Questions tagged [enthalpy]
A thermodynamic state function describing the total energy content of a system.
388
questions
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How do you determine whether is it [products] - [reactants] or [reactants] - [products] when calculating the enthalpy change of a reaction?
In the reaction above, knowing the enthalpy of formation of each individual compound, how do I determine whether it is:
[products] - [reactants] or
[reactants] - [products]
To calculate the standard ...
0
votes
2answers
38 views
Which of the following reaction will have greater enthalpy and why? [closed]
Which of the following reaction will have greater enthalpy and why?
$\ce{C (g) + 4 H (g) -> CH4(g)}$ (reaction enthalpy = $x\ \mathrm{kJ/mol}$)
$\ce{C (graphite) + 2H2 (g) -> CH4 (g)}$ ...
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3answers
79 views
Need help understanding Enthalpy
My chemistry textbook says that:
Enthalpy (Internal Energy) = chemical potential energy + thermal energy
Now, I know that Enthalpy and Internal Energy are different so I really don't understand ...
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votes
1answer
30 views
how to compute enthalpy of combustion for methanoic acid [closed]
My data source gives the following values for various enthalpies of formation and combustion:
formation of CO2 = -393.5
formation of HCOOH = -425
combustion of HCOOH = -254.3
Wikipedia gives ...
1
vote
0answers
39 views
Enthalpy of reaction between ammonia and oxygen
I have to ask this here because I asked literally everybody I know and I still disagree with them.
Using the information in the table below, calculate the heat of released from the following ...
0
votes
0answers
10 views
Enthalpy of formation for Lithium ion in ethylene carbonate
The following link has the enthalpy of formation for aqueous lithium http://nshs-science.net/chemistry/common/pdf/R-standard_enthalpy_of_formation.pdf
I am wondering how to get the enthalpy of ...
3
votes
1answer
22 views
How do you find the bond enthalpy from an equation?
Which equation represents the $\ce{N-H}$ bond enthalpy in $\ce{NH3}?$
$$
\begin{align}
&\textbf{A.} &\ce{NH3(g) &-> N(g) + 3 H(g)}\\
&\textbf{B.} &\ce{1/3 NH3(g) &-> ...
1
vote
1answer
38 views
Ground state elevation for phase transition
I have a question about the illustration from chem.libretexts.org:
the ground states of the liquid and gas are offset from that of the previous state by the heats of fusion and vaporization, ...
0
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1answer
32 views
Is the heat for constant pressure the same as for constant volume in the formation of nitric oxide?
In the reaction $\ce{N2 (g) + O2(g) -> 2NO (g)}$
my textbook[1] says $q_v$ and $q_p$ are not the same, without any explanation.
I know that
\begin{align}
\Delta U &= q_p + w\\
\Delta U &=...
0
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0answers
24 views
How to calculate the temperature change when CaCl2, NaCl, and water added together at an initial temperature of 298 K?
My question: How to calculate the temperature change when adding $\pu{0.1 kg}$ of $\ce{CaCl2}$ and $\pu{0.1 kg}$ of $\ce{NaCl}$ to $\pu{1 kg}$ water with an initial temperature of $\pu{298 K}$?
I ...
2
votes
1answer
31 views
Calculating ΔH with non-integer coefficients in the balanced equation
I am working a practice problem out Ebbing's General Chemistry and for part of the problem I needed to calculate $ΔH.$ The problem considers the burning of ethane in oxygen to yield carbon dioxide and ...
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1answer
24 views
Enthalpy Changes [closed]
0.50 g of Mg is added to a 50 mL HCl solution. The temperature increased by 5.2 degree Celsius. Calculate the enthalpy change for the reaction per mole of Mg that reacted.
1
vote
1answer
43 views
What does higher electron gain enthalpy mean here?
While talking about ionic bonds my book states :
Higher the value of electron gain enthalpy of the atom, greater the ease of formation of the anion from it, i.e., other atom should have high value ...
1
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0answers
25 views
How to get the enthalpy of a reaction from the enthalpy of formations with Hess' law?
Let's consider a generic reaction:
$$\sum_{i=0}^{i=I}n_{i}A_{i} \rightarrow \sum_{j=0}^{j=J}m_{j}B_{j} $$
where $n$ and $m$ are the
stoichiometric coefficients, while $A$ and $B$ are reagents and ...
1
vote
0answers
35 views
Thermochemical data for thiocyanatoiron(III) ion
I am currently writing a lab report about the formation constant of $\ce{FeSCN^2+}.$ All I could find online was a formation constant at $\pu{25 °C}$ $(890),$ which was 70% higher than the ...
