H2 + F2--> 2HF deltaH = 518.0 kJ
H2 ----> 2H deltaH = 436.8 kJ
F2 ---> 2F deltaH = 158.2 kJ
(a) calculate Delta H for H + > HF
(b) what is the standard enthalpy change of formation of F2
Help please!!! i think this is easier than i'm making it.
for (b) would i take 158.2/2 to get 79.1 kJ ?
For b, isn't delta Hf for elements in the free state zero?
For a, I would do this.
eqn1 H2 + F2 ==> 2HF -518.0 kJ(did you omit the minus sign)?
eqn 2 2F ==> F2 -158.2
eqn 3 2H ==> H2 -431.8
Now add eqn 1 to 2 and 3 and you should end up with
2H + 2F ==> 2HF which is just twice what you want. Add delta H values and take 1/2 the total.
RRYTGYYTYB
To calculate the enthalpy change for the reaction H + F ---> HF, you can use Hess's law. Hess's law states that the total enthalpy change for a reaction is equal to the sum of the enthalpy changes for the individual steps of the reaction.
(a) To find the enthalpy change for the reaction H + F ---> HF, you need to combine the given reactions:
H2 + F2 ---> 2HF ΔH = 518.0 kJ
H2 ---> 2H ΔH = 436.8 kJ
F2 ---> 2F ΔH = 158.2 kJ
You can see that the reaction H2 + F2 ---> 2HF involves the formation of H-H and F-F bonds, so you need to reverse the second and third reactions and adjust the coefficients to match the overall reaction:
2H ---> H2 ΔH = -436.8 kJ
2F ---> F2 ΔH = -158.2 kJ
Now, add the reversed reactions together with the original reaction:
2H + 2F + H2 + F2 ---> HF + HF + H2 + F2
Simplifying, you get:
H + F ---> 2HF ΔH = -436.8 kJ + (-158.2 kJ) + 518.0 kJ = -76.0 kJ
Therefore, the enthalpy change for the reaction H + F ---> HF is -76.0 kJ.
(b) The standard enthalpy change of formation (ΔHf) of F2 is the enthalpy change when one mole of F2 is formed from its elements in their standard states. It is equal to the enthalpy change for the reaction F2 ---> 2F.
Given that the enthalpy change for the reaction F2 ---> 2F is ΔH = 158.2 kJ, you do not divide it by 2 to get the standard enthalpy change of formation of F2. The enthalpy change of formation is the enthalpy change per mole of the compound formed.
So, the standard enthalpy change of formation of F2 is 158.2 kJ.