Questions tagged [enthalpy]

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

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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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What is change in enthalpy in energy profile? [closed]

Consider an energy profile diagram. My doubt is that why does change in potential energy (PE) equal change in enthalpy here? At constant pressure change in enthalpy equals the net heat exchanged ...
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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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Why does it seem that the heat of combustion is higher with substances of greater molecular weights?

Why does it seem that the heat of combustion is higher with substances of greater molecular weights? How do the intermolecular forces or bonds of the reactants affect the heat of combustion? I have ...
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Sign of Proton Affinity

I was searching for reactions with hydrocarbon anions and hydrogen cation/proton and noticed that the enthalpy change of a reaction is always positive or likewise proton affinity associated with the ...
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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 quantity is one mole in the reaction in enthalpy change? [duplicate]

For example, I have a reaction: $H_2(g)+\frac{1}{2}O_2(g)→H_2O(l),𝛥H=-285.8 kJ mol^{-1}$ What does this "per mole" in the unit represent? What quantity is one mole?
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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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I am unable to figure out the reason for first bond enthalpy being higher than second one in water

In $\ce{H2O}$, I am able to understand that the enthalpy needed to break two $\ce{OH}$ bonds are not the same. But I don't understand why is the bond energy for breaking the first $\ce{O-H}$ bond more ...
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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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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 ...
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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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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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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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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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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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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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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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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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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 ...
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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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