
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### Enthalpy change of dissolving air in water?

I'm looking for a value of the enthalpy change of dissolving air in water but can't find it. In other words, if I pull a vacuum over some water to remove all the air, per Henry's Law, how much energy ...
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### How to calculate the variation of enthalpy in a reaction that does not occur in standard conditions?

Standard enthalpy of formation is a value that represents the enthalpy variation in the formation of compounds with the purpose of using this values to know the enthalpy of a reaction where these ...
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### Rubbing alcohol gets warm after diluting with water

I've just diluted a 95% rubbing alcohol bottle with water to bring it down to 70%. However, the bottle got noticeably warmer after I did so. I've searched online and I couldn't find any exothermic ...
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### Does the absolute value of enthalpy have any physical significance?

I know it is pretty hard to measure the change in volume in open systems. Accordingly, calculating the change in internal energy using the first law of thermodynamics is pretty hard also. Is this the ...
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### Is the magnitude of ionisation enthalpy and ionisation energy is same?

In my textbook it is written that ionisation energy and ionisation enthalpy are two different quantities. ionisation energy is the amount of energy provided to extract an electron from the outermost ...
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### Why do stable compounds have a lower heat of combustion?

My brain somehow doesn't accept that more stable compounds require have less enthalpy. Isn't it like, if a compound is stable, it requires more energy to break the bonds because the compound doesn't ...
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### Hyperconjugation, stability of molecules, and heat of formation

I recently asked a question about why the heat of formation of organic radicals and positive ions decreases with their size and degree of branching at the radical or ionic site. The user "Buttonwood" ...
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### Why does the heat of formation of organic radicals and positive ions decrease with their size and degree of branching at the radical or ionic site?

I am currently studying the textbook Mass Spectrometry, third edition, by Jürgen H. Gross. Chapter 2.4.3 Bond Dissociation Energies and Heats of Formation says the following: The heat of formation ...
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### Mechanistic explanation of salt lowering temperature of ice slush

As we all know, adding salt to ice water lowers its temperature. I've read plenty of system-level accounts of energy balances, enthalpies, vapor pressures, phase equilibria and freezing points—I ...