Hess’ cycle Flashcards
1
Q
What is Hess’ Law?
A
- Hess’ Law is the idea that the enthalpy change for a chemical reaction is independent of the route taken (it doesn’t matter which way you go- the enthalpy change will always be the same.)
2
Q
What does a combustion cycle look like?
A
- 2 arrows pointing downwards to CO2 and H20 (these will always be at the bottom of combustion cycle.)
Refer to image in pink folder
pst… pst “burn to the ground!”
3
Q
What does a formation cycle look like?
A
- 2 arrows pointing upwards to reactants/ products from the elements (which are at the bottom.)
Refer to image in pink folder
pst… pst “built the house back up.”
4
Q
What do we use, from the Q, to determine whether we draw formation/ combustion cycle?
A
- If data given is combustion data, (ie. symbol in table =∆cHƟ) - you draw combustion cycle.
- If data given is formation data (ie. symbol in table = ∆fHƟ- you draw a formation cycle.
5
Q
If Q asked for enthalpy of formation but gave you combustion data, which cycle would you draw?
A
- Draw a combustion cycle.
6
Q
What condition must the elements written at the bottom of formation cycle be written as?
A
- Must be written as their standard states.
7
Q
What do the two large arrows added together in combustion/ formation cycle add up to?
A
- They add up to equal the enthalpy change written on arrow from reactants to products.
8
Q
How would we work out enthalpy change given the enthalpies of formation?
A
- Draw formation cycle.
- Go around cycle with finger (from reactants to products), if you are going against an arrow in formation cycle, reverse the sign in front of the no associated with arrow.
- Add the two enthalpies of formation together to give enthalpy change of reaction.
9
Q
True or False
F2 has an enthalpy of formation of 67kJmol-1
A
- False.
- Elements have an enthalpy of formation of 0 (ie. can’t form an element from an element.)
