# Can you explain the notation of enthalpy and how it is denoted and used? Very confused at the moment :/

Sep 14, 2017

I will give this a try, but you will need to consult a text......

#### Explanation:

Let's take first an exothermic combustion reaction.....

$C {H}_{4} \left(g\right) + 2 {O}_{2} \left(g\right) \rightarrow C {O}_{2} \left(g\right) + 2 {H}_{2} O \left(l\right)$

$\Delta {H}_{\text{reaction}}^{\circ} = - 891 \cdot k J \cdot m o {l}^{-} 1$

And what does we mean by $m o {l}^{-} 1$? This means $\text{PER MOLE of reaction as written}$. That is the combustion of $16 \cdot g$ methane, with $64 \cdot g$ dioxygen gives $44.0 \cdot g$ carbon dioxide, and $36.0 \cdot g$ of water PLUS $891 \cdot k J$ of energy, which we can use for our hot water service, or to cook our meals, or even drive a car.....

Another way I could look at the reaction is to designate the energy as a product of the reaction.....which indeed it is.

$C {H}_{4} \left(g\right) + 2 {O}_{2} \left(g\right) \rightarrow C {O}_{2} \left(g\right) + 2 {H}_{2} O \left(l\right) + 891 \cdot k J$

And we could also calculate the energy output in terms of $J \cdot {g}^{-} 1$ of methane, i.e. here we gets $55.7 \cdot k J \cdot {g}^{-} 1$ of methane upon complete combustion. And thus when we use quoted values, we must be mindful of the reaction we perform. And these combustion data are routinely quoted in these terms.

And so in your given example.....

A+BCrarrB+AC;DeltaH^@=100*kJ*mol^-1

...metathesis of $\text{MOLAR QUANTITIES}$ of $A$ with $B C$ to give stoichiometric $B$ and $A C$ REQUIRES AN ENERGY INPUT of $100 \cdot k J$; and so again it is a case of moles of reaction AS WRITTEN.

Are you happy with this spray?