Questions tagged [enthalpy]
A thermodynamic state function describing the total energy content of a system.
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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 $\Delta U > 0$, ...
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1answer
19 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 ...
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1answer
30 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. ...
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1answer
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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 ...
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205 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 ...
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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 ...
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1answer
42 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?
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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 ...
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1answer
75 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 ...
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1answer
32 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
$$
\begin{align}
\ce{4 NH3 + 3 O2 &-&...
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1answer
41 views
How is “the surrounding” and its temperature defined to calculate entropy change during a reaction?
Lets say supercooled liquid water at $263\ \mathrm K$ isobarically changes to solid ice at the same temperature. I wish to calculate the change in entropy of the surroundings and I happen to know the $...
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2answers
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Why enthalpy is defined at a constant pressure?
How absolute enthalpy and enthalpy change are defined? What is the clear difference between enthalpy and heat? Is it the constancy of pressure what makes the enthalpy a state function?
My teacher ...
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1answer
162 views
Explanation of the strength of phosphorus-oxygen bond
When discussing the Wittig reaction, Clayden's Organic Chemistry cites the strength of the $\ce{P=O}$ formed in triphenylphosphine oxide as a driver of the reaction through enthalpy:
The $\ce{P=O}$ ...
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1answer
38 views
Heat of formation method
For enthalpy calculations why is it that in some cases the "heat of formation method" is used? I know that $ΔH_f$ represents the heat required to form the compound at standard temperature and pressure ...
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1answer
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Confusion in change in enthalpy relation
In the equation
$$\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 ...
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1answer
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Calculating enthalpy change for a real gas
I know this question would be closed citing it to be a homework question. I posted it earlier also. But believe me I have tried it for myself before and that too many times but then couldn't arrive at ...
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1answer
39 views
How do you calculate the delta H for multiple reactions?
I am having difficulty in a question:
Given that
1. $\ce{N_2(g) \rightarrow 2N(g), \Delta H = 941 kJ/mol}$
2. $\ce{N_2(g) \rightarrow N_2^+ + e^-, \Delta H = 1501 kJ/mol}$
3. $\ce{N(...
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2answers
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When to use internal energy and enthalpy in balances
I am confused as to when you use ΔU and when to use ΔH, I don’t really understand the difference. Is it that ΔU is used for closed systems whilst ΔH is used for open systems?
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Experimental measurement on the ring-strain
for my thesis i calculated the ring-strain of a molecule by MP2 on the computer. I was wondering if there is an easy possibility to get the experimental value of the ring strain. I guess I have to ...
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1answer
113 views
Using thermodynamics to predict the acid-base character of fluoride ion in water
I was working through the end-of-chapter exercises of the acid-base chapter in Shriver's Inorganic Chemistry when I came across the following problem:
4.7. The effective proton affinity $\ce{A^{'}_{...
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1answer
74 views
What is the molar enthalpy? [closed]
In a practice test there's the question, 'Given that the combustion of.305g of glucose causes a raise of temperature of 6.30C of a caloromiter with total heat capacity C=755J/C and the molecular ...
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1answer
59 views
How can there be an enthalpy of formation for gaseous water?
Under standard conditions( 1 bar and 298.15 K), water will be in the liquid state. So how can you find the enthalpy of formation for gaseous water at standard conditions(as found in the back of all ...
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1answer
124 views
Why do we use pV term independent of U in the equation H = U + pV? [duplicate]
I have read the following definition of enthalpy in my textbook:
A substance has to occupy some space in its surroundings depending upon its volume ($V$). It does against the compressing influence ...
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1answer
67 views
thermochemistry problem confusion
I can understand upto where the ideal gas equation is used to calculate volume of steam. But on trying to find out $ \Delta V$ , i saw that there are no gases on the reactant side so then $P\Delta V=(\...
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1answer
136 views
Insoluble salts and Gibbs free energy
Is it true that according to gibbs free energy equation, insoluble salts such as AgCl don't dissolve because the process would be too endothermic? The right side of the equation, TΔS, would have to be ...
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1answer
293 views
How to calculate the enthalpy of reaction using Gaussian 09?
I'm having a hard time trying to understand what's the correct way to calculate the enthalpy of reaction given that I've already calculated the frequencies of my compounds in Gaussian. Below I show ...
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0answers
109 views
How to calculate the enthalpy of fusion at a different temperature?
The heat capacity of liquid iodine is $80.7$ $\ce{J*K^{-1}*mol^{-1}}$, and its enthalpy of vaporization is $41.96$ kJ $\ce{*mol^{-1}}$ at its boiling point ($184.3$ $\ce{^oC}$). Furthermore, the ...
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2answers
976 views
Why is creating water exothermic?
I have to calculate the bond dissociation energy of steam. I'm a total noob, so don't go too hard on me.
(I'm translating the exercise by myself, sorry if there is a mistake.)
Calculate the bond ...
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0answers
452 views
Enthalpy of neutralization of strong acid and strong base differing from enthalpy of formation of water from ions
If the neutralization between strong acid and bases has enthalpy of neutralization of around $\pu{-57.1 kJ mol^-1}$, why is it that when I try to calculate the enthalpy of formation of water from $\ce{...
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0answers
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What factors contribute in making a solution more endothermic?
For example, a solution between potassium nitrate and water is more endothermic than sodium nitrate in water. Can someone explain?