2
votes
2answers
99 views
Why when deriving the Clapeyron equation can you assume that dH = dq, as surely the pressure is not constant?
Part of the Clapeyron equation involves replacing $\Delta S$ with $\Delta H/T$. I can see the step from $\Delta S$ to $\Delta q/T$ but I'm not sure why you can replace $\Delta q$ with $\Delta H$. From ...
0
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0answers
52 views
Is it possible to use the laws of termodynamics for non-ideal systems?
For the reaction
$$\ce{2CO(g) + O2(g) -> 2CO2(g)}\quad\Delta H = \pu{−560 kJ}$$
it is given that the reaction occurs at constant volume of $\pu{1L}$ and the initial and final pressures are $\pu{...
6
votes
1answer
165 views
Heat involved in isenthalpic process
Question 28 from [1, p. 4.4]:
An ideal gas is expanded irreversibly from $\pu{5 L}$ to $\pu{10 L}$ against a constant external pressure of $\pu{1 bar}.$ The value of heat involved $(q)$ in this ...
2
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3answers
85 views
Is enthalpy of combustion the 'heat released' or 'enthalpy change'?
In a constant volume calorimeter, $\pu{3.5 g}$ of a gas with molar weight $\pu{28 g}$ was burnt in excess $\ce{O2}$ at $\pu{298 K}.$ Temperature of calorimeter increased from $298$ to $\pu{298.45 K}$ ...
0
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1answer
28 views
Change in enthalpy equation [closed]
If the formula for change in enthalpy is ΔH=ΔU+pΔV, and ΔU = q - pΔV then shouldn't the formula be simplified to ΔH = q regardless of whether the volume or pressure is constant? Is this only true when ...
7
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108 views
Why are IUPAC's definitions of exo- and endothermic disconnected from the direction of heat flow?
I'm wondering if anyone can provide a rationale for IUPAC's definitions of exothermic and endothermic, since they disconnect these terms from the direction of heat flow.
Specifically, here are IUPAC'...
0
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1answer
62 views
Enthalpy and heat relationship
They say that enthalpy of reaction is the heat when at constant temperature and pressure. So if enthalpy is
$$\mathrm dH = \mathrm dU + p\,\mathrm dV = T\,\mathrm dS - p\,\mathrm dV + p\,\mathrm dV + ...
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1answer
50 views
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 = ...
2
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0answers
91 views
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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votes
1answer
51 views
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 ...
1
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1answer
97 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
$$\log P(\pu{mm Hg}) = -6620/...
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vote
2answers
41 views
Spontaneity of an exothermic chemical reaction
When the process is exothermic (ΔHsystem<0), and the entropy of the system increases (ΔSsystem>0), the sign of ΔGsystem is negative at all temperatures. Thus, the process is always spontaneous.
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Why can't calorimetry be used to measure fast reactions?
As my previous post was a bit unclear and unspecific, I am reformulating it. First of all this is a question that I had while reading some concepts my book introduces, so this isn't a homework ...
0
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1answer
66 views
I was told that the more exothermic the solution, the more soluble the salt. Is this statement wrong?
I was also told that $$\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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0answers
20 views
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 ...
2
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0answers
92 views
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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0answers
31 views
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 ...
4
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1answer
188 views
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 ...
3
votes
1answer
48 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 ...
0
votes
1answer
58 views
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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1answer
215 views
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 ...
1
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1answer
172 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,$$
and the change in temperature is $\mathrm dT = 0.$
0
votes
1answer
142 views
Calculating bond dissociation enthalpy of F-F bond
$$\ce{N2 + 3 F2 -> 2 NF3} \quad ΔH = \pu{-263 kJ mol-1}$$
The bond dissociation enthalpies of the nitrogen triple bond and nitrogen–flourine bond are $\pu{946 kJ mol-1}$ and $\pu{272 kJ mol-1}....
2
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2answers
58 views
I can't find reaction enthalpy ∆H for water formation reaction
$$\ce{1/2 O2(g) + H2(g) → H2O (l)}$$
The tabulated value is $\Delta H = \pu{−285.8 kJ/mol}$
I'm using the bond energies values for each molecule and for some reason, I am getting a different result, ...
1
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1answer
45 views
Question about calculating enthalpy change
$$\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 ...
4
votes
2answers
110 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 ...
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0answers
32 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 ...
1
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1answer
379 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 ...
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0answers
23 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 ...
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0answers
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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 ...
2
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1answer
85 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
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1answer
87 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
763 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 ...
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0answers
22 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
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0answers
23 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 ...
