Questions tagged [enthalpy]
A thermodynamic state function describing the total energy content of a system.
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What is the enthalpy change of neutralisation of vinegar (diluted acetic acid) and calcium carbonate?
If anyone has conducted an experiment that determined this value or knows reliable sources that show the literature/thereotical value of this enthalpy change, please share your answer. Thank you very ...
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35 views
Calculating energy density and carbon footprint given heats of formation
This is the problem that I'm attempting to solve:
Calculate for the energy density and carbon footprint of a reformate manufactured via continuous
catalytic regenerative reformer (Benzene, C6H6 – 17%;...
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Enthalpy of Atomisation [closed]
Is there any difference between the enthalpy of atomisation of H2 molecule and enthalpy of atomisation of H..Why or Why not ??
Because The Wikipedia definition says that for a molecule it is defined ...
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When, if ever, can standard enthalphy be subsituted for standard chemical potiential?
I'm reading through a combustion modeling paper by Gordon & McBride which is a bit scant on some of the algebra & calculus used to derive their equations. My advisor and I have derived nearly ...
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Is Bond energy in PCl5 5*P-Cl or P-Cl+4*Cl-Cl? [closed]
i ran into a problem where they gave a reaction formula PCl5->PCl3+Cl2 (all in gaz form)and they wanted to calculate bond enthalpy of the whole reaction at 298K Can you give me the exact formula of ...
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47 views
Can a combustion process be more energy efficient with oxygen enrichment?
I wonder if a combustion process with air as oxidant can benefit by oxygen enriching the air intake? By benefit I mean if the net energy output would be positive, or negative, since the enrichment ...
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What is the enthalpy change for the reaction $\ce{2 OH- -> H2O2 + 2 e-}$?
The reaction $\ce{2 OH- -> H2O2 + 2 e-}$ produces hydrogen peroxide that decomposes into water and oxygen in the exothermic reaction $\ce{2 H2O2 + 4 e- -> 2 H2O + O2 + 4 e-}$ with an enthalpy ...
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75 views
A Curious (definitely wrong) result of Absolute Entropy [closed]
The Gibbs free energy is defined as
$$G=H-TS$$
Differentiating both sides
$$dG=dH-TdS-SdT$$
For an adiabatic reversible expansion of an ideal gas, we would have $dG=0$, and $dS=0$.
Thus we can write, $...
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1answer
42 views
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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1answer
49 views
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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36 views
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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27 views
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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57 views
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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1answer
44 views
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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1answer
34 views
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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1answer
39 views
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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1answer
68 views
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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1answer
42 views
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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1answer
50 views
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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1answer
33 views
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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1answer
51 views
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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Is the total ionic-dipole bond (hydrogen bonds) acting on an ion stronger than the lattice energy holding the ions together?
When magnesium chloride dissolves in water, its $\Delta H^⦵_\mathrm{sol}$ is negative.
Since $\Delta H^⦵_\mathrm{sol} = \Delta H^⦵_\mathrm{hyd} - \Delta H^⦵_\mathrm{latt},$ this means $-\Delta H^⦵_\...
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1answer
65 views
Change in Entropy and Enthalpy found over a temperature range and asked to compute the change in Gibb's Energy
I was given the following question by my tutor:
"The variation of enthalpy ($\Delta H$) and entropy ($\Delta S$) in a reaction carried out at
constant pressure in the temperature range between 300 ºC ...
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Why is average bond enthalpy measured by homolytic fission?
Average bond enthalpy: the energy required for 1 mole of a specific gaseous bonds to undergo homolytic fission
Why specifically homolytic fission? Is it to allow comparison, but if that were the case,...
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Change in temperature if salt is dissolved in water with different initial temperatures
I will mix a salt with room-temperature water and 8 degree Celsius water. Then I will compare the heat of solution with each other.
Does the initial temperature of the water affect how much the heat ...
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0answers
25 views
Combining enthalpies of formation and combustion in the context of Hess's law
I know that you can use enthalpy of combustion to work out formation, and Formation to work out enthalpy change. But can you use enthalpy of formation and combustion data is the same Hess's cycle to ...
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What is the lattice enthalpy of zinc chloride?
I am trying to find a literature value for the enthalpy of solution of zinc chloride to compare with my experimental value but I can't seem to find it. So I'm trying to use the formula: enthalpy of ...
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Resolving ambiguity around Lattice Enthalpy in Born-Haber Cycle
According to my textbook,
Lattice enthalpy is the enthalpy change that occurs when one mole of a solid ionic compound is separated into gaseous ions under standard conditions.$^1$
According to the ...