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1answer
41 views
Oxidation of glucose
By oxidation of 9 g of glucose it is released 150 kJ of heat. Calculate the standard enthalpy of derivation of glucose (in kJ/mol), if the given enthalpies of the reactants are ΔfH (H2O(l)) = -285,8 ...
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Why are negative enthalpies becoming more positive described as decreasing?
This website and my lecturer have describe negative enthalpies that are becoming less exothermic as decreasing. A relevant diagram from the linked to website to illustrate what I mean:
I see the ...
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1answer
43 views
Where can I find the total enthalpy and heat of dissolution in water of a given components?
I don't really have a background in chemistry but I'm actually studying in aerospace engineering, sorry if this question may seem trivial.
I am interested in the decomposition process of the hydrogen ...
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1answer
37 views
How can change in Gibbs free energy ever be positive?
I have a question regarding this formula for Gibbs free energy of a system: $\Delta$G = $\Delta$H - T$\Delta$S.
According to the second law of thermodynamics: T$\Delta$S $\ge$ $\Delta$Q and to my ...
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1answer
43 views
Predicting the temperature of an object using the heat of its surroundings after a certain amount of time?
how would you predict what the temperature of something would be if it spends a certain amount of time exposed to heat from its surroundings? is this possible?
ex. what temperature would a hot ...
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0answers
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Why is ΔH (enthalpy)=Σreactants-Σproducts and not Σproducts-Σreactants? [duplicate]
I was told that ΔH=Σreactants-Σproducts in regards to finding the enthalpy, given average bond enthalpies.
Why is the case? It seems counterintuitive, because of the delta, which would imply ΔH=...
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1answer
163 views
Finding Gibbs energy at different temperatures
How can I find $∆G$ at a higher/lower temperature if I am given $∆G°$ and $∆H°$ $\pu{298 K}$, and a reaction equation (solubility)?
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1answer
604 views
Calculating the enthalpy of polymerisation of ethylene given the bond strengths
Given the average bond dissociation enthalpies of a $\ce{C-C}$ bond (say $x$) and a $\ce{C=C}$ bond (say $y$), find the enthalpy of the following polymerisation reaction (per mole of ethylene):
$$...
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Anomalous trend in formation enthalpy of alkali metal halides
I read that in alkali metal halides, the formation enthalpy for fluorides become less negative as we go down the group, whereas the reverse is true for chlorides, bromides and iodides.
Why is it so? ...
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1answer
157 views
The sign of enthalpy of formation of magnesium oxide
I'm currently doing a lab to calculate the enthalpy of formation for $\ce{MgO}$. However at the moment me and my lab partner are having a disagreement. We've both calculated and agreed upon the same ...
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1answer
156 views
Would ionic compounds dissolve in methanol?
Question:
Determine whether liquid hexane ($\ce{C6H14}$) or liquid methanol ($\ce{CH3OH}$) is the more appropriate solvent for the substances grease ($\ce{C20H42}$) and potassium iodide ($\ce{KI}$)....
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What really are the units of enthalpy change? [duplicate]
A reasonable definition of standard enthalpy change of formation would be:
'The enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions'
...
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2answers
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Is Kirchoffs law valid at all pressures?
$$\Large H_{T_\mathrm{f}, p}=H_{T_\mathrm{i},p}+\int_{T_\mathrm{f}}^{T_\mathrm{i}}c_p(T)dT$$
This can be derived by integrating: $$C_v =\left(\frac{\mathrm{d}U}{\mathrm{d}T}\right)_V$$
Applying this ...
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1answer
89 views
Why are there two contradicting formulas for Enthalpy (Delta H): [duplicate]
1) Delta H = Bonds broken - Bonds Formed
2) Delta H = $H_{products} - H_{reactants}$
In the first formula, bonds break in the reactants and form in the products, so its basically saying "bonds ...
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0answers
75 views
How do the pressure acting on the reactants and the initial temperature of the reactants affect the enthalpy of reaction?
In an exothermic reaction that occurs under constant pressure, does the pressure acting on the reactants affect the enthalpy of reaction? Does the initial temperature of the reactants affect the ...
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1answer
255 views
Is the bond dissociation enthalpy of Cl-F greater than that of Cl-Cl?
In my book, it is mentioned that $\ce{Cl-F}$ has greater bond dissociation enthalpy than $\ce{Cl-Cl}$. Is it true? I know that interhalogen molecules usually have weaker bonds than dihalogen molecules ...
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2answers
92 views
Why don't we consider VΔP when we define Q?
We only define $Q = \Delta U + W_\text{exp}$ (expansion work = $-P\Delta V$).
If heat can cause $\Delta U$ and work, why work is defined only as expansion work in the first place where there are other ...
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1answer
105 views
What is V times dP?
Work is defined when there is a change of volume. Mathematically, $W=-\int P\, dV$. When pressure is constant $W= P\, \Delta V$. For an ideal gas expansion we have $W= nRT\ln\frac{V_2}{V_1}$. If its ...
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Why is the enthalpy of a reaction equal to the difference between the enthalpies of combustion of the reactants and the products?
My textbook gives me the following formula for calculating the enthalpy change of any reaction:
$$\Delta H_\mathrm{r}^\circ=\sum(\Delta H_\mathrm{c}^\circ)_\text{reactants}-\sum(\Delta H_\mathrm{c}^\...
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1answer
1k views
What is the difference between enthalpy of reaction and standard enthalpy?
What is the difference between enthalpy of reaction ($\Delta H$) and standard enthalpy ($\Delta H^\circ$)? I was told that the standard enthalpy of reaction is the change in heat when one mole of ...
