# Questions tagged [enthalpy]

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

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### Exothermic: enthalpy or heat?

I am used to thinking of exothermic reactions as being defined as having negative $ΔH.$ However, I also am used to hearing/thinking of exothermic reactions as "releasing heat". Since change in ...
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### What makes determining entropy changes theoretically so hard compared to enthalpy?

It is really hard to determine the entropy change of a reaction accurately because of subtle solvent effects and a lack of standard entropies. On the other hand, enthalpy is relatively easy and ...
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### Enthalpy of combustion of 1-butanol

1-butanol $\ce{C4H10O}$ So I have absolutely no idea how to find the enthalpy and I did some looking on the internet with no use since I don't understand. I came here hoping someone generous could ...
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### Reaction enthalpy using bond energy in resonating molecules

To find out and enthalpy of a reaction using bond energy we use the formula $$\Delta H = \sum\Delta_\mathrm{bond}H_\mathrm{reactants} - \sum\Delta_\mathrm{bond}H_\mathrm{products}$$ since we use all ...
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### Why are Born-Haber enthalpy calculations inaccurate for transition metal compounds?

A senior high/secondary school level exam question is about discrepancies between calculated and empirical enthalpies of formation. It asks why a discrepancy “usually” exists for d-block compounds ...
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### Iron(III) nitrate and hydrogen peroxide reaction

When iron(III) nitrate is added to hydrogen peroxide as a catalyst, the temperature initially decreases and begins to increase after 30 seconds. Why is this the case? (is there any intermediate ...
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### Energy absorbed by a system in isothermic process in which the phase changes from liquid to gas

A system contains $x$ mole of material, with surface area $S$ is pressed by a mass $M_1$ (no fraction and no any other outer forces involves). Let $\Delta H_\mathrm{l\rightarrow g}$ be the transition ...
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### Relation between equilibrium constant and entropy change

Like the Van't Hoff equation, which relates change in enthalpy to equilibrium constant, is there a similar equation for the relation between change in entropy and equilibrium constant? Consider the ...
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### Need to normalize lattice enthalpy by no. of nearest-neighbor (NN) interactions to equate to NN interaction strength?

I've seen electrostatic interaction energy (from $E\sim \frac{q_1 q_2}{r}$) used to explain differences in lattice enthalpies between different crystalline solids. For example, I've seen it explained ...
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### Thermodynamic data related to diazotization reaction

Is there any thermodynamic data available for diazotization of aniline and also for the reduction of the diazonium salt using sodium sulfite? I also want to know the heat of reaction for both the ...
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### Temperature dependency of enthalpy H and entropy S [closed]

In textbooks, I often read that the temperature dependency of the enthalpy of reaction $\Delta_\mathrm{r}H$ and the entropy of reaction $\Delta_\mathrm{r}S$ can be neglected (in a limited $T$-range). ...
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### Change in Gibbs free energy vs. change in enthalpy

When learning about other thermodynamic quantities, like $\Delta H$, I was taught that this represents the energy absorbed or released by a system after the reaction has gone 100% to completion. For ...
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### Are precipitations exothermic and/or endothermic?

Should be an easy one. I'm fumbling a concept. I've read precipitations are exothermic. Is this accurate? Why would there be no endothermic precipitation reactions?
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### Which substances have the highest/lowest enthalpy of formation per gram?

Which substances have the most extreme enthalpies of formation per gram at standard temperature and pressure?
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### Is heating water endothermic?

So I got a problem recently which asked to calculate the amount of energy needed to increase the temperature of 200 g of water from 26 to 51 degrees Celsius. The value I got was 20900 J. So far I ...
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### Heat of formation of aqueous H⁺

In my book it is given that by convention heat of formation of aqueous $\ce{H+}$ is taken to be zero. Heat of formation is energy released or absorbed when 1 mole of a compound is formed from its ...
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### Combining Bond Enthalpies and Enthalpies of Formation

Can I mix formation enthalpies and bond enthalpies in the same calculation? I think the answer is no, since they are relative measurements, and possibly relative to different things, but I haven't ...
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### Do endothermic reactions have a higher absolute change in temperature than exothermic reactions? [closed]

or does it depend on the reaction itself. I am conducting an experiment and the endothermic reactions have temperature changes of at least 8 degrees while the exothermic reactions have a change in 1 ...
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### How to describe the enthalpy of acid dissociation in aqueous solutions?

How to describe the enthalpy of acid dissociation in aqueous solutions? If we had a weak acid "PB" that dissociates to give a proton and a conjugate base. Classically it would be described like the ...
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### Can an exothermic (endothermic) process increase (decrease) internal energy?

We classify exothermic and endothermic processes based on enthalpy change, $\Delta H < 0$ and $\Delta H > 0$ respectively. I assume this translates to $\Delta U < 0$ and $\Delta U > 0$, ...
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### Overall enthalpy balance on a mixing process

Let's say I have a mixing process involving $\ce{A}$ and $\ce{B}$ at temperatures $T_\ce{A}$ and $T_\ce{B}$, respectively. The $\ce{AB}$ mix is at temperature $T_p$. Our $ΔH_\mathrm{mix}$ is defined ...
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### Does enthalpy of dissolution change with temperature?

Dissolution can be either endothermic or exothermic. At higher temperature dissolution is much faster. However, supplying heat also leads to an increase in the internal energy of the system. ...
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### How does the proticity of an acid affect the enthalpy change of its reaction with a base?

For an experiment I've carried out a series of reactions between monoprotic bases (ammonia and potassium hydroxide) and several acids. Two of these acids were monoprotic (hydrochlori acid and ethanoic ...
So the bond association enthalpy for ionic compounds like $\ce{NaCl}$ and $\ce{NaBr}$ can be easily calculated from a Born-Haber cycle. But the way a Born-Haber cycle is constructed it uses info that ...