Enthalpy is the amount of heat energy in a system. Denoted by $H$.$^1$
Enthalpy change:$^1$
$$ \Delta H = H_2 - H_1 $$
Where $\Delta H$ is Enthalpy change, $H_2$ is final Enthalpy, $H_1$ is initial Enthalpy.
Example problem: $^1$
Calculate the amount of $\Delta H$ that takes place when 10 grams of ammonia chloride is dissolved in 500 $\text{cm}$ of water(density is 1 $\text{gcm}^{-3}$, specific heat capacity is 4.18 $\text{Jg}^{-1}\text{k}^{-1}$), the $\Delta T$ is 25 K to 23 K. Also determine the type of energy change.
It is simply the enthalpy change per mole of the substance. It is denoted by $\Delta H^{\minuso}$.$^1$
In general,
$$ \Delta H^{\minuso} = \frac{\Delta H}n $$
Where $\Delta H$ is the enthalpy change, and $n$ is the number of moles in the substance.$^1$
For example, if you wanted to calculate the amount of energy released from dissolving one mole of ammonia in water(from the previous problem), you would do the following:$^1$
$M_\text{ammonia chloride} = 28+4(1)+ 35.5$
$n = \frac{10}{53.5}$
$\Delta H^{\minuso} = \frac{4180}{\frac{10}{53.5}}$ $\Delta H^{\minuso} = 22363\text{ J}$
Enthalpy change of a combustion reaction is the amount of energy change when 1 mole of a substance is combusted in the presence of excess oxygen under standard conditions. It is donated by $\Delta H^{\minuso}_{c}$.$^1$
However, in order to solve problems using this, there is one more concept that needs to be introduced, which is the Law of Conservation of Energy.