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Questions tagged [enthalpy]

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

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Standard Enthalpy of Formation of Refrence state(A consequence or assumption) and H+(aq)

Ok, this has been bugging me for a while now. Our teacher taught us that the standard enthalpy of formation of the reference states of elements is "assumed" to be zero. However, I've read in ...
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Enthalpy change with uncertainties [closed]

Mg + CuSO4 -> MgSO4 + Cu 1.0g Mg and 50ml(=50g) 0.500mol/dm^3 CuSO4 CuSO4 is limiting reactant use q=mcT/mol of limiting reactant and enthalpy change= - q/mol to find the enthalpy change of this ...
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Two different approaches for calculating enthalpy change for methane + HCl

I know that $\Delta{H}=H_{products}-H_{reactants}$ and also we can use $\Delta{H}=\Sigma{H}_{bonds broken}-\Sigma{H}_{bonds formed}$. Take the example of methane + HCl. If you calculate the total ...
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The correct way of formulating change in enthalpy

I came across this question: One mole of a non-ideal gas undergoes a change of state $(2.0 atm, 3.0L, 95K)$ to $(4.0 atm, 5.0L, 245K)$ with a change in internal energy = $30L atm$. Find the change in ...
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If $\Delta$Hsoln < 0, will the conditions for solubility always be met?

If $\Delta$Hsoln < 0, will the conditions for solubility always be met? This was on an exam, and the answer was yes. It could be true, but it seems wrong to me because G=H-TS. So it just seems as ...
Shay's user avatar
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Why are internal energy and work mentioned in a question about exothermic or endothermic systems? [closed]

This is a question from my chemistry homework. Note the explanation below. A 100.0-g bar of gold is heated from 25°C to 50°C during which it absorbs 322 J of heat. The volume of the gold bar remains ...
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Does pressure affect the enthalpy of formation?

In a complete combustion reaction, coal and air (both at $300K$ and $2$ atm) react to produce carbon dioxide which exits the reactor at the same temperature and pressure as it entered. $$\mathbf{C+O_2}...
Eric Brown's user avatar
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Will a reaction happen slower if it has a higher enthalpy change than its counterparts?

Suppose both reactions are conducted in identical conditions and they have the same activation energy, and B has 0 enthalpy. Which one will happen faster? My experiment said reaction A happen slower ...
Chris Nalusaku's user avatar
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Why does enthalpy change and not internal energy help in determination of spontaneity?

Spontaneity of a process is measured by Gibbs Free Energy $$\Delta G= \Delta H-T\Delta S$$ $$\Delta H=\Delta U+\Delta (PV)$$ Is there an intuitive explanation for why Gibb's energy depends upon ...
Portuguese Man Of War's user avatar
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Stability comparison between Acetylene/Ethyne (C2H2) and Nitrogen Gas (N2) [closed]

N≡N is more stable than H2N—NH2 (hydrazine) But HC≡CH is less stable than H3C—CH3 Why is it so? Even though the size of the corresponding compounds and hybridization is the same? (Usually ethane is ...
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Heat capacity of calorimeter

The problem is: $\pu{30 mL}$ of water at $\pu{24.00 °C}$ were stored in a calorimeter in a laboratory experiment. Then, $\pu{40 mL}$ of water at $\pu{55.00 °C}$ were added to the calorimeter ...
Sherry Zakhari's user avatar
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2 answers
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Doubt regarding definition of enthalpy

We define enthalpy change as heat released at constant pressure but it is also defined for process where pressure varies. How do I understand this?
Aarush Saharan's user avatar
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Work from a spontaneous endothermic reaction?

Some reactions are spontaneous ($\Delta_\mathrm{r}G < 0$) although they are endothermic ($\Delta_\mathrm{r}H > 0$), e.g. the plastic packs that once cracked they get cold. I saw that it is often ...
Olfaction's user avatar
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Estimation methods for temperature evolution of a high $\Delta H_{mix}$ dilution ($\ce{KOH}$ in water)

Question up front: Can temperature evolution in solutions be short-hand-estimated by simple state based calculations - with reasonable expectations of precision? (+/- 10° C) I am trying to calculate ...
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What is the enthalpy change for elimination reactions of alkyl halide - exothermic or endothermic?

There are a few questions in thermodynamics (with unverified data) that the addition reaction of ethene to give ethyl chloride has a +ve enthalpy change, thereby elimination to give alkene must be ...
Che Mistry's user avatar
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Why exactly does copper have a higher melting point than zinc?

It is said that copper has a higher melting point than zinc because of the d electrons in copper being involved in metallic bonding. However, copper has no unpaired electrons in its d subshell just ...
wonderingwhy's user avatar
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Temperature dependence on the feasibility of an equilibrium

If we consider a reaction A + B ⟶ C + D, with $\Delta$S $>0$, then regardless of whether $\Delta$H is positive or negative, increasing the temperature will make $\Delta$H - T$\Delta$S more negative,...
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visualization problem of dilution of enthalpy $\Delta_{dil} H$ data with different initial concentrations

description I have a question about the $m_f$ vs. $\Delta_{\text{dil}} H$ plot in some experimental studies. For example, the dilution-of-enthalpy data at 300 °C with two initial concetration ($m_i$) ...
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How does enthalpy of a reaction vary with pressure?

