# Questions tagged [enthalpy]

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

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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 ...
118 views

### 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 ...
387 views

### 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 ...
284 views

### 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. ...
475 views

### 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 ...
291 views

### Calculating the strength of an ionic bond that contains poly-atomic ions

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 ...
148 views

### What is the difference between ΔfH and H-H° in NIST–JANAF tables?

Can someone explain the difference between $Δ_\mathrm{f}H$ and $H-H^\circ$ in NIST–JANAF tables?
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### What is the molar enthalpy of reaction

When 0.45 g of Zn is added to 50.0 ml of 0.95 M HCl solution, the solution inside the calorimeter heats up by 12 °C. What is the molar enthalpy of the reaction (in kJ/mol)? I know how to find the ...
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### Calculating ΔH from bomb calorimetry

Suppose we carry out a reaction in a bomb calorimeter whose starting temperature is $298.15\ \mathrm K$. Here we assume $\Delta V$ is close enough to zero that we consider the process to be at ...
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### Anomalous trend in formation enthalpy of alkali metal halides

I read that in alkali metal halides, the formation enthalpy for fluorides become less negative as we go down the group, whereas the reverse is true for chlorides, bromides and iodides. Why is it so? ...
290 views

### The sign of enthalpy of formation of magnesium oxide

I'm currently doing a lab to calculate the enthalpy of formation for $\ce{MgO}$. However at the moment me and my lab partner are having a disagreement. We've both calculated and agreed upon the same ...
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### Would ionic compounds dissolve in methanol?

Question: Determine whether liquid hexane ($\ce{C6H14}$) or liquid methanol ($\ce{CH3OH}$) is the more appropriate solvent for the substances grease ($\ce{C20H42}$) and potassium iodide ($\ce{KI}$)....
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### What really are the units of enthalpy change? [duplicate]

A reasonable definition of standard enthalpy change of formation would be: 'The enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions' ...
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### Is Kirchoffs law valid at all pressures?

$$\Large H_{T_\mathrm{f}, p}=H_{T_\mathrm{i},p}+\int_{T_\mathrm{f}}^{T_\mathrm{i}}c_p(T)dT$$ This can be derived by integrating: $$C_v =\left(\frac{\mathrm{d}U}{\mathrm{d}T}\right)_V$$ Applying this ...
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### Why are there two contradicting formulas for Enthalpy (Delta H): [duplicate]

1) Delta H = Bonds broken - Bonds Formed 2) Delta H = $H_{products} - H_{reactants}$ In the first formula, bonds break in the reactants and form in the products, so its basically saying "bonds ...
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### How do the pressure acting on the reactants and the initial temperature of the reactants affect the enthalpy of reaction?

In an exothermic reaction that occurs under constant pressure, does the pressure acting on the reactants affect the enthalpy of reaction? Does the initial temperature of the reactants affect the ...
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### Is the bond dissociation enthalpy of Cl-F greater than that of Cl-Cl?

In my book, it is mentioned that $\ce{Cl-F}$ has greater bond dissociation enthalpy than $\ce{Cl-Cl}$. Is it true? I know that interhalogen molecules usually have weaker bonds than dihalogen molecules ...
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### Why don't we consider VΔP when we define Q?

We only define $Q = \Delta U + W_\text{exp}$ (expansion work = $-P\Delta V$). If heat can cause $\Delta U$ and work, why work is defined only as expansion work in the first place where there are other ...
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### What is V times dP?

Work is defined when there is a change of volume. Mathematically, $W=-\int P\, dV$. When pressure is constant $W= P\, \Delta V$. For an ideal gas expansion we have $W= nRT\ln\frac{V_2}{V_1}$. If its ...
My textbook gives me the following formula for calculating the enthalpy change of any reaction: \Delta H_\mathrm{r}^\circ=\sum(\Delta H_\mathrm{c}^\circ)_\text{reactants}-\sum(\Delta H_\mathrm{c}^\...
What is the difference between enthalpy of reaction ($\Delta H$) and standard enthalpy ($\Delta H^\circ$)? I was told that the standard enthalpy of reaction is the change in heat when one mole of ...