Enthalpy measures the total amount of energy of a thermodynamics system which is in the form of heat or volume multiplied by the constant pressure.
The formula to calculate the enthalpy is along the lines:
H = Q + pV
Q is the internal energy
p is the vpressure
V is the volume
H is the enthalpy.
To calculate the change in enthapy, you need initial and final values with constant pressure. The formula for enthalpy change is ΔH = (Q₂ - Q₁) + p * (V₂ - V₁) or
ΔH = ΔQ + p * ΔV
Q₁, Q₂ are the internal energies of the products and reactants.
V₁, V₂ are the volume of the products and reactants
p is the pressure
ΔH is change in enthalpy
ΔQ is change in inyternal energy
ΔV is the change in volume
Standard Enthalpy of Formation
It is another method used to compute the enthalpy change. The standard enthalpy of formation of a compound is ΔH°f. It is nothing but the change in enthalpy for forming one mole of substance in the standard state.
The standard enthalpy of formation formula is given as
ΔH°reaction = ∑ΔH°f(products) - ∑ΔH°f(reactants)
∑ΔH°f(reactants) is the sum of the standard enthalpes of the reactants.
∑ΔH°f(products) is the sum of the standard enthalpies of the products
ΔH°reactionis the enthalpy change
The standard enthalpy of each chemical substance is collected from the standard enthalpy of formation table mentioned below:
||Change in Enthalpy in kJ/mol
Question: Calculate the change in enthalpy for 2H2 + O2 → 2H2O?
Given chemical reaction is 2H2 + O2 → 2H2O
The standard enthalpy of formation of substances is here.
H2 = 0, O2 = 0, H2O = −285.8
Change in enthalpy formula is ΔH°reaction = ∑ΔH°f(products) - ∑ΔH°f(reactants)
∑ΔH°f(reactants) = 2(0) + 0 = 0
∑ΔH°f(products) = 2(-285.8) = -571.7
ΔH°reaction = -517.7 - 0 = -517.7
Therefore, change in enthalpy of the reaction is -517.7 kJ.
Avail free online calculators to learn physics concepts from our website Physicscalc.Com. You will also get instant results by using our handy calculator tools.