Kirchhoff's law gives us a way to find variation in enthalpy of a reaction with temperature But is there some equivalent law for pressures? Ie, given a reaction and it's enthalpy at a pressure P1, is ...
ThatApollo777's user avatar
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Why is the standard enthalpy of formation temperature-dependent in JANAF tables?

I'm unsure why $\Delta_\mathrm{f}H^\circ$ within the NIST-JANAF Thermochemical Tables [1] is temperature-dependent. Wouldn't the $\Delta_\mathrm{f}H^\circ$ represent the enthalpy of formation at the ...
Jacob Ivanov's user avatar
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How can enthalpy of an ideal gas be independent of pressure?

I know that when one applies a manipulation of the Ideal Gas Law to the Maxwell Relations the result that enthalpy is independent of pressure tumbles out of it, i.e., $(\mathrm dH/\mathrm dp)=0$. I ...
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Change in internal energy at constant pressure versus heat released at constant volume

I am trying to solve a question that involves studying the heat released of a combustion process at constant pressure versus at constant volume. The question is The heat of combustion of ethanol ...
mark's user avatar
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Is ΔH = ΔU + ΔPV only valid for gases?

In this question, formula ΔH = ΔU + ΔPV is used. In the solution, ΔPV is replaced for RTΔng. As there are solid products also in this equation, don't we consider ΔPV for them. Also if there were ...
RandomGuy's user avatar
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Sign of ∆S in conversion of diamond to graphite

It is said that ∆S is positive as randomness is increasing from diamond to graphite as in diamond molecules are tightly packed together but we know that ∆S=∆H/T for phase transition and that ...
S K's user avatar
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How do I approach the problem if Cp is given as a function of volume?

Here is the problem I'm trying to solve: A system undergoes Isothermal Expansion under the atmosphere. For the system, $ C_p = a + \frac{b}{V} \ $. The process is given by $ P_{ATM}, V_1, T_1 \...
poirot_06's user avatar
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How to treat enthalpy change and pressure in reactions involving a liquid and solid state

In physical chemistry, it is often stated that "at constant pressure, the change in enthalpy gives the amount of heat transferred into the system" and the justification for this claim is $$...
Neel's user avatar
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What is the difference between zero-point and total energy of a molecule in the output of an ORCA FREQ calculation?

I am trying to find the enthalpies of formation for a couple organometallics in ORCA and as a first time user I am a bit bogged down in the terminology. The Gaussian manual has a section on these ...
Spencer's user avatar
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How is temperature inside a bomb calorimeter constant?

I've looked around and have been unable to find an answer. When determining molar reaction enthalpy, you first perform the reaction in the steel bomb then measure the temperature change of the water ...
heyhellohi2's user avatar
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1 answer
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Is ionisation enthalpy physical property or chemical property?

Ionisation enthalpy means energy change when an electron is removed. According to Wikipedia A chemical property is any of a material's properties that becomes evident during, or after, a chemical ...
Harjot Dhillon's user avatar
1 vote
1 answer
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Why does bond energy not vary with position?

Consider the formation of a hydrogen bond: $$\ce{H + H -> H2}$$ The $\Delta H$ is $\pu{-436 kJ mol-1}$, so we can equally write the equation as: $$\ce{H + H -> H2 + \pu{436 kJ}}$$ But depending ...
user137472's user avatar
1 vote
2 answers
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Understanding the Role of Gibbs Free energy and Enthalpy as the energy required to drive a reaction

I was looking at the enthalpy change for water-splitting reaction: $$ \Delta H^o_R = [\Delta H^0_{H_2(g)} +\frac{1}{2}\Delta H^0_{O_2(g)}]-\Delta H^0_{H_2O(l)} = \pu{285.83 kJ/mol}$$ According to ...
RMS's user avatar
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Enthalpy change of a solution of 1-pentanol in 1-butanol

I came across a question asking to indicate the enthalpy of solution for a solution of 1-pentanol in 1-butanol. The options were the following: $\Delta H < 0$ $\Delta H \approx 0$ $\Delta H >...
Cold_Spaghetti's user avatar
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What exactly is meant by standard state in thermodynamics? [duplicate]

I was told that the standard enthalpy of a reaction is the enthalpy change when the reaction occurs with all reactants and products being in there standard states and under standard conditions(1 bar ...
Aditya Mukherjee's user avatar
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1 answer
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calculation of enthalpy change

