# Questions tagged [enthalpy]

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

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### 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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### Standard Enthalpy of Formation for Branched Alkanes

I'm looking at the heat of formation for branched and straight alkanes and I've noticed a few odd trends I'm having trouble explaining. For straight-chained alkanes, the addition of carbon atoms to ...
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### Why are absolute enthalpies of reactants not available?

The standard enthalpy of reaction is given as $$\Delta_r H = \sum\nu\; H_m(\text{products})- \sum\nu\;H_m(\text{reactants}).$$ Peter Atkins in his book Elements of Physical Chemistry then writes ...
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### Enthalpy of mixing in a regular solution

In a mixture of $A$ and $B$ where $A$ and $B$ are randomly mixed and the average coordination number is $z$. If there are $n_B$ molecules of $B$ in the mixture these will be in contact with a total of ...
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For example, I have a question below that I know the correct answer to, but I don't know why it's positive rather than negative. When $\ce{NaOH}$ is neutralized by $\ce{HCl}$: $$\ce{NaOH(aq) +... 0answers 2k views ### Why does it not matter what mass of NaOH is used to solve for q? So we recently did a lab in Chem to prove Hess's law. The reactions we performed were$$\begin{align} \ce{NaOH + HCl &-> NaCl + H2O}\tag1\\ \ce{NaOH(s) + HCl &-> NaCl + H2O}\tag2\\ \ce{...
I have a simple lab problem: $30\ \mathrm{mL}$ of $1\ \mathrm{mol/L}$ $\ce{H2SO4}$ reacts with $50\ \mathrm{mL}$ of $1\ \mathrm{mol/L}$ $\ce{NaOH}$ in a neutralization reaction as per the following ...
I am having trouble calculating the value of $\Delta H_\mathrm r$ for the reaction: $$\ce{Mg + HCl}$$ The total heat capacity ($C_\text{cal} + C_\mathrm r$) is $6.88\ \mathrm{kJ/{^\circ C}}$, and the \$...