Questions tagged [enthalpy]
A thermodynamic state function describing the total energy content of a system.
356
questions
1
vote
1answer
58 views
Why enthalpy changes in phase transition when there is no change of temperature?
Why there is a change in enthalpy in phase transition?
I'm asking because enthalpy depends only on temperature:
$$\mathrm dH = C_p\,\mathrm dT,$$
and the change in temperature is $\mathrm dT = 0.$
0
votes
0answers
23 views
Writing the formation enthalpy of ammonium
Is it correct to write the equation for the formation enthalpy of $\ce{NH4+}$ as follows
$$\ce{NH3(g) + H+(g) -> NH4+(g)}$$
or should it be
$$\ce{1/2 N2(g) + 2 H2(g) -> NH4+(g)}$$
0
votes
1answer
23 views
Calculating bond dissociation enthalpy of F-F bond
$$\ce{N2 + 3 F2 -> 2 NF3} \quad ΔH = \pu{-263 kJ mol-1}$$
The bond dissociation enthalpies of the nitrogen triple bond and nitrogen–flourine bond are $\pu{946 kJ mol-1}$ and $\pu{272 kJ mol-1}....
2
votes
2answers
43 views
I can't find reaction enthalpy ∆H for water formation reaction
$$\ce{1/2 O2(g) + H2(g) → H2O (l)}$$
The tabulated value is $\Delta H = \pu{−285.8 kJ/mol}$
I'm using the bond energies values for each molecule and for some reason, I am getting a different result, ...
1
vote
1answer
32 views
Question about calculating enthalpy change
$$\Delta H = \left( \begin{array}{c} \text{total enthalpy of}\\ \text{bonds broken}\end{array}\right)-\left( \begin{array}{c} \text{total enthalpy of}\\ \text{bonds made}\end{array}\right)$$
$$\Delta ...
-2
votes
0answers
46 views
Enthalpy of an ideal gas undergoing change in pressure and temperature
A 1.50 mol sample of an ideal gas at 245 K and 0.75 bar pressure is altered by some process that takes it to a state in which its temperature is increased to 295 K and its pressure is increased to 0....
4
votes
2answers
77 views
Exothermic: enthalpy or heat?
I am used to thinking of exothermic reactions as being defined as having negative $ΔH.$ However, I also am used to hearing/thinking of exothermic reactions as "releasing heat".
Since change in ...
-1
votes
0answers
32 views
Standard Free Energy of Formation and Reaction Coefficients [duplicate]
Consider the following reaction:
$$\ce{N2 + 3H2 \to 2NH3}$$
I want to calculate the enthalpy involved in the reaction.
I have been given the standard free energy of formation of $\ce{NH3}$, $\ce{N2}...
1
vote
0answers
28 views
What makes determining entropy changes theoretically so hard compared to enthalpy?
It is really hard to determine the entropy change of a reaction accurately because of subtle solvent effects and a lack of standard entropies. On the other hand, enthalpy is relatively easy and ...
0
votes
1answer
42 views
Enthalpy of combustion of 1-butanol
1-butanol $\ce{C4H10O}$
So I have absolutely no idea how to find the enthalpy and I did some looking on the internet with no use since I don't understand.
I came here hoping someone generous could ...
1
vote
0answers
14 views
Reaction enthalpy using bond energy in resonating molecules
To find out and enthalpy of a reaction using bond energy we use the formula
$$\Delta H = \sum\Delta_\mathrm{bond}H_\mathrm{reactants} - \sum\Delta_\mathrm{bond}H_\mathrm{products}$$
since we use all ...
4
votes
0answers
10 views
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 ...
2
votes
1answer
37 views
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 ...
2
votes
1answer
79 views
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 ...
3
votes
1answer
52 views
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 ...
0
votes
0answers
20 views
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 ...
0
votes
0answers
13 views
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 ...
1
vote
2answers
62 views
Relation between ΔH and ΔE for the reaction in gaseous phase at constant temperature and pressure
Which of the following statements is correct for the reaction at constant temperature and pressure:
$$\ce{CO(g) + \frac{1}{2}O_2(g) -> CO2(g)}$$
$\Delta H = \Delta E$
$\Delta H > \...
0
votes
1answer
65 views
Comparing strength of the C-H, N-H, and O-H bonds
$$
\begin{align}
\ce{CH4(g) + Cl(g) &-> CH3(g) + HCl(g)} &\quad ΔH^\circ &= \pu{-14 kJ mol-1}_\mathrm{rxn}\\
\ce{NH4(g) + Cl(g) &-> NH2(g) + HCl(g)} &\quad ΔH^\circ &= \...
