Questions tagged [enthalpy]
A thermodynamic state function describing the total energy content of a system.
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Irregular trend in the first ionisation enthalpy of 3d elements
I was studying my textbook and I stumbled upon this part.
The irregular trend in the first lonisation enthalpy of the 3d metals, though of little chemical significance, can be accounted for by ...
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Where is my non-expansive work in combustion?
Consider if I burn some octane.
The enthalpy released is $5430 \;\text{kJ/mol}$. The Gibbs free energy released is $5537 \;\text{kJ/mol}$.
From my understanding, enthalpy is the heat change in the ...
4
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2answers
166 views
Does entropy contribute work?
I've seen that Gibbs free energy is defined as the maximum amount of work that a system can do. From this I gather, this is how much energy it will release - right?
But, I've also learnt that the heat ...
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1answer
37 views
What's the equilibrium pressure of CO2 gas over sodium hydroxide?
Many (too many) decades ago in physical chemistry lessons, I seem to recall that given the enthalpy changes of reactions, and maybe phase diagrams, one could work out what equilibrium a reaction would ...
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Why is the enthalpy of formation of 2,2-dimethylpentane larger than that of 2,2,3-trimethylbutane?
Firstly, it is commonly known and taught that branched alkanes are more stable than linear ones, and so have larger enthalpies of formation. But why is that the case? Are there some steric ...
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Are spontaneous exothermic reactions spontaneous due to more stable products or to an increase in the entropy of the surroundings? [closed]
The lattice formation of an ionic compound from its infinitely separated gaseous ions is spontaneous under standard conditions, but I cannot figure out the 'driving force' behind this.
One way to look ...
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83 views
How change in potential energy corresponds to change in enthalpy?
So while reading about activation energy , I came across this graph :
Here, change in potential energy of reactant and product is depicted as ${ΔH}$, but at constant pressure and temperature , ...
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2answers
63 views
Why is enthalpy change = heat change only for constant pressure?
Please help me, a mathematician doing his elective course on Physical Chemistry, out with this very simple question: why does the relation between enthalpy and heat $$\Delta H = \Delta U + p\,\Delta V ...
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What is the difference between ΔH = ΔU +Δ(PV) and ΔU = q + w? [closed]
Here, ΔH = Enthalpy change, ΔU = Change in internal energy, P = pressure and V = volume.
I know that ΔH(Enthalpy) is heat given/taken to/from system at constant pressure. But I have been practicing ...
2
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1answer
54 views
Enthalpy in the van't Hoff equation
In the van't Hoff equation, why do we say that the change in enthalpy at standard pressure is constant? My book derives the equation for a mixture of ideal gases reacting with each other. The proof ...
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106 views
Relation between enthalpy and internal energy for chemical reaction at constant volume
Question
The combustion of benzene(l) gives $\ce{CO2(g)}$ and $\ce{H2O(l)}.$ Given that the heat of combustion of benzene at constant volume is $\pu{-3263.9 kJ mol-1}$ at $\pu{25 ^\circ C}$; heat of ...
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Is it the solution energy or the reduction potentials (or both) that drives a galvanic cell?
I have received conflicting explanations for why the redox reaction in a galvanic cell occurs.
Explanation 1:
Some say that it occurs because the anodic metal dissolves more easily than the cathodic ...
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17 views
Are there any increment systems for guessing the heat of formation for organic compounds that have C, H, O in them?
In the link, you can find two methods of approximation which can help one calculate the heat of formation for Alkanes. I do realize that with the addition of O, the position of the functional group ...
4
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1answer
222 views
Why is delta-H negative here?
Very basic question here, but I'm confused why Delta-H seems to be the reverse of what I would expect for bond enthalpy. For example, here's a problem in my textbook where the goal is to find delta-H:
...
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3answers
345 views
Does the entropy of a chemical system increase in an endothermic reaction?
Since entropy increases when temperature increases, the entropy of surroundings increases during an exothermic reaction. When I apply the same logic, it seems like the entropy of a chemical system ...
2
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35 views
Is it possible to measure the entropy of a chemical reaction through calorimetry?
My broad question is how do you measure entropy change? I was doing a bit of digging into how thermodynamic tables are developed at I got this from NIST. Basically it seems like you need to indirectly ...
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Cooling down from −78 °C to −88 °C without cryogenics
Say I have a gas cylinder with nitrous oxide and cool it down to −78 °C with dry ice. Is there an endothermic process/reaction that would extract the remaining heat from $\ce{N2O}$ and bring it down ...
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1answer
73 views
How to find enthalpy of solvation for gas phase cation using Hess's law?
