Chem Dimensions enthlapy question

[darkmaster25]

For those who have chem dimensions this question is from, pg 214 Q13(e)

A+2B --->C

In this reaction the total entlapy of the reactants is 80kJ/mol, the total enthlapy of products of -90kJ/mol and the activation evergy for the forward reaction is 120kJ/mol. (e) Calculate the activation energy of the reverse reaction C-->A+2B. I got 210kJ adding the 90kJ to get to 0 and the 120kJ. However the book gives 290kJ as an answer. I just want to make sure I am right, and If I am not why?

[Jenny_2108]

For those who have chem dimensions this question is from, pg 214 Q13(e)

A+2B --->C

In this reaction the total entlapy of the reactants is 80kJ/mol, the total enthlapy of products of -90kJ/mol and the activation evergy for the forward reaction is 120kJ/mol. (e) Calculate the activation energy of the reverse reaction C-->A+2B. I got 210kJ adding the 90kJ to get to 0 and the 120kJ. However the book gives 290kJ as an answer. I just want to make sure I am right, and If I am not why?

