Questions tagged [enthalpy]
A thermodynamic state function describing the total energy content of a system.
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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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Kirchhoff's law (Thermodynamics) [closed]
For the following question:
The standard enthalpy of formation of the metallocene bis(benzene)chromium was measured
in a calorimeter. It was found for the reaction $\ce{Cr(C6H6)2(s) → Cr(s) + 2C6H6(g)...
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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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Calculating the enthalpy of sweat evaporation
The following is a textbook question and answer:
Evaporation of sweat requires energy and thus take excess heat away
from the body. Some of the water that you drink may eventually be
converted into ...
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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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Enthalpy of crystal solids using Density Functional Theory
I wanna see the feasibility of a chemical reaction
$$A \rightarrow B +C$$
where A,B and C are crystal systems. Now I am using VASP in order to calculate the internal energy U, pressure and volume of ...
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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 ...
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hot pack experiment: calcium chloride vs magnesium chloride
why is calcium chloride preferred in hot packs even though magnesium chloride produces higher negative enthalpy? Experiment performed at school shows higher temperature change and more quicker ...
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Why is it necessary for the standard enthalpy of fusion to change at the transition temperature?
I have a table here:
For argon, for example, shouldn't the standard enthalpy of fusion @ 83.81K be 0 J/mol?
After all, it is already at the respective transition temperature.
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In regards to the Gibbs free energy equation, it seems that the two terms oppose one another, Enthalpy and -TS. Can you explain this?
So for example, if H is endothermic, then the enthalpy term is positive, meaning energy is absorbed by system overall, meaning net bonds broken, meaning the equation leans towards nonspontaneity. If ...
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Comparison of enthalpy change for the dissolution of alkaline earth metal oxides
Compare the heat liberated per mole for the following reactions:
$\ce{MgO + H2O -> Mg(OH)2}$
$\ce{CaO + H2O -> Ca(OH)2}$
$\ce{SrO + H2O -> Sr(OH)2}$
$\ce{BaO + H2O -> Ba(OH)2}$
Data for ...
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Understanding isosteric enthalpy (heat) of adsorption
I am trying to understand what is isosteric heat of adsorption. Based on van't Hoff equation:
$$ \left(\frac{\partial \ln K}{\partial T}\right)_θ = \frac{ΔH^\circ}{RT^2} $$
and $ΔH^\circ$ is defined ...
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Is there a need to change sign for enthalpy value for a reaction that appears in reverse when using Hess's law?
Problem
Given the enthalpies of the following reactions
\begin{align}
\ce{2H(g) &-> H2(g)} &\quad \Delta_\mathrm{r}H^\circ &= \pu{-437.6 kJ mol^-1} \tag{R1}\\
\ce{C(s) + 2 H2(g) &-&...
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Atomization enthalpy trends. The case of Phosphorus
I was looking again at the periodic trends like Ionization Energy, Electron Affinity, Radius and so on. I understood all the general trends and the exeptions for these ones, but I can't fully ...
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How should I evaluate $\left(\frac{\partial U}{\partial V}\right)_T$ with van der waals equation: $P = \frac{nRT}{V-nb}-a(\frac{n}{V})^2$?
So in my homework I was able to get this-
$\left(\frac{\partial U}{\partial V}\right)_T = T(\frac{\partial P}{\partial T})_V - P$
but now I'm trying to plug in $P = \frac{nRT}{V-nb}-a(\frac{n}{V})^2$ ...
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How does the enthalpy change of bond enthalpy relate to enthalpy changes of formation? What's the difference? [duplicate]
The enthalpy change = sum of bond energies broken (reactants) - sum of bond energies made (products).
Then I read
The enthalpy change = sum of the enthalpy change of formation of products - sum of the ...
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In acids why does polarising the O-H bond weaken rather than strengthen it?
In hexaaquaions it is generally true that aqueous solutions of complex ions with a higher positive charge on the central metal ion will have a lower pH. Explanations typically note that the greater ...
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Why is octasulfur present in equation for standard enthalpy of formation of sulfuric acid?
