Hess’ cycle

Question 1

What is Hess’ Law?

Answer 1

• Hess’ Law is the idea that the enthalpy change for a chemical reaction is independent of the route taken at constant pressure. (it doesn’t matter which way you go- the enthalpy change will always be the same.)

() - extra info for clarification.

Question 2

What does a combustion cycle look like?

Answer 2

• 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!”

Question 3

What does a formation cycle look like?

Answer 3

• 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.”

Question 4

What do we use, from the Q, to determine whether we draw formation/ combustion cycle?

Answer 4

• 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.

Question 5

If Q asked for enthalpy of formation but gave you combustion data, which cycle would you draw?

Answer 5

• Draw a combustion cycle.

Question 6

What condition must the elements written at the bottom of formation cycle be written as?

Answer 6

• Must be written as their standard states.

Question 7

What do the two large arrows added together in combustion/ formation cycle add up to?

Answer 7

• They add up to equal the enthalpy change written on arrow from reactants to products.

Question 8

How would we work out enthalpy change given the enthalpies of formation?

Answer 8

• 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.

Question 9

True or False

F2 has an enthalpy of formation of 67kJmol-1

Answer 9

• False.
• Elements have an enthalpy of formation of 0. (ie. can’t form an element from an element.)