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1answer
119 views
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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1answer
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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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1answer
41 views
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 ...
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1answer
60 views
Endothermic or Exothermic [closed]
Is an endothermic reaction always endothermic at all conditions of pressure, temperature etc.? Similarly for exothermic... I mean is sign of ∆H always the same as sign of ∆H°?
This might seem to be a ...
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2answers
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Are there any reactions that are endothermic and exothermic? [closed]
As it says: are there any reactions that require heat and produce heat simultaneously?
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Values for heat of reaction example
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:
Energetics of $\ce{H}^\...
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1answer
47 views
Why are there so many different formulations for energy? When do I use each one? [closed]
I understand that energy is a fairly arbitrary concept, and that it completely depends on your definition of energy. So far in my class, we have discussed:
Entropy, $S$ (not really energy, but it ...
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2answers
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Which change occurs in all exothermic reactions? [closed]
Which change occurs in all exothermic reactions?
A) Bubbles of gas are released from the mixture.
B) Light energy is produced.
C) The temperature of the mixture decreases.
D) The temperature of the ...
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1answer
60 views
Understanding the term 'heat of hydrogenation'
From Chemistry Libretexts:
Heat of hydrogenation (symbol: $ΔH_\mathrm{hydro}$) of an alkene is the standard enthalpy of catalytic hydrogenation of an alkene.
Does the heat of hydrogenation include ...
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2answers
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Water becomes cold on mixing energy drink [closed]
I noticed that the temperature of water decreases noticeably when I mix an energy drink (Glucon-D) in it. The ingredients mentioned on the pack are majorly glucose (52%) and sucrose (45%). I believe ...
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3answers
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Is the kinetic energy change of a system (q) equal to the enthalpy change of a system (∆H)?
When silver nitrate and hydrochloric acid are put together in a container, they form silver chloride. If you're given information about the reactants (including temperature change, heat capacity, and ...
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1answer
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Heats of formation of neutral molecules and homolytic vs heterolytic bond dissociation in mass spectrometry
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:
Great efforts have been ...
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1answer
46 views
Why for compounds of elements in a group with larger anions have nearly constant lattice enthalpy?
Why do compounds of elements in a group with larger anions have nearly constant lattice enthalpy?
In the reasons for the decrease of the solubility of sulphates of alkaline earth metals, it is given ...
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How to determine the formula for calculating the enthalpy change of a reaction? [closed]
$$\ce{4 HCl(g) + O2(g) -> 2 Cl2(g) + 2H2O(g)}$$
In the reaction above, knowing the enthalpy of formation of each individual compound, in order to calculate the standard enthalpy change of the ...
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2answers
51 views
Which of the following reaction will have greater enthalpy and why? [closed]
Which of the following reaction will have greater enthalpy and why?
$\ce{C (g) + 4 H (g) -> CH4(g)}$ (reaction enthalpy = $x\ \mathrm{kJ/mol}$)
$\ce{C (graphite) + 2H2 (g) -> CH4 (g)}$ (reaction ...
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3answers
105 views
Need help understanding Enthalpy
My chemistry textbook says that:
Enthalpy (Internal Energy) = chemical potential energy + thermal energy
Now, I know that Enthalpy and Internal Energy are different so I really don't understand what ...
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1answer
123 views
how to compute enthalpy of combustion for methanoic acid [closed]
My data source gives the following values for various enthalpies of formation and combustion:
formation of CO2 = -393.5
formation of HCOOH = -425
combustion of HCOOH = -254.3
Wikipedia gives ...
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1answer
109 views
Enthalpy of reaction between ammonia and oxygen
I have to ask this here because I asked literally everybody I know and I still disagree with them.
Using the information in the table below, calculate the heat of released from the following ...
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Enthalpy of formation for Lithium ion in ethylene carbonate
The following link has the enthalpy of formation for aqueous lithium http://nshs-science.net/chemistry/common/pdf/R-standard_enthalpy_of_formation.pdf
I am wondering how to get the enthalpy of ...
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1answer
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How do you find the bond enthalpy from an equation?
Which equation represents the $\ce{N-H}$ bond enthalpy in $\ce{NH3}?$
$$
\begin{align}
&\textbf{A.} &\ce{NH3(g) &-> N(g) + 3 H(g)}\\
&\textbf{B.} &\ce{1/3 NH3(g) &-> 1/3 ...
