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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51 views

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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Why is the standard enthalpy change of formation of C3H8 different when calculating the same enthalpy change using bond dissociation values? [closed]

The $\Delta H_r$ (change in enthalpy of reaction) in the following reaction: $\ce{3C(s) + 4H2(g) -> C3H8(g)}$ is $\pu{-2356 kJ/mole}$ when using the below dissociation energy values: However the $\...
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35 views

How can you calculate the tendency of one compound to become another?

Excuse my inexperience with chemistry here. I've been poring over papers with terms like "Standard Gibbs free energy of formation", "Hess's Law", "standard enthalpy of ...
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Thermochemistry: On Units of Enthalpy Change and Implicit Given of Initial Temperature of Calorimeter

I would like to ask two separate questions. But first, so that we are on the same page, the experiment used a constant-pressure calorimeter, and it is a neutralization reaction of 1M of HCl and 1M of ...
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23 views

Why does lattice enthalpy decrease with increasing ionic size?

Lattice enthalpy decreases as ions get larger, but I have found two explanations: The charge density is greater in smaller ions, so greater attraction The ions are themselves able to get closer ...
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Why are branched alkanes used as fuels instead of straight chain alkanes?

My textbook states that branched alkanes are thermodynamically more stable than straight chain alkanes. Of course, iso-octane has an enthalpy of formation of $\Delta H_\mathrm f$= $\pu{-259.3.40 kJ ...
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234 views

Kinetics Vs Thermodynamics : Apparent contradiction in the definition of Enthalpy in the two disciplines

My book gives the following curve: It gives the following relation :$$ΔH= E_{\mathrm{activation,forward}}-E_{\mathrm{activation,backward}} \tag{1}$$ But I suspect that $ E_{\mathrm{activation,forward}...
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Why is the zero of standard enthalpy of formation a convention?

The standard enthalpy of formation $\Delta H_f^°$ of pure elements is zero by definition. Why is that a convention? It is true that enthalpy is defined unless a constant (like energy and entropy), but ...
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Predicting sign of enthalpy of allotropic transition of phosphorus

How to predict the sign of enthalpy during the transition $$\ce{P(s, white) -> P(s, red)}?$$ $\ce{P(s, red)}$ is more stable than $\ce{P(s, white)}$ due to release in angle strain. Since during ...
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Why do internal energy/heat and enthalphy differ? What is the physical significance of the $PV$ term for enthalphy of an ideal gas? [duplicate]

Let it be noted that this is not a duplicate of this downvoted-to-hell question, simply because I hope to ask it better! I've been reading a thermo textbook and I've got a simple question. The ...
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Why isn't the mass of the things dissolved in water not added to the mass of the water in an enthalpy calc?

Take a change in enthalpy question. When $\pu{5 g}$ of powder is dissolved in $\ce{100 ml}$ of water, the added substance raises the temperature of the water by $\ce{5 ^\circ C}$ and you are told to ...
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Feasibility of a Reaction [closed]

I understand for a reaction to be feasible, $\Delta$H needs to be negative and $\Delta$S positive. But in terms of Gibbs free Energy Change, without considering kinectic factors, If $\Delta$H is ...
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What is the mixing enthalpy of oxygen and water? [closed]

What I am actually interested in is making a fluid that can oxygenate humans through the skin to couteract ischaemia like gangrene and covid. So if the question has to be narrowed down it means ...
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Sign of change in enthalpy and change in entropy

Why is it wrong to assert that the change in entropy and the change in enthalpy must always have the same sign? What makes me think that they must have the same sign is the fact that every reaction ...
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The relation between q and T when the volume is constant [closed]

$$ \begin{align} q_V &= ΔU \tag{1}\\ q_p &= ΔH \tag{2}\\ C_V &= \frac{\mathrm dq_V}{\mathrm dT} = \left(\frac{\partial U}{\partial T}\right)_V \tag{3}\\ C_p &= \frac{\mathrm dq_p}{\...
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Significance of pV work in enthalpy [closed]

If we already have $pV$ work included in the internal energy $U = q + w,$ then what is the $pV$ work in the equation for enthalpy $H = E + pV?$
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480 views

Why does spontaneity of a reaction depend on temperature?

As we know, $$\Delta G=\Delta H - T\,\Delta S.$$ Both in my textbook and on the internet, it was given that temperature was a determining factor in the spontaneity of a reaction. For example, when $\...
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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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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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51 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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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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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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56 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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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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87 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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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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42 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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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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207 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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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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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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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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138 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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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 ...
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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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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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