0
votes
0answers
44 views
Trying to understand rationale behind Joule-Thomson experiment
The purpose of the original Joule (as opposed to JT) expansion experiment was to assess intermolecular interactions in real gases. In that experiment, Joule immersed twin glass globes into a water ...
1
vote
1answer
36 views
Change in heat of photosynthesis
Consider a plant that uses the energy it receives from the sun to do work (to undergo photosynthesis) to produce glucose. The minimum amount of work this plant can do to produce 1 mole of glucose is ...
1
vote
1answer
35 views
Heat and work of glucose combustion
$$\ce{C6H12O6(s) + 6 O2(g) -> 6 CO2(g) + 6 H2O(g)}$$
When looking at glucose combustion I found that the change in Gibbs free energy is greater than the change in enthalpy. If we assume the process ...
0
votes
1answer
38 views
Is the enthalpy of a molecule determined by the energy of the electrons? [closed]
I understand that the strength of the bond of a molecule is determined by the potential and kinetic energy of the electrons. I also understand that a reaction where the bond strength of the products ...
0
votes
0answers
20 views
why is white phosphorous given to have ZERO standard heat of formation [duplicate]
why is white phosphorous given to have ZERO standard heat of formation??
while it is well known fact that white phosphorous is highly reactive and is obviously less stable than red and black ...
3
votes
1answer
194 views
What is meant by electrons moving to lower potential energy?
I am currently learning about chemical bonds in chemistry, and came across somebody who wrote this (source):
"Chemical bonds certainly "contain" potential energy, and the atoms want to move to a ...
4
votes
1answer
157 views
Would the change in enthalpy (ΔH) for the dissolution of urea in water be positive or negative?
To test the properties of a fertilizer, $15.0\ \mathrm g$ of urea, $\ce{NH2CONH2(s)}$, is dissolved in $150\ \mathrm{mL}$ of water in a simple calorimeter. A temperature change from $20.6\ \mathrm{^\...
1
vote
1answer
64 views
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).
...
2
votes
1answer
61 views
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 ...
1
vote
1answer
201 views
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?
2
votes
2answers
56 views
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?
0
votes
2answers
61 views
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 ...
1
vote
1answer
64 views
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 ...
0
votes
1answer
33 views
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 ...
-2
votes
1answer
61 views
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 ...
1
vote
0answers
27 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 ...
3
votes
2answers
110 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 $\Delta U > 0$, ...
1
vote
1answer
22 views
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 ...
1
vote
1answer
49 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. ...
0
votes
1answer
101 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 ...
8
votes
2answers
237 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 ...
0
votes
0answers
22 views
Is there a website or book that contains accepted values for the enthalpy change of the reactions between acids and bases?
I am looking for a website/book/journal/etc. which contains the accepted values for the enthalpy change of the follwoing reactions:
Hydrochloric acid with potassium hydroxide
Hydrochloric acid with ...
0
votes
1answer
61 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?
-3
votes
1answer
206 views
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 ...
2
votes
1answer
130 views
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 ...
0
votes
1answer
55 views
Using Hess's law to find molar enthalpy of formation without an initial reaction [closed]
I'm having issues relating Hess's law to this question:
The given chemical equation represent the combustion of ammonia and the combustion of hydrogen
$$
\begin{align}
\ce{4 NH3 + 3 O2 &-&...
2
votes
1answer
48 views
How is “the surrounding” and its temperature defined to calculate entropy change during a reaction?
Lets say supercooled liquid water at $263\ \mathrm K$ isobarically changes to solid ice at the same temperature. I wish to calculate the change in entropy of the surroundings and I happen to know the $...
1
vote
2answers
94 views
Why enthalpy is defined at a constant pressure?
How absolute enthalpy and enthalpy change are defined? What is the clear difference between enthalpy and heat? Is it the constancy of pressure what makes the enthalpy a state function?
My teacher ...
11
votes
1answer
516 views
Explanation of the strength of phosphorus-oxygen bond
When discussing the Wittig reaction, Clayden's Organic Chemistry cites the strength of the $\ce{P=O}$ formed in triphenylphosphine oxide as a driver of the reaction through enthalpy:
The $\ce{P=O}$ ...
1
vote
1answer
48 views
Heat of formation method
For enthalpy calculations why is it that in some cases the "heat of formation method" is used? I know that $ΔH_f$ represents the heat required to form the compound at standard temperature and pressure ...
-1
votes
1answer
86 views
Confusion in change in enthalpy relation
In the equation
$$\Delta H = \Delta U + \Delta PV,$$
under what circumstances can we simplify it into
$$\Delta H = \Delta U + P\,\Delta V + V\,\Delta P$$
by product rule?
In this article (pdf ...
