Questions tagged [enthalpy]
A thermodynamic state function describing the total energy content of a system.
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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 ...
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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., (dH/dP) = 0.
I don't understand ...
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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 ...
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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 ...
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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 ...
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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 \...
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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
$$...
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What is the difference between zero-point and total energy of a molecule in the output of an ORCA FREQ calculation?
I am trying to find the enthalpies of formation for a couple organometallics in ORCA and as a first time user I am a bit bogged down in the terminology. The Gaussian manual has a section on these ...
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How is temperature inside a bomb calorimeter constant?
I've looked around and have been unable to find an answer.
When determining molar reaction enthalpy, you first perform the reaction in the steel bomb then measure the temperature change of the water ...
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Is ionisation enthalpy physical property or chemical property?
Ionisation enthalpy means energy change when an electron is removed.
According to Wikipedia
A chemical property is any of a material's properties that becomes evident during, or after, a chemical ...
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Why does bond energy not vary with position?
Consider the formation of a hydrogen bond:
$$\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}}$$
But depending ...
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Understanding the Role of Gibbs Free energy and Enthalpy as the energy required to drive a reaction
I was looking at the enthalpy change for water-splitting reaction:
$$ \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}$$
According to ...
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Enthalpy change of a solution of 1-pentanol in 1-butanol
I came across a question asking to indicate the enthalpy of solution for a solution of 1-pentanol in 1-butanol.
The options were the following:
$\Delta H < 0$
$\Delta H \approx 0$
$\Delta H >...
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What exactly is meant by standard state in thermodynamics? [duplicate]
I was told that the standard enthalpy of a reaction is the enthalpy change when the reaction occurs with all reactants and products being in there standard states and under standard conditions(1 bar ...
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calculation of enthalpy change
So for for the calculation of $\Delta{H}$,we have two formulaes in our book:
$\Delta{H}=E_{2}-E_{1}$
where $E_{2},E_{1}$ are the internal energies of the product and reactants respectively.
Now this ...
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Discrepancy between calculated and experimental enthalpy change of formation? Hydrogen Iodide
Afterthought: See edits below!
$$\ce{1/2 H2(g) + 1/2 I2(g) -> HI(g)} \qquad \Delta_\mathrm f H^\circ=26.5\ \mathrm{kJ\ mol^{-1}}$$
source: https://atct.anl.gov/Thermochemical%20Data/version%201....
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Physical meaning of term T∆S
Plot
"I assume the role of an attentive observer, carefully watching a chemical reaction unfold within a sealed container. This enclosed environment, completely closed ( no matter but energy
can ...
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Why does Cr have higher melting point in the 4 period among the transition metal? [duplicate]
There is a lot of answer stating that the low enthalpy of atomisation in Cr is due to the reason of partially filled d orbital, thus having extra stability. This makes the orbital more attracted ...
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How to Calculate Entropy Changes at Non-Standard Conditions
I am trying to calculate the values of $\Delta H$ and $T\Delta S$ for the reaction taking place at $\pu{800 ^\circ C}$
$$
\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}{...
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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 "...
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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 ...
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Calculating the enthalpy of reaction at non-standard concentration for reactions in aqueous phase
Standard conditions for a chemical reaction in the aqueous phase, for instance, a chemical reaction in solution, consider that reagents should be at a 1 M concentration. I was wondering how to ...
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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:
$$H=U+PV$$
...
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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 to be change in enthalpy, ...
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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 ...
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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 ...
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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 ...
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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=∆...
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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 ...
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Determine enthalpy and entropy from measurements of Kd at different temperatures?
In biochemical experiments, it is very common to measure IC50 (half inhibitory concentration) eg by adding different amounts of an inhibitor to an enzyme+substrate and measuring the reaction rate or ...
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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 $...
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What is the chemical equation for enthalpy of solution of hydrated salts? [closed]
If I take the anhydrous salt $\ce {CuSO_4}$, the equation for its dissolution in water would be:
$$ \ce {CuSO_4}_{(s)} + \ce {aq}.\rightarrow \ce {CuSO_4}_{(aq)} $$
Now for the hydrated salt $\ce {{...
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Enthalpy and Bond Energy Sign Convention
I read that a negative enthalpy of formation means an exothermic reaction. Does that mean this reaction is true? The enthalpy of formation for $\ce{H_2O}$ is -285.8 kJ/mol.
$$\ce{2H2(g) + O2(g) -> ...
