Answer to Question #105939 in Organic Chemistry for Tasheni

Question

Using Hess law to calculate delta H for the reaction 2C (s)+3H2 (g)-2H2O (g)

Answer 1

As the chemical equation is not qiven correctly, we can look through some other example of how to use Hess Law.

For example to calculate the enthalpy for this reaction:

2C(s) + H2(g) ---> C2H2(g)

Given the following thermochemical equations:

C2H2(g) + 5⁄2O2(g) ---> 2CO2(g) + H2O(ℓ)ΔH° = −1299.5 kJ

C(s) + O2(g) ---> CO2(g)ΔH° = −393.5 kJ

H2(g) + 1⁄2O2(g) ---> H2O(ℓ)ΔH° = −285.8 kJ

Solution:

1) Determine what we must do to the three given equations to get our target equation:

a) first eq: flip it so as to put C2H2 on the product side

b) second eq: multiply it by two to get 2C

c) third eq: do nothing. We need one H2 on the reactant side and that's what we have.

2) Rewrite all three equations with changes applied:

2CO2(g) + H2O(ℓ) ---> C2H2(g) + 5⁄2O2(g) ΔH° = +1299.5 kJ

2C(s) + 2O2(g) ---> 2CO2(g) ΔH° = −787 kJ

H2(g) + 1⁄2O2(g) ---> H2O(ℓ)ΔH° = −285.8 kJ

Notice that the ΔH values changed as well.

3) Examine what cancels:

2CO2 ⇒ first & second equation

H2O ⇒ first & third equation

5⁄2O2 ⇒ first & sum of second and third equation

4) Add up ΔH values for our answer:

+1299.5 kJ + (−787 kJ) + (−285.8 kJ) = +226.7 kJ