Question

Find an example of a chemical reaction, and find its enthalpy of reaction

at 25 deg C and 1 atm pressure using Hess' Law.

Answer 1

In chemical reactions the initial state is the reactants and the final state is the products.

E = E_final - E_initial E = E_products - E_reactants

According to the first law

E_system +E_surroundings = 0

or

E_system = -E_surroundings

If the reactants and products are at 25 ^oC and 1 atmosphere they are referred to standard state enthalpies and the enthalpy of the reaction is written as ΔH.

another example is:

Given that for the combustion of glucose

C₆H₁₂O₆(s) + 6O₂(g) --->6CO₂(g) + 6H₂O(g) ΔH = -2816 kJ

and for the combustion of ethanol,

C₂H₅OH(l) + 3O₂(g) ----->2CO₂(g) + 3H₂O(l) ΔH = -1372 kJ

Calculate ΔH (in kJ) for the fermentation of glucose:

C₆H₁₂O₆(s) ---->2C₂H₅OH(l) + 2CO₂(g) ΔH = ?

Sol:

C₆H₁₂O₆(s) + 6O₂(g)---> 6CO₂(g) + 6H₂O(g) ΔH = -2816 kJ

4CO₂(g) + 6H₂O(l) ----> 2C₂H₅OH(l) + 6O₂(g) ΔH = - 2(-1372)= +2744 kJ

C₆H₁₂O₆(s) ----> 2C₂H₅OH(l) + 2CO₂(g) ΔH = -72 kJ