U3-3-3 - Hess's Law
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State Hess’s law
The enthalpy change for a chemical reaction is independent of the route taken. Overall ∆H is the same regardless of any intermediates formed
∆H1 = …
∆H1 = ∆H2 + ∆H3
In the type of diagram shown, how do you know if the sign (+ or −) for ∆H should be reversed or not?
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
What change needs to be applied to the highlighted equation’s ∆H to calculate enthalpy change for the top reaction?
x3 (3 moles of C in target equation) so -394 x 3
What change needs to be applied to the highlighted equation’s ∆H to calculate enthalpy change for the top reaction?
x2 (2 moles of H2 in target equation) so -286 x 2
What change needs to be applied to the highlighted equation’s ∆H to calculate enthalpy change for the top reaction?
reverse (C3H4 is a product in target equation) so +1939