U3-3-3 - Hess's Law

Question 1

State Hess’s law

Answer 1

The enthalpy change for a chemical reaction is independent of the route taken.

Overall ∆H is the same regardless of any intermediates formed

Question 2

∆H₁ = …

Answer 2

∆H₁ = ∆H₂ + ∆H₃

Question 3

In the type of diagram shown, how do you know if the sign (+ or −) for ∆H should be reversed or not?

Answer 3

Going in same direction as arrow (e.g. W to Z) - sign stays as is

Going in opposite direction to arrow (e.g. Z to W) - sign reversed

Question 4

What change needs to be applied to the highlighted equation’s ∆H to calculate enthalpy change for the top reaction?

Answer 4

x3
(3 moles of C in target equation)

so -394 x 3

Question 5

What change needs to be applied to the highlighted equation’s ∆H to calculate enthalpy change for the top reaction?

Answer 5

x2
(2 moles of H₂ in target equation)

so -286 x 2

Question 6

What change needs to be applied to the highlighted equation’s ∆H to calculate enthalpy change for the top reaction?

Answer 6

reverse
(C₃H₄ is a product in target equation)

so +1939