Questions tagged [enthalpy]
A thermodynamic state function describing the total energy content of a system.
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Heat involved in isenthalpic process
Question 28 from [1, p. 4.4]:
An ideal gas is expanded irreversibly from $\pu{5 L}$ to $\pu{10 L}$ against a constant external pressure of $\pu{1 bar}.$ The value of heat involved $(q)$ in this ...
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3answers
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Is enthalpy of combustion the 'heat released' or 'enthalpy change'?
In a constant volume calorimeter, $\pu{3.5 g}$ of a gas with molar weight $\pu{28 g}$ was burnt in excess $\ce{O2}$ at $\pu{298 K}.$ Temperature of calorimeter increased from $298$ to $\pu{298.45 K}$ ...
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Calculating the enthalpy and entropy [on hold]
I would like to know the answer and comment of this question
Caculate the enthalpy and entropy changes of1mol Diopside as it is heated at 1atm from 600k to 1000K.
Standard state of 298.15K and 0....
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1answer
24 views
Change in enthalpy equation [on hold]
If the formula for change in enthalpy is ΔH=ΔU+pΔV, and ΔU = q - pΔV then shouldn't the formula be simplified to ΔH = q regardless of whether the volume or pressure is constant? Is this only true when ...
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Why are IUPAC's definitions of exo- and endothermic disconnected from the direction of heat flow?
I'm wondering if anyone can provide a rationale for IUPAC's definitions of exothermic and endothermic, since they disconnect these terms from the direction of heat flow.
Specifically, here are IUPAC'...
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1answer
42 views
Enthalpy and heat relationship
They say that enthalpy of reaction is the heat when at constant temperature and pressure. So if enthalpy is
$$\mathrm dH = \mathrm dU + p\,\mathrm dV = T\,\mathrm dS - p\,\mathrm dV + p\,\mathrm dV + ...
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1answer
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Enthalpy change of water [closed]
A given amount of (liquid) water in an open vessel is heated from 20 degree celsius to 50 degree celsius. Suppose it absorbs x joule in the process. What is the enthalpy change in the process? Is it = ...
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Why do alkyl substituents stabilize alkenes?
My textbook states that because $sp^2-sp^3$ bonds are stronger than $sp^3-sp^3$ bonds, alkenes are more stable than alkanes. The rule given is that bonds with more $s$ character are stronger.
However,...
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1answer
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Chemistry 12: Calculating changes in enthalpy [closed]
I was wondering if I could please get some help with this:
In a coffee-cup calorimeter $\pu{100.0 mL}$ of $\pu{1.0 M}\ \ce{NaOH}$ and $\pu{100.0 mL}$ of $\pu{1.0 M}\ \ce{HCl}$ are mixed. Both ...
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57 views
How to find the Cp, difference of Cp and heat of fusion from the dependence of vapor pressure on temperature?
The vapor pressure of zinc varies with temperature as
$$\log P(\pu{mm Hg}) = -6850/T - 0.755\log T + 11.24 \label{eqn:1}\tag{1}$$
and that of liquid zinc as
$$\log P(\pu{mm Hg}) = -6620/...
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Spontaneity of an exothermic chemical reaction
When the process is exothermic (ΔHsystem<0), and the entropy of the system increases (ΔSsystem>0), the sign of ΔGsystem is negative at all temperatures. Thus, the process is always spontaneous.
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Why can't calorimetry be used to measure fast reactions?
As my previous post was a bit unclear and unspecific, I am reformulating it. First of all this is a question that I had while reading some concepts my book introduces, so this isn't a homework ...
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1answer
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I was told that the more exothermic the solution, the more soluble the salt. Is this statement wrong?
I was also told that $$\Delta H(\text{solution}) = \Delta H(\text{hydration}) - \Delta H(\text{lattice E})$$
However, for $\Delta H(\text{solution})$ to be exothermic, it has to have a negative value ...
