Questions tagged [enthalpy]

A thermodynamic state function describing the total energy content of a system.

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What is the unit of enthalpy? [closed]

I have seen both kJ and kJ/mol as units for enthalpy and enthalpy of formation and enthalpy of combustion in my book, can both be used? And also what is the unit of Gibbs free energy, kJ or kJ/mol? ...
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Atomization enthalpy trends. The case of Phosphorus

I was looking again at the periodic trends like Ionization Energy, Electron Affinity, Radius and so on. I understood all the general trends and the exeptions for these ones, but I can't fully ...
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How should I evaluate $\left(\frac{\partial U}{\partial V}\right)_T$ with van der waals equation: $P = \frac{nRT}{V-nb}-a(\frac{n}{V})^2$?

So in my homework I was able to get this- $\left(\frac{\partial U}{\partial V}\right)_T = T(\frac{\partial P}{\partial T})_V - P$ but now I'm trying to plug in $P = \frac{nRT}{V-nb}-a(\frac{n}{V})^2$ ...
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Calculating Enthalpy Change from Calorimetry [closed]

What calculating the enthalpy change of a reaction, for example 5g of CaCl2 dissolving in 100 mL of water, is m the mass of water or the mass of water plus the mass of the salt. q = (105g)(4.18)T or q ...
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How does the enthalpy change of bond enthalpy relate to enthalpy changes of formation? What's the difference? [duplicate]

The enthalpy change = sum of bond energies broken (reactants) - sum of bond energies made (products). Then I read The enthalpy change = sum of the enthalpy change of formation of products - sum of the ...
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In acids why does polarising the O-H bond weaken rather than strengthen it?

In hexaaquaions it is generally true that aqueous solutions of complex ions with a higher positive charge on the central metal ion will have a lower pH. Explanations typically note that the greater ...
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Which is chemical reaction has the highest energy density? [duplicate]

Lets say X and Y react to form XY, and release E Joules of energy in total, not per mole. If molar weight of X is 1 g and Y is 2 g So the energy density is 1000E/3 J/Kg. Which chemical reaction has ...
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Why is octasulfur present in equation for standard enthalpy of formation of sulfuric acid?

Write a balanced chemical equation corresponding to the standard enthalpy of formation of $\ce{H2SO4}.$ \eqref{rxn:r1} is the given answer, \eqref{rxn:r2} is my answer: \begin{align} \ce{1/8 S8 (s) + ...
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What do they mean with temperature in the formula for enthalpy?

I'm studying enthalpy and I'm having a real hard time trying to conceptualize it. I can do the exercises and know the formulas but I feel like I lack an understanding of what it actually is. One of my ...
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What is the most stable cycloalkene? [closed]

I'm looking specifically at cyclopentene and cyclohexene. Supposedly, greater instability would lead to a more exothermic enthalpy of combustion. However, based on my data, cyclopentene is more ...
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Enthalpy Departure

I'm trying to fill in the steps from my textbook on the derivation of enthalpy departure from ideal gas behavior. My textbook gives the variation of mass-specific enthalpy with temperature pressure as ...
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Enthalpy contribution and heat capacity

Imagine having at the input of a burner 105 mol/h of $\ce{O_2}$ at 50 °C. The enthalpy contribution of this input mass is calculated using the Kirchhoff equation $$\Delta H = \Delta H^° + \int_{25 °C}^...
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What is the difference between heat of reaction (q) and enthalpy of reaction ∆H?

I am confused as I can’t understand the difference between the thermodynamic terms q and ∆H. In my book,“general chemistry” by Ebbing and Gammon,it is stated that at constant pressure Qp=∆U+P∆V=∆H. ...
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Exothermic temperature rise on NaOH + Methanol

When mixing NaOH (w/w 0.05%) with methanol, there is an increase of temperature around 20 C I wonder if we keep this ratio in large amount of methanol, will there be any spike in temperature increase ...
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Why can't the Fajans' rule and "Lattice energy" theory be used to explain the difference in thermal stability of MgCO3 and CaCO3?

CaCO3 is more thermally stable than MgCO3, I do understand that a larger cation would be able to stabalise a larger anion to a greater extent but why can't we use the above theories to explain?
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Calculating Enthalpy of formation versus Calculating Enthalpy of a reaction not occurring at standard conditions

My understanding of Hess's law and its use in determining the change of enthalpy of a reaction has been challenged recently. Up until recently, I thought that the only way to calculate a chemical ...
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Can reverse micelle's exist outside solution?

