# Questions tagged [enthalpy]

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

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### Why are my calculations showing an exothermic reaction as endothermic?

I recently did a lab on the effect of a variable on the hydration enthalpies of salt, but my final enthalpies turned out to be positive. Shouldn't the hydration enthalpies be negative, as bonds are ...
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### Question to find increment in energy of a sample of He gas [closed]

If a closed rigid wall container filled with 20 gram of Helium gas moving at a constant speed of 10 m/sec is stopped abruptly to stand still, find the increment in temperature of this He sample. Way I ...
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### Enthalpy and Hess’s law experiment question [closed]

I think the first equation should be + delta-H, and the correct answer is the fourth equation. I think if the first equation is correct, then the second and third equations are also correct. From the ...
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### Why is enthalpy change at constant pressure not only internal energy change? [closed]

Okay so this might seem a bit Weird but the formula for change in enthalpy has been given the following way- ∆H=∆U+∆PV so in this case when we say ∆V is zero we get that ∆PV is also zero and thus ∆H=∆...
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### What makes the enthalpy of hydration of calcium chloride dihydrate unique? [closed]

I have recently done an experiment in an attempt to find the relationship between the water ratio in a hydrate and the enthalpy of hydration, and my results showed a correlation, but an anomaly for ...
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### why is change in Enthalpy of a galvanic cell negative?

my teacher told me that change in enthalpy of a cell reaction is always negative but i can't get in what respect he said and why. Someone please give me a good thought?
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### Why is the change in enthalpy computation for a change of state done like this?

Suppose that a system whose initial state is Pressure = $P_1$, Volume = $V_1$; and it is taken through a process after which final pressure and volume are $P_2$ and $V_2$ respectively. Now change in ...
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### What is the mechanism of endothermic dissolution?

My real question is in fact "How can dissolution happen if it's endothermic?" However, there have been many questions regarding this exact topic and the answers about Gibbs free energy ...
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### Why does enthalpy of hydration increase as water ratio in hydrate increases?

I recently did an experiment for a lab report where I found that hydrates with a larger water ratio (like how CaCl2⋅2H2O has a water ratio of 2, and Na2S2O3⋅10H2O has a ratio of 10) have a higher ...
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### Significance of enthalpy of hydration of salts [closed]

I've been researching for weeks but can't find anything on a real life process of hydration, as my experiment is determining the relationship between the enthalpy of hydration and different types of ...
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### Why does increasing charge density matter more for the hydration enthalpy compared to the lattice energy?

For the solvation of ionic salts, if we look down a group, say Group II, and we keep the anion constant, we find that the solubility decreases going down the group. This is because the hydration ...
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### Specific Heat Derivation

The specific heat capacity at constant volume and the specific heat capacity at constant pressure are intensive properties defined for pure, simple compressible substances as partial derivatives of ...
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### How would I correct this Hess cycle?

Given $$\ce{2 SO2(g) + O2(g) <=> 2 SO3(g)} \quad \Delta_\mathrm{r}H = \pu{-196.0 kJ mol^-1}$$ and $\Delta_\mathrm{f}H(\ce{SO2}) = \pu{-296.0 kJ mol^-1},$ find $\Delta_\mathrm{f}H(\ce{SO3}).$ I ...
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### rate of combustion of alcohols and hydrocarbons in relation to the number of carbons in the chain

I understand that the heat released when burning alcohols is greater as long as the number of carbons in the alcohol chain is increased (i.e the enthalpy of butanol is greater than propanol > ...
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### Why is the first ionization enthalpy of molecular O2 less than atomic O?

A cousin of mine asked for my help to answer one of his homework questions. It stated- Explain why- The first ionization energy of molecular oxygen $\ce{O2}$ ($\pu{1175 kJ mol-1}$), is lesser than ...
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### How does the calorimeter affect heat released by the system (q)? [closed]

Here are the 2 questions I am pondering about How would the calculated quantity of heat (q) released by a system differ if the calorimeter's heat capacity was taken into account? My thought process ...
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### How can you calculate the tendency of one compound to become another?

Excuse my inexperience with chemistry here. I've been poring over papers with terms like "Standard Gibbs free energy of formation", "Hess's Law", "standard enthalpy of ...
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### Thermochemistry: On Units of Enthalpy Change and Implicit Given of Initial Temperature of Calorimeter

I would like to ask two separate questions. But first, so that we are on the same page, the experiment used a constant-pressure calorimeter, and it is a neutralization reaction of 1M of HCl and 1M of ...
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### Why does lattice enthalpy decrease with increasing ionic size?

Lattice enthalpy decreases as ions get larger, but I have found two explanations: The charge density is greater in smaller ions, so greater attraction The ions are themselves able to get closer ...
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### Why is the zero of standard enthalpy of formation a convention?

The standard enthalpy of formation $\Delta H_f^°$ of pure elements is zero by definition. Why is that a convention? It is true that enthalpy is defined unless a constant (like energy and entropy), but ...
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### Predicting sign of enthalpy of allotropic transition of phosphorus

How to predict the sign of enthalpy during the transition $$\ce{P(s, white) -> P(s, red)}?$$ $\ce{P(s, red)}$ is more stable than $\ce{P(s, white)}$ due to release in angle strain. Since during ...
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### Why do internal energy/heat and enthalphy differ? What is the physical significance of the $PV$ term for enthalphy of an ideal gas? [duplicate]

Let it be noted that this is not a duplicate of this downvoted-to-hell question, simply because I hope to ask it better! I've been reading a thermo textbook and I've got a simple question. The ...
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### Why isn't the mass of the things dissolved in water not added to the mass of the water in an enthalpy calc?

Take a change in enthalpy question. When $\pu{5 g}$ of powder is dissolved in $\ce{100 ml}$ of water, the added substance raises the temperature of the water by $\ce{5 ^\circ C}$ and you are told to ...
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### Feasibility of a Reaction [closed]

I understand for a reaction to be feasible, $\Delta$H needs to be negative and $\Delta$S positive. But in terms of Gibbs free Energy Change, without considering kinectic factors, If $\Delta$H is ...
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### What is the enthalpy change of solution of oxygen and water? [closed]

What is the enthalpy change of solution of oxygen dissolved into water?
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### Sign of change in enthalpy and change in entropy

Why is it wrong to assert that the change in entropy and the change in enthalpy must always have the same sign? What makes me think that they must have the same sign is the fact that every reaction ...