IS the formula different?

Question

Consider the following balanced reaction:

2 A₂ + 3 B → C₂ + 2 D

What is the enthalpy of reaction if the enthalpy of formation of the compounds is given as

Compound	Enthalpy of Formation (kJ/mol)
A2	4.15
B	-14.54
C2	0.00
D	23.20

 ^kJ/_mol

Answer 1

∆Hrxn = ∑∆H products - ∑∆H reactants

∑ for the products = 0 + 2x23.2 = 0 + 46.4 = 46.4

∑ for reactants = 2x4.15 + 3x-14.54 = 8.3 -(-43.62) = 8.3 + 43.62 = 51.92

Products - reactants = 46.4 - 51.92 = -5.52

∆Hrxn = - 5.52 kJ/mol

Answer 2

If we call the stoichiometric coefficient n_i , the formula for the calculation of ΔH_rxn in terms of heats of formation of all the species in the reaction is...

Summation from 1 to n_p of n_iΔH_i - Summation from 1 to n_r of n_iΔH_i (most chemistry books write this badly)

In your case: 2(ΔH_f,D) + ΔH_f,C2 - (3(ΔH_f,B + 2(ΔH_f,A2))

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