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Lattice energy as a factor that helps determine which charge the atoms take on?
I do not quite understand the following line of reasoning:
Why does Na not form $\ce{Na^{2+}}$ ions? To obtain $\ce{Na^{2+}}$ ions, the first and second ionization energy must be applied: 496 + 4563 ...
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Why can we add/substract/cross out chemical equations for Hess law?
Suppose I take the model example:
(1) $\ce{C + 1/2 O2 -> CO}$
(2) $\ce{CO + 1/2 O2 -> CO2}$
Adding both together:
(3) $\ce{C + CO + 1/2 O2 + 1/2 O2 -> CO + CO2}$
(=> $\ce{C + 1/2 O2 + 1/2 ...
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Find the percentage of dissociation of nitrogen tetroxide given pressure, temperature, enthalpy and entropy
Given
$$
\begin{align}
\Delta_\mathrm f H^\circ(\ce{N2O4}) &= \pu{9.16 kJ mol^-1}
&\quad
\Delta_\mathrm f H^\circ(\ce{NO2}) &= \pu{33.18 kJ mol^-1}
\\
S^\circ(\ce{N2O4}) &= \pu{304.3 ...
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Why would a metal form an ionic compound with a nonmetal from a lower period?
Take sodium iodide for example. I understand both elements can obtain a full outer shell via the electron transfer. But doesn't the electron move from a lower-energy orbital ($3s$ in sodium) to a ...
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Intuitive explanation for enthalpy of hydration
In my chemistry course, we were discussing types of enthalpy and the enthalpy of hydration came up. My instructor defined this as
heat associated with adding water to an anhydrous salt.
However, I ...
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Standard Enthalpy of Formation
I have hard time understanding why in the definition of standard enthalpy of formation (SEF), the reactants and the products are required to have fixed temperature and pressure. Specifically, ...
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How do I calculate enthalpy change calculations questions?
Below is a question I'm struggling with.
When $\pu{0.1 mol}$ of aqueous potassium hydroxide was added to $\pu{0.1 mol}$ of nitric acid, $\pu{5200 J}$ were transferred to the surroundings. What is the ...
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Difference between change of enthalpy of neutralization between copper and manganese carbonate
I was doing a calorimetry experiment to calculate the change in enthalpy during neutralization reactions between metal carbonates and hydrochloric acid. I experimented with 5 metal carbonates: ...
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Comparing the enthalpy change between the formation of water in two states
The enthalpy of the reaction $\ce{H2(g) + 1/2 O2 (g) -> H2O (g)}$ is $\Delta H_1$ and that of $\ce{H2(g) + 1/2 O2 (g) -> H2O (l)}$ is $\Delta H_2$. Then,
a) $\Delta H_1 < \Delta H_2$
b) $\...
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What makes a compound high or low energy and why does that relate to reactivity? [closed]
After reading about thermodynamic stability, it now makes sense to me how the enthalpy of a reaction relates to the thermodynamic stability of a compound.
However, when looking for a clear way to ...
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Spontaneity criterion for phase transformations and chemical reactions [closed]
Why is the Gibbs free energy (G) considered a spontaneity criterion for phase transformations and chemical reactions? Why are other thermodynamic parameters such as enthalpy (H), entropy (S), and ...
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Alternate formulation of h(P,T)?
I'm using the following handbook "Engineering and Chemical Thermodynamics 2nd edition" by Koretsky. In the book they have an alternate formulation for h(P,T) though it was never explained ...
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Is there an equivalent term for favourable and non-favourable entropy?
For Gibbs energy, we have 'exergonic' or 'endergonic'
For Enthalpy, we have 'exothermic' or 'endothermic'
It seems logical there should be an equivalent pair of terms for entropy, but I can't seem to ...
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Should change in enthalpy be zero in galvanic cells?
When an exothermic redox reaction happens by just mixing the reagents, the change in enthalpy at constant pressure equals heat which is consistent with the fact that:
For a closed system at constant ...
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How do I calculate the temperature change in a gaseous reaction?
So, with $\pu{3 mol}$ of $\ce{H2}$ and $\pu{3 mol}$ of $\ce{Cl2}$ and given the reaction $\ce{H2(g) + Cl2(g) -> 2 HCl(g)}$ with $\Delta H=\pu{-184.6 kJ/mol}$, I am supposed to calculate the work ...
