Before, going into details of thermodynamics and the enthalpy formation, it is important to understand the meaning of two specific words:
It is said to be a condition that has 1 atmospheric pressure and 25oC.Â The symbol is denoted as H0f where f stands for formation and 0 as standard conditions.
This is the term refers to substance and it is identified as product of any chemical equation which can be either formed directly retrieving from element that is involved. The substance involved will always be along with coefficient of 1. The meaning is further illustrated by examples:
C(s, graphite) + 2H2Â (g) + (1/2)O2(g) —> CH3OH(â„“)
C(s. graphite) + O2(g) —> CO2(g)
H2(g) + O2(g) —> H2O2(â„“)
For a particular isomer of C8H18 the following reaction produces 5104.1kj of heat per mole of C8H18 (g) that is consumed under any standard conditions.
C8H18 + 25/202(g) —Â® 8CO2 (g) + 9H2O (g)
deltaHrxn= 5104.1 kj
âˆ†Hrxn=9âˆ†H (H2O) +8âˆ†H (CO2) âˆ’âˆ†H (C8H18)
âˆ†H (C8H18) =9âˆ†H (H2O) +8âˆ†H (CO2) âˆ’âˆ†Hrxn
âˆ†H (C8H18) =9(âˆ’241.8) +8(âˆ’393.5) +5108.7=âˆ’215.5kJ/mol
How to calculate standard enthalpy of formation?
We probably know that when compounds are formed there is a chance of retrieving enthalpy value and further it helps to calculate change in enthalpy which is denoted as delta H for any kind of chemical reaction.
But, why does it work? This is because you should know that enthalpy is in a property state which means that it is not important to know how we reach there, but what really matters is from where we start and end. So, for any kind of reaction, delta H can take place in two different ways:
- Elements can further be converted to products
- Reactants also get converted to elements
Actual concept behind enthalpy calculations
While you want to know for a particular isomer of C8H18 the following reaction produces, it is also vital to know that reaction might be reversed while keeping the magnitude of Delta H constant, but the sign may change.
- Elements while being in their standard states do not contribute to any enthalpy calculations in order to get reactions as enthalpy of any element is witnessed as in its standard state which signifies as zero.
- Allotropes of any element apart from any standard state would be denoted as non-zero standard enthalpies of any formation.
In case the balanced equation for any reaction is multiplied by any integer, then it is also essential that corresponding value of Î”HÂ which should also be multiplied by integer.
While defining standard enthalpy of formation it can be used only to measure energy that is either being released or consumed when a mole of standard is produced under any standard conditions from pure elements. In order to define it through symbol, it is denotes as Î”H.
- Î”= change in the enthalpy
- f= it says that the substance itself is formed from elements
- o = it indicates that there is a change in standard enthalpy
You should know that every substance would appear in its standard state. Whether it is in solid, liquid or gas, the substance would always be under standard conditions.
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