hess' law Flashcards
What is hess’s law?
The enthalpy change of a reaction is independent of the route taken
What are enthalpy cycles?
An enthalpy cycle is a pictorial representation showing the alternative routes of reaction between reactants and products
Why do we use enthalpy cycles?
The enthalpy change of a reaction can’t always be direct;y measured so we use enthalpy cycles to help us indirectly calculate the enthalpy change
What are some conditions of enthalpy cycles?
- they are triangular
- in each corner there should be the same umber of each atom as well as energy, matter is also conserved .
- the sides of the triangle are arrows
What is the enthalpy cycle (combustion) using the equation below?
[enthalpy change of the reaction]
6C(s)+ 3H2(g)-> C6H6(l)
Carbon and hydrogen can be combusted to form CO2 +H2O
Benzene can be combusted to form CO2+H2O
Delta H 6C(s)+ 3H2(g)→→→ C6H6(l)
[pretend there are arrows here, A and B, A from the 6c to the 6co2 and B from the C6 to the 3h2o]
6CO2(g)+3H2O(l)
Delta H= A-B (vectors) KJ/mol C6H6 : -3267 C:-394 H2: -286
Calculating A, you know what just look at 4:19 of
https://www.youtube.com/watch?v=f0B3zPJPk2U&list=PLkocNW0BSuEFvnpnhj8fKN-KFUOlnKiT0&index=16&t=183s
What are enthalpy cycles basically
Vectors but not fun
What is the standard enthalpy of formation (define)
The enthalpy change when one mole of a substance is formed from it’s constituent elements in their standard states under standard conditions
The experimentally determined values for the enthalpy change of combustion does not always agree with values in the data booklet. Suggest 2 reasons why?
This could be because non-standard conditions were used, incomplete combustion of the glucose may have also occurred
Glucose can be formed according to the equation below: 6C+6H2+3O2→C6H12O6
The enthalpy change of formation of glucose cannot be determined directly.
Suggest a reason why this is the case.
- Rate is too slow
- activation energy too high
- CO2 and H2O formed instead
