Hess’s Law Flashcards
What does Hess’s Law state
That the enthalpy change in converting reactants to products is the same regardless of the route taken, provided the initial and final conditions are the same
When can Hess’s law be used
It means it is possible to bring together data and calculate enthalpy changes which cannot be measured directly by experiments(don’t occur under standard conditions/difficult to perform in practice)
Two things Hess’s law can be used to calculate
-standard enthalpy changes of FORMATION from standard enthalpy changes of COMBUSTION
-standard enthalpy changes of REACTION from standard enthalpy changes of FORMATION
Three components of triangle and way arrows are pointing
Reactants (arrow towards products), products, elements (arrows towards both reactants and products)
What is the enthalpy change of elements
Zero as they already exist in stated conditions (eg oxygen as O2(g))
Equation for enthalpy change of combustion
(Sum of enthalpy changes of combustion of reactants) - (sum of enthalpy changes of combustion of products)
How is the cycle different when enthalpy change of combustion
Oxides at bottom have arrows from both reactants and products(enthalpies of combustion) with arrow from reactants to products (often an enthalpy of formation from elements to a compound)
Why do the arrows point down in combustion enthalpy cycle
As it is possible to combust all the reactants and products to form carbon dioxide and water at the bottom
Why do the arrows point upwards in standard enthalpy of formation cycles
As can use the elements at the bottom to make both the reactants and products