So for for the calculation of $\Delta{H}$,we have two formulaes in our book: $\Delta{H}=E_{2}-E_{1}$ where $E_{2},E_{1}$ are the internal energies of the product and reactants respectively. Now this ...
aiman's user avatar
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Discrepancy between calculated and experimental enthalpy change of formation? Hydrogen Iodide

Afterthought: See edits below! $$\ce{1/2 H2(g) + 1/2 I2(g) -> HI(g)} \qquad \Delta_\mathrm f H^\circ=26.5\ \mathrm{kJ\ mol^{-1}}$$ source: https://atct.anl.gov/Thermochemical%20Data/version%201....
Cheng's user avatar
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Physical meaning of term T∆S

Plot "I assume the role of an attentive observer, carefully watching a chemical reaction unfold within a sealed container. This enclosed environment, completely closed ( no matter but energy can ...
Bharat Prajapat's user avatar
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1 answer
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Why does Cr have higher melting point in the 4 period among the transition metal? [duplicate]

There is a lot of answer stating that the low enthalpy of atomisation in Cr is due to the reason of partially filled d orbital, thus having extra stability. This makes the orbital more attracted ...
Satyadarshi's user avatar
1 vote
1 answer
255 views

How to Calculate Entropy Changes at Non-Standard Conditions

I am trying to calculate the values of $\Delta H$ and $T\Delta S$ for the reaction taking place at $\pu{800 ^\circ C}$ $$ \ce{CO2 + H2O + 2CH4 -> 3CO + 5H2} $$ I calculated $\Delta H$ to be $\pu{\...
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Enthalpy of adsorption and adsorption rate

What I want to know is about the adsorption enthalpy change by the change of surface coverage and the adsorption rate. In general situations, adsorption would be exothermic. So in the Gibbs free ...
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Is ionization energy enthalpy change?

In physics, we learn how to calculate the ionization energy of a hydrogen atom using the Bohr model. The result is the following equation: $$ E = - \frac{1}{n^2} \frac{Z^2e^2}{2a_0} = - \frac{Z^2 R_H}{...
Yusuke Terada's user avatar
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Is the activation energy for vaporization equal to the enthalpy of vaporization?

When ethanol, say, evaporates, the intermolecular forces between molecules need to be broken. As a vapour, there are no new forces being created, so it is a purely endothermic process, with no "...
Rafael's user avatar
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2 answers
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Reversibility for Entropy Changes in Chemical reaction

There is a problem in my textbook that asks to calculate the entropy change in surroundings for the Haber process, assuming that the enthalpy change is -92.4 kJ/mol. The solution to this problem is to ...
Manan Vij's user avatar
-1 votes
1 answer
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Understanding the definition of enthalpy [duplicate]

My chemistry text describes Internal Energy (U) of a substance as the sum of all categories of energy, i.e., Kinetic (temperature), potential, bonding, etc. Then, Enthalpy is defined as: $$H=U+PV$$ ...
Dennyc39's user avatar
2 votes
2 answers
563 views

What do the units of enthalpy kJ/mol represent?

Enthalpy is making me slightly confused. Here $\Delta H$ refers to the change in enthalpy. Does this refer to the enthalpy change of reaction? Are they the same? It seems that change in enthalpy, ...
Stark's user avatar
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Enthalpy change of neutralisation?

Consider this problem. The enthalpy of neutralisation is defined as the enthalpy change when one mole of water is formed through the react of an acid and base at standard conditions. As the ...
Ca Sagan's user avatar
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1 answer
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How is the reason for “why neutralizing weack acids with strong bases gives less heat” that the heat is used in ionizing the acid?

In my lab manual this is written but I can’t see how this is true The heat of neutralization of weak acids by strong bases is less than 13.7 Cal/mole because the weak acid becomes completely ionized ...
Jay's user avatar
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1 answer
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Why does an exothermic/endothermic reaction have a negative/positive enthalpy

So regarding change in enthalpy and exothermic/endothermic reactions, I think I have some conceptual understanding but I'm missing some things. My current understanding is that: Given a reaction, if ...
KI.'s user avatar
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2 answers
182 views

Is change in enthalpy heat supplied at constant internal or external pressure? [closed]

From what I understood, we introduced enthalpy to work with reactions that were carried out at constant external pressure. But then I came across this formula (for ideal gas): ∆H=∆U + p∆V where p∆V=∆...
S123's user avatar
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1 vote
1 answer
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Why is the first electron affinity exothermic and successive ones endothermic?

I am having trouble wrapping my head around electron affinities. And the textbook explanations aren't very helpful. So, the textbook says that the 1st electron affinity is generally exothermic. The ...
John Smith's user avatar
1 vote
2 answers
69 views

Should I give an endothermic reaction the exact energy it needs to occur or can I give it in parts?

I know this might be a silly question, but should I sustain the exact energy or more for the enthalpy of endothermic reaction to occur or can I give it in parts? Let's say a certain reaction requires $...
Belal Bahaa's user avatar

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