Write a balanced chemical equation corresponding to the standard enthalpy of formation of $\ce{H2SO4}.$
\eqref{rxn:r1} is the given answer, \eqref{rxn:r2} is my answer:
\begin{align}
\ce{1/8 S8 (s) + ...
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What is the most stable cycloalkene? [closed]
I'm looking specifically at cyclopentene and cyclohexene. Supposedly, greater instability would lead to a more exothermic enthalpy of combustion. However, based on my data, cyclopentene is more ...
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Enthalpy Departure
I'm trying to fill in the steps from my textbook on the derivation of enthalpy departure from ideal gas behavior. My textbook gives the variation of mass-specific enthalpy with temperature pressure as
...
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Enthalpy contribution and heat capacity
Imagine having at the input of a burner 105 mol/h of $\ce{O_2}$ at 50 °C. The enthalpy contribution of this input mass is calculated using the Kirchhoff equation
$$\Delta H = \Delta H^° + \int_{25 °C}^...
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What is the difference between heat of reaction (q) and enthalpy of reaction ∆H?
I am confused as I can’t understand the difference between the thermodynamic terms q and ∆H.
In my book,“general chemistry” by Ebbing and Gammon,it is stated that at constant pressure Qp=∆U+P∆V=∆H.
...
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Exothermic temperature rise on NaOH + Methanol
When mixing NaOH (w/w 0.05%) with methanol, there is an increase of temperature around 20 C
I wonder if we keep this ratio in large amount of methanol, will there be any spike in temperature increase ...
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Calculating Enthalpy of formation versus Calculating Enthalpy of a reaction not occurring at standard conditions
My understanding of Hess's law and its use in determining the change of enthalpy of a reaction has been challenged recently.
Up until recently, I thought that the only way to calculate a chemical ...
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Can reverse micelle's exist outside solution?
As in reverse micelle's, the polar liquid remains inside the micelle while the exterior is one of lower dielectric constant; if the liquid were to be carefully removed, would the micelle structure ...
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Why is it that the enthalpy of hydration equals the difference of solvation enthalpies of anhydrous and hydrous salts
Taking an example from my textbook
At finite dilution of CuSO4, heat is liberated
$$\ce{CuSO4(s) -> CuSO4 (aq)}.$$ ΔH of solvation = -15.8 kcal (anhydrous CuSO4 solvation reaction)
and when it is ...
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Combustion Temperature Calculation Using C_p and Heat of Formation
I am simulating the combustion temperature for various compositions of syngas and finding the product temperature. Part one is varying the ratio of CO and H2 from 3:1 to 1:3 in the syngas, burning it ...
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enthalpy of formation of hydronium
Can someone explain how the enthalpy of formation of aqueous hydronium was determined. It appears to be exactly the same as the enthalpy of formation of liquid water. I know this must have something ...
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How to find the temperature for when Kc = 1
The formulas I can use are $\Delta$G = H - T$\Delta$S, and $\Delta$G = $\Delta$G(s.t.d) + RTlnK
I understand that when Kc is 1, the reaction is barely spontaneous and thus I can use the first formula ...
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What is the effect of temperature on the enthalpy of reaction?
In the study of reaction mechanisms, enthalpy of formation for intermediate compounds at various temperatures is studied. I understand the reason to do that.
How much (and what) effect can temperature ...
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Why is the the temperature change due to the combustion of paraffin wax so high (10000 Kelvin)? [closed]
I'm trying to work out the temperature change (I got something like 10k Kelvin,hotter than the Sun's core...) caused by the combustion of paraffin wax which has a molar enthalpy of combustion $E^{wax}...
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Why is my ethanol–water mixture showing negative change in enthalpy of mixing? [duplicate]
I am conducting an experiment where I mix two solutions $\text{A}$ and $\text{B}$ and then I am measuring the temperature change (for real solutions, i.e., $\Delta_\mathrm{mix}H\neq 0).$ I took the ...
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What is an example of an exothermic reaction with negative ∆S?
In high school we learn that when a reaction has both negative $\Delta{}S$ (entropy change) and negative $\Delta{}H$ (enthalpy change) it occurs spontaneously at lower temperatures, but becomes non-...