Find enthalpy of solvation $\Delta_\mathrm{solv}H$ using Hess's law, and show the expression that allows to calculate the enthalpy of solvation.
$$\ce{Ca^2+(g) -> Ca2+(aq)}$$
I am not told which ...
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1answer
68 views
How can change in Enthalpy for a reaction be written like this?
If a gas expands in a container with constant external pressure then:
$W=-P_{ext}\Delta V$
Instead of $P_{ext}$ if we use $P_{int}$ then we will not get the same value because $P_{int}$ is constantly ...
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1answer
119 views
Reactions with a positive Delta H and a negative Delta S are said to be nonspontaneous at ALL temperatures. But what if the temperature IS low enough? [closed]
What I mean is, that if you use G = H-TS and make the temperature low enough, couldn't you make any reaction spontaneous? For example, you have a Delta H of 85 kJ and a Delta S of -66 J/K. I've been ...
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108 views
Enthalpy of dissolution of NaOH in small amount of water
is there a way to calculate enthalpy of dissolution of NaOH in small amount of water?
I know that enthalpy of dissolution of sodium hydroxide in water is –44,5 kJ/mol. However it assumes the orders of ...
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175 views
What would be the physical interpretation of the equation dG = Vdp − SdT?
I was trying to understand the derivation of the formula $G=G^\circ+RT\ln (p/p^\circ)$ starting with the following relationships:
$$
\begin{align}
\mathrm dU&=T\,\mathrm dS-p\,\mathrm dV\tag{1}\\
\...
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1answer
66 views
Meaning of the symbol “$\Delta_\mathrm{sub}H^\circ[\ce{I2,s}]$”
One of the back exercises (13(C)) from the book for practice provided at my school asked whether the following relation is true:
$\Delta_\mathrm{f}H^\circ(\ce{I2,g}) = \Delta_\mathrm{sub}H^\circ[\ce{...
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65 views
Enthalpy, heat of solution test: Calcium chloride vs magnesium chloride in water
I did a school experiment to measure the heat of solution (enthalpy) of various salts dissolved in water.
My results are completely different to the teacher's answer.
Specifically my test showed ...
3
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79 views
How to find the equilibrium constant under non-standard conditions?
Given the following thermodynamic data at $\pu{298 K}:$
$$
\begin{array}{lrrr}
\hline
\text{Compound} & \Delta_\mathrm{f}H^\circ/\pu{kJ mol^-1} & S^\circ/\pu{J K^-1 mol^-1} & C_p^\circ/\pu{...
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Calculating Enthalpy
I’m typing this from my phone...sorry if the quality is sub par.
I have a very involved spreadsheet from a colleague of mine that was put together in 1994. The spreadsheet calculates the excess air ...
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1answer
65 views
Does the enthalpy of formation include translational and rotational energy?
I can look up the enthalpy of formation of water vapor and get -241,826 KJ/mol. I want to use this to compute the total energy of water vapor at STP from the energies of O2 and H2 gas. What I don't ...
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91 views
Why is the bond energy for a C=O bond higher in CO2?
I was looking at the table of bond energies, and what caught my eye was how the bond energy for a $\ce{C=O}$ bond is $\pu{736 kJ/mol}$ with the exception of the $\ce{C=O}$ bonds in $\ce{CO2}$ being $\...
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Why is the standard enthalpy of formation of aluminium oxide higher than magnesium oxide?
$$\begin{align}
&\text{Aluminium oxide:} &\ce{4Al + 3O_2 &-> 2Al2O3} &(\Delta H_\mathrm f &= \pu{-1675 kJ/mol})\\[1.5em]
&\text{Magnesium oxide:} &\ce{2Mg + O_2 &-&...
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431 views
Change in enthalpy and entropy when sugar dissolves in water
We know that dissolution of sugar in water is a spontaneous process.
So, change in Gibbs free energy $(ΔG)$ must be negative for the overall process:
$$ΔG = ΔH - TΔS < 0$$
Hence either the enthalpy ...
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How do I go about calculating the enthalpy of a solution with an unknown salt?
I have the $\Delta T$, I have the specific heat of the calorimeter, I have known weights/mass. I know the salt is lithium-based. What steps do I take to calculate the enthalpy of the solution so I can ...
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1answer
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Why constant volume combustion gives higher energy than constant pressure?
I have noticed when I have done some combustion equilibrium that enthalpy is higher when doing a constant volume combustion than pressure constant combustion.
Why is it so ? What are the reasons ...
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1answer
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Does the enthalpy of solution formula (LE + Hyd) change depending on the question?
Hey guys. I have been working on enthalpies and literally every video and website I’ve seen,
the formula for calculating enthalpy of solution = lattice enthalpy of dissociation + enthalpy of hydration,...
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136 views
Why does Delta H change when the coefficients change in a reaction?
So I understand that if twice as much of the reactants are present, then twice as much energy is released. But isn’t the energy released per mole of reactant still the same? You are just scaling up ...
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2answers
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Change in Enthalpy in Free Expansion of Ideal and Real gas against Vacuum
By free expansion, I am referring to gas allowed to expand freely against vacuum in a Joule Expansion.
If gas is ideal then change in Internal Energy '∆U' and change in Enthalpy '∆H' is zero.
(By ∆H = ...
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Calorimetry Question - determine temperature change without knowing enthalpy change?
I have a lab that I honestly cannot figure out.
Basically, we have a video where we watch 200 ml of NaOH (2M) and HCl (1M) combined in a calorimeter with a magnetic mixer. Pretty standard. This video ...
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Verify heat produced by flame using enthalpy of fomation
I'm simulating using a CFD solver a pure oxygen/methane flame. However, I doubt that the combustion model does not produce enough heat. The heat from reaction is defined by $S_h=-\sum_{j} \frac{h_j^0}{...
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1answer
72 views
Do most chemical reactions happen at constant pressure?
While studying enthalpy $H=U+PV$ and its changes, I realized I am not clear on the following: chemical reactions happen with the external pressure being constant and equal to the atmospheric pressure $...
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1answer
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Definition of enthalpy of vaporization
The amount of heat required to vapourize one mole of a liquid at
constant temperature and under standard pressure (1 bar) is called its
standard enthalpy of vapourization.
The enthalpy of vaporization ...
6
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1answer
513 views
What is the bond energy of H-O? Wikipedia disagrees with itself
What is the bond energy of H-O? Wikipedia disagrees with itself:
On English Wikipedia:
467 kJ/mol
On French Wikipedia:
366 kJ/mol
What is the right value?
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1answer
56 views
A lot of confusion in 1st law of thermodynamics [closed]
My sir told me that Total energy of system = K.E + P.E in starting.
Then Change in energy = F(External force on body ) * displacement of walls.
Then from here , change in energy = W+q.
(Don’t ...
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1answer
170 views
Why does enthalpy change differ in a chemical reaction because of temperature?
If enthalpy is a state function, shouldn't the change in enthalpy be the same between a given initial and final state, regardless of how the change took place? Kirchoff's equation describes this ...
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Why is dilution exothermic?
The breaking of bonds usually absorbs energy and forming bonds usually releases energy.
Then, why is the dilution of acids and bases exothermic?
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Why can the standard enthalpy of formation be calculated for different states of matter when the temperature should be constant?
If the standard enthalpy of formation needs to be determined at $25ºC$ and 1 atm of pressure then why is it possible for the enthalpy of formation to be calculated for the liquid, solid, and/or gas ...
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Why does the C−H bond dissociation energy vary in a homologous series of primary alcohols?
Specifically, for the primary carbon atom in the alcohol. Here is bond dissociation energy (BDE) data from chapter three of Luo's Comprehensive handbook of chemical bond energies [1] (boldface refers ...
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1answer
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Question to find increment in total energy of a sample of He gas? [closed]
If a closed rigid wall container filled with 20 gram of Helium gas moving at a constant speed of 10 m/sec is stopped abruptly to stand still, find the increment in temperature of this He sample.
Way I ...
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1answer
107 views
Why is enthalpy change at constant pressure not only internal energy change? [closed]
Okay so this might seem a bit Weird but the formula for change in enthalpy has been given the following way-
∆H=∆U+∆PV
so in this case when we say ∆V is zero we get that ∆PV is also zero and thus ∆H=∆...
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1answer
106 views
What makes the enthalpy of hydration of calcium chloride dihydrate unique? [closed]
I have recently done an experiment in an attempt to find the relationship between the water ratio in a hydrate and the enthalpy of hydration, and my results showed a correlation, but an anomaly for ...
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1answer
33 views
Why is the change in enthalpy computation for a change of state done like this?
Suppose that a system whose initial state is Pressure = $P_1$, Volume = $V_1$; and it is taken through a process after which final pressure and volume are $P_2$ and $V_2$ respectively. Now change in ...
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1answer
60 views
What is the mechanism of endothermic dissolution?
My real question is in fact "How can dissolution happen if it's endothermic?" However, there
have been many questions regarding this exact topic and the answers about Gibbs free energy
...