As in reverse micelle's, the polar liquid remains inside the micelle while the exterior is one of lower dielectric constant; if the liquid were to be carefully removed, would the micelle structure ...
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Why is it that the enthalpy of hydration equals the difference of solvation enthalpies of anhydrous and hydrous salts

Taking an example from my textbook At finite dilution of CuSO4, heat is liberated $$\ce{CuSO4(s) -> CuSO4 (aq)}.$$ ΔH of solvation = -15.8 kcal (anhydrous CuSO4 solvation reaction) and when it is ...
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Combustion Temperature Calculation Using C_p and Heat of Formation

I am simulating the combustion temperature for various compositions of syngas and finding the product temperature. Part one is varying the ratio of CO and H2 from 3:1 to 1:3 in the syngas, burning it ...
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enthalpy of formation of hydronium

Can someone explain how the enthalpy of formation of aqueous hydronium was determined. It appears to be exactly the same as the enthalpy of formation of liquid water. I know this must have something ...
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How to find the temperature for when Kc = 1

The formulas I can use are $\Delta$G = H - T$\Delta$S, and $\Delta$G = $\Delta$G(s.t.d) + RTlnK I understand that when Kc is 1, the reaction is barely spontaneous and thus I can use the first formula ...
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What is the effect of temperature on the enthalpy of reaction?

In the study of reaction mechanisms, enthalpy of formation for intermediate compounds at various temperatures is studied. I understand the reason to do that. How much (and what) effect can temperature ...
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Why we can use ΔH to determine reaction rate in free radical bromination reaction steps?

I am trying to understand why we can use ΔH to determine reaction rate in this a free radical halogenation reaction. Given: (CH3)2CH2 + Br• → (CH3)2CH• + HBr = 397 - 368 = +29 kJ/mol = ΔH (CH3)2CH• + ...
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Why is the the temperature change due to the combustion of paraffin wax so high (10000 Kelvin)? [closed]

I'm trying to work out the temperature change (I got something like 10k Kelvin,hotter than the Sun's core...) caused by the combustion of paraffin wax which has a molar enthalpy of combustion $E^{wax}...
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Why is my ethanol–water mixture showing negative change in enthalpy of mixing? [duplicate]

I am conducting an experiment where I mix two solutions $\text{A}$ and $\text{B}$ and then I am measuring the temperature change (for real solutions, i.e., $\Delta_\mathrm{mix}H\neq 0).$ I took the ...
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What is an example of an exothermic reaction with negative ∆S?

In high school we learn that when a reaction has both negative $\Delta{}S$ (entropy change) and negative $\Delta{}H$ (enthalpy change) it occurs spontaneously at lower temperatures, but becomes non-...
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Physical meaning of enthalpy change when internal pressure Is not constant [closed]

My chemistry textbook states the following "general enthalpy" formula for the change in enthalpy when the internal pressure is not constant: $∆H= ∆U + ∆(PV)$ How is the internal pressure ...
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How strongly affected is the buffering capacity of sodium carbonate - sodium bicarbonate by temperature changes?

I would like to do a lab regarding the affects of temperature on the buffering capacity of sodium carbonate - sodium bicarbonate in particular for sulfuric acid. Currently I have two competing ...
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How to calculate enthalpy of mixing for non-ideal solutions?

I have conducted a series of experiments where I basically mixed two solutions to form a non-ideal mixture and then I measured the temperature rise/fall (since both positive and negative deviation are ...
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Enthalpy/Temperature change in a chemical reaction

I was wondering how exactly absolute temperature change in a chemical reaction could be calculated. For example, consider this reaction: 3H₂+N₂→2NH₃, ΔH=-46kJ/mol (298.15K) Assuming the given reaction ...
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Why does the trend of the enthalpy of reaction per atom of carbon in hydrocarbons approach a near-constant value?

I was looking at trends for enthalpies of hydrocarbons for a high school project and found an interesting paper regarding it, stating that the enthalpy per carbon atom approaches a constant. I'm ...
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Endothermic reaction with dV < 0 [closed]

I am looking for an endothermic reaction dH > 0 for which the net volume change is negative, e.g. gases reacting to form a liquid. Does anybody have an illustrative example?
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At equilibrium, which one of the following is always Zero? [duplicate]

At equilibrium, which one of the following is always zero? (A)∆H (B)∆S(Total) (C)∆S(System) (D)∆G° I know that at equilibrium ∆G=0, and was facing difficulty in connecting the above mentioned ...
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Databases for Enthalpy of Solution and Ionic Radii for Ionic Salts

I am looking to investigate the relation between experimental enthalpies of solution and theoretical enthalpies of solution calculated using theoretical equations for the lattice enthalpy and ...
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Justifying assumptions about method to find equilibrium pressure for co-existence of graphite and diamond

The standard state Gibbs free energies of formation of $\ce{C(graphite)}$ and $\ce{C(diamond)}$ at $T = \pu{298 K}$ are $\pu{0 kJ mol-1}$ and $\pu{2.9 kJ mol-1}$, respectively. The conversion of ...
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Why is it sometimes kJ only, and in other times kJ/mol? What's the difference? [closed]

Here is an example in my book. Computing the enthalpy of formation $\Delta{}H_\mathrm{f}$ via multiple enthalpies of reaction $\Delta{}H_\mathrm{r}$ I notice some of the equations run by $\pu{kJ/mol}$...
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Why is the enthalpy change of an ideal gas a function only of temperature?

While teaching us thermodynamics our chemistry teacher told us that enthalpy change of a real gas is a function of temperature, pressure etc. but the enthalpy change of an ideal gas is only a function ...
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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 ...
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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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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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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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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 ...
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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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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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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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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 ...
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