Enthalpy

Tags

Chem/Reaction

Cegep/2

Word count

189 words

Reading time

2 minutes

Thermodynamic function equal to internal energy plus pressure times volume

$$H=E+PV$$

==At constant pressure, enthalpy change equals heat:==

$$\begin{array}{rl}\mathrm{\Delta }H& =q_P=\mathrm{\Delta }E+P\mathrm{\Delta }V=\mathrm{\Delta }E+RT\mathrm{\Delta }{n}_g\\ \mathrm{\Delta }H& =q_{rxn}=-(\underset{\approx 0}{\underbrace{q_{cal}}}+q_{sol})=-m_{sol}{s}_{sol}\mathrm{\Delta }T\end{array}$$

Not actually the total heat of a system

Standard enthalpy of formation

Enthalpy change when one mole of a particular compound is formed from its elements in standard states
Sym. $\mathrm{\Delta }{H}_f^{\circ }$

$$\mathrm{\Delta }{H}_{f_{element}}^{\circ }=0$$

[!example] Write the equation for the formation of $\mathrm{MgCO}{\phantom{A}}_3{\phantom{A}}_{(\mathrm{s})}$.

$$2\mathrm{Mg}{\phantom{A}}_{(\mathrm{s})}+2\mathrm{C}{\phantom{A}}_{(\mathrm{graphite})}+3\mathrm{O}{\phantom{A}}_{2(\mathrm{g})}\to 2\mathrm{MgCO}{\phantom{A}}_3\mathrm{\Delta }{H}_{rxn}^{\circ }=2\mathrm{\Delta }{H}_f^{\circ }$$

Bond enthalpy

Enthalpy change to break a particular bond in one mole of gas
Measure of molecular stability
Sym. $BE$

Bond enthalpies of diatomic molecules are directly measurable, thus they have more significant figures.

Standard enthalpy of reaction

Enthalpy change of a particular reaction
Sym. $\mathrm{\Delta }{H}_{rxn}^{\circ }$

$$\mathrm{\Delta }{H}_{rxn}^{\circ }=\sum _{i\in \text{products}}{n}_i\mathrm{\Delta }{H}_{f_i}^{\circ }-\sum _{i\in \text{reagents}}{n}_i\mathrm{\Delta }{H}_{f_i}^{\circ }$$

For gases:

$$\mathrm{\Delta }{H}_{rxn}^{\circ }=\sum _{i\in \text{reagent bonds}}B{E}_i-\sum _{i\in \text{product bonds}}B{E}_i$$

Contributors

Ajitani Hifumi

Changelog

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