# Questions tagged [enthalpy]

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

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### Why is my ethanol–water mixture showing negative change in enthalpy of mixing?

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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I am making homemade flash paper (nitrocellulose) and had a question regarding the safety and stability of the mixture. I have purchased a respirator, beakers, stirring rods ect so i stay safe. After ...
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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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### Confusion on when to use mass of entire solution when performing q=mc∆T

I hope you are having a good day. I am studying for the AP Chemistry exam in May 2020. While studying, I encountered a doubt. In Problem 1 (the question on the top), the educator saw that the ...
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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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### 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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### Enthalpy Change

If I mixed $25.0\ \mathrm{cm^3}$ of $0.350\ \mathrm{mol\space dm^{-3}}$ sodium hydroxide solution with $25.0\ \mathrm{cm^3}$ of $0.350\ \mathrm{mol \space dm^{-3}}$ hydrochloric acid. The temperature ...
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### Is the standard enthalpy of formation always non-positive?

If the standard enthalpy of formation is defined as the energy change when 1 mole of a substance is formed from its elements in their standard states and forming chemical bonds is an exothermic ...
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### How to calculate what temperature change in entropy for the reaction equal to the change in entropy for the surroundings given Enthalpy and entropy

A reaction has $\Delta H_\textrm{rxn} = \pu{-107 kJ}$ and $\Delta S_\textrm{rxn} = \pu{285 J/K}$. At what temperature is the change in entropy for the reaction equal to the change in entropy for the ...
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### Joules to kJ/mol conversion

In my AP Chem class we are working on testing Hess's Law and conducted three reactions. Then enthalpy changes of the 1) $\ce{NaOH + HCl}$ and 2) $\ce{NaOH + NH4Cl}$ to predict the enthalpy change of 3)...
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### Why won't water freeze if you put ice in it, despite ice being frozen?

Just now I got some room temperature water and added some ice to cool it, and it just came to me, like the scientist I wish I was ... why doesn't frozen (freezing point) ice added to higher ...
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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 ...
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 ...