# Questions tagged [enthalpy]

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

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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 ...
734 views

### 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 ...
263 views

### 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 ...
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### How do scientist obtain the value of enthalpy of formation? [closed]

Do they do it experimentally or do/can they use bond enthalpy to figure it out? For example $\ce{C2H4 + H2 -> C2H6}$. The enthalpy of formation, $\Delta_fH$ of formation for the product is listed ...
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### Which of the following species has an enthalpy change of formation = 0 kJ/mol?

Below was a question in the 2012 Australian Chemistry Olympiads: The enthalpy change of formation ($\Delta_\mathrm{f}H^\circ$) for a species at $\pu{298 K}$ is defined as the enthalpy change that ...
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### How can I calculate the standard reaction enthalpy if I lack one formation enthalpy?

I have this reaction, for which I need the standard enthalpy of reaction: $$\ce{CuO (s) + H2S (g) \rightarrow CuS (s) + H2O (g)}$$ This is the data from the NIST web book (standard formation ...
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### Why is the enthalpy of combustion higher for n-butane as compared to 2-methylpropane?

N-butane has an enthalpy of formation of $\pu{-125.6 kJ/mol}$, whereas it is $\pu{-134.2kJ/mol}$ for 2-methylpropane. When comparing the heat set free at the combustion process, why is it higher for n-...
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### Enthalpy - unit confusion

I think I have trouble interpreting reaction enthalpy in these kind of situations: I am given that $\Delta _rH^0 = 25,9$kJ/mole in the reaction: $$\ce{1/2 H2 + 1/2 I2 \rightarrow HI}$$ which I ...
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### How do I calculate the standard enthalpy of formation for liquid ethanethiol? Do I use Hess's law? [closed]

Calculate a value for the standard enthalpy of formation for liquid ethanethiol, $\ce{C2H5SH}$. Use the equation given below and enthalpy of combustion data from the following table. Just assume ...
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### Why is $\Delta G = -T \Delta S_{\mathrm{total}}$ valid only at constant pressure?

According to my book(Elements of Physical Chemistry by Atkins and de Paula, 5th ed.), $\Delta G = - T \Delta S_{\mathrm{total}}$ is valid only for constant pressure and temperature. Enthalpy is ...
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### Does temperature change pressure in isochoric system?

Consider the following equilibrium reaction: $$\ce{2 NOCl (g) <=> 2 NO (g) + Cl2 (g)} \qquad \Delta H > 0$$ My textbook has a statement that: If you decrease the temperature when ...
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### What exactly does the “per mole” unit of heat of atomization mean?

I am facing a confusion in the definition of the standard enthalpy of atomization. The confusion arises because of the "per mole" in its units ($\pu{kJ mol^-1}$). My question: When I say "per mole", ...
284 views

### Why must NaOH be in excess if we wish to measure the effect acid strength has on the enthalpy of neutralisation?

A few different acids are given i.e $\ce{HCl}$ , ethanoic acid, $\ce{H2SO4}$, ethanedioic acid. We want to conduct an experiment in which we measure the temperature rise for neutralisation as a ...
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### Why does the standard enthalpy of formation diverge so far from the standard Gibbs free energy of formation for some substances?

If you look at a Table of Thermodynamic Values for chemical substances, most substances have very close values for their standard enthalpy of formation ($\Delta H_{\mathrm{f}}^\circ$) and their ...
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### Why is the standard enthalpy of formation of black phosphorus not zero? [duplicate]

Black phosphorus is the most stable allotrope of phosphorus. Then why is its standard enthalpy of formation not taken as zero?
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### Enthalpy Change

In calculating the change of enthalpy in converting Fulminic to Isofulminic acid, would one simply take the sum of the negative enthalpies of the bonds of Isofulminic acid minus the sum of the ...
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### Why increase in enthalpy leads to increase in entropy? [closed]

Given statement: Enthalpy of adsorption is the strength at which molecule (say $\ce{H2}$) is bound to the adsorbent (say some metal center). For a free $\ce{H2}$ molecule, the entropy contribution is ...
I have seen the equation $S(T_2)=S(T_1)+C_p\ln(T_2/T_1)$ where $C_p$ is the molar heat capacity at a constant pressure. I understand that this assumes that the temperature range is sufficiently small ...