Hi! I was studying for an exam (lol) and I came across something I don't quite understand.. could someone help me with Standard Enthalpy of Formation? The equation in my notes looks like this:
ΔHºrxn = ΣΔHºf(products) - ΣΔHºf(reactants)
Thanks in advance!
That is Hess' Law. IT states that the heat of reaction is equal to the sum of the heats of formations of the products minus the heats of formation of reactions.
Example
2HNO3 + Fe >> Fe(NO3)2 + H2 (g)
The heat of reaction is equal to the heat of fromation of IronII nitrate-2*heatformation nitric acid. (the elments have zero Hf)
I think understand the concept but could you give me an example with numbers?
Hello! I'd be happy to help you understand the concept of Standard Enthalpy of Formation (ΔHºf) and how the equation you mentioned relates to it.
The Standard Enthalpy of Formation (ΔHºf) is the change in enthalpy that occurs when one mole of a compound is formed from its elements in their standard states, under standard conditions (usually 298 K and 1 atm pressure). It is a measure of the heat energy released or absorbed during the formation of a compound.
The equation you provided, ΔHºrxn = ΣΔHºf(products) - ΣΔHºf(reactants), relates to Hess's Law, which states that the enthalpy change for a reaction depends only on the initial and final states of the system and is independent of the pathway taken.
In this equation:
- ΔHºrxn refers to the standard enthalpy change of the reaction you are studying.
- ΣΔHºf(products) represents the sum of the standard enthalpies of formation for all the products involved in the reaction.
- ΣΔHºf(reactants) represents the sum of the standard enthalpies of formation for all the reactants involved in the reaction.
To calculate the standard enthalpy change of a reaction using this equation, you need to know the standard enthalpies of formation for each compound participating in the reaction. These values can be found in reference tables or databases.
To use the equation properly, follow these steps:
1. Identify the balanced chemical equation for the reaction.
2. Determine the standard enthalpies of formation (ΔHºf) for all the reactants and products involved in the reaction.
3. Substitute these values into the equation.
4. Calculate the sum of the standard enthalpy of formation for products and subtract it from the sum of the standard enthalpy of formation for reactants.
5. The result will be the standard enthalpy change of the reaction (ΔHºrxn).
Remember that the standard enthalpy of formation values are always given per mole of the compound, so be sure to use the correct stoichiometric coefficients in the equation to account for the number of moles involved in the reaction.
I hope this explanation helps! Let me know if you have any further questions.