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Computing enthalpy of formation of dissolved gas from its enthalpy of dissolution
Provided that I know the enthalpy of formation $\Delta H_f^0(X_{g})$ of a substance $X$ in gas phase and its enthalpy of dissolution $\Delta H^0_{sol}(X)$ in the same conditions, considering that the ...
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What is total enthalpy?
I've seen that it's defined as $h_0 = h + \frac{v^2}{2}$, where $h_0$ is the total enthalpy per unit mass. I can see that the $\frac{v^2}{2}$ term probably has something to do with kinetic energy, but ...
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How do you find the heat capacity ratio of combustion products?
For example, if gaseous oxygen $(O_2)$ and paraffin wax $(C_{32}H_{66})$ were to stoichiometrically combust at STP, how would one go about finding the heat capacity ratio ($\gamma={c_p \over c_v}$) of ...
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1answer
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Infinite Increase in Entropy when Energy added to Absolute Zero
My textbook states the following:
If a system were at absolute zero, an additional small amount of heat energy would lead to an infinite increase in entropy. Such a state is impossible. Absolute ...
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1answer
42 views
Facing a problem in enthalpy calculation
If 1 mole of gaseous carbon atoms are converted into diamonds, calculate enthalpy change of process given that bond enthalpy of $\ce{C-C}$ bond is $\pu{400 kJ mol-1}.$
According to me, the answer ...
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1answer
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Calculating bond energy given the enthalpy value of burning reaction
The burning enthalpy (exothermic) of $\ce{NO +O2 ->NO2}$ is $\pu{57 kJ mol^-1}$ and $\ce{O=O}$ bond energy is $\ce{494 kJ mol^{-1}}$, then what is the bond energy of $\ce{N-O}$ ?
So I balanced the ...
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Reaction of sulphur with calcium carbonate
Can sulphur react with calcium carbonate to form sulphur trioxide and calcium carbide, as the energy of 3(S=O) is greater than ((C=O) +2(C-O))?
EDIT: i forgot to say that this is in an oxygen-free ...
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1answer
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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 $\mathrm dT = 0.$
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1answer
36 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}....
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2answers
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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, ...
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1answer
37 views
Question about calculating enthalpy change
$$\Delta H = \left( \begin{array}{c} \text{total enthalpy of}\\ \text{bonds broken}\end{array}\right)-\left( \begin{array}{c} \text{total enthalpy of}\\ \text{bonds made}\end{array}\right)$$
$$\Delta ...
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2answers
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Exothermic: enthalpy or heat?
I am used to thinking of exothermic reactions as being defined as having negative $ΔH.$ However, I also am used to hearing/thinking of exothermic reactions as "releasing heat".
Since change in ...
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What makes determining entropy changes theoretically so hard compared to enthalpy?
It is really hard to determine the entropy change of a reaction accurately because of subtle solvent effects and a lack of standard entropies. On the other hand, enthalpy is relatively easy and ...
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1answer
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Enthalpy of combustion of 1-butanol
1-butanol $\ce{C4H10O}$
So I have absolutely no idea how to find the enthalpy and I did some looking on the internet with no use since I don't understand.
I came here hoping someone generous could ...
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Reaction enthalpy using bond energy in resonating molecules
To find out and enthalpy of a reaction using bond energy we use the formula
$$\Delta H = \sum\Delta_\mathrm{bond}H_\mathrm{reactants} - \sum\Delta_\mathrm{bond}H_\mathrm{products}$$
since we use all ...
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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 ...
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1answer
55 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 ...
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1answer
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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 ...
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1answer
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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 ...
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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 ...
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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 ...
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2answers
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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)}$$
$\Delta H = \Delta E$
$\Delta H > \...
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1answer
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Comparing strength of the C-H, N-H, and O-H bonds
$$
\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 &= \...
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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 ...
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1answer
37 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 ...
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1answer
67 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 ...
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1answer
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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 ...
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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 ...
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1answer
198 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 ...
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1answer
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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{^\...
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1answer
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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).
...
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1answer
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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 ...
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1answer
556 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?
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2answers
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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?
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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 ...
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1answer
197 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 ...
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1answer
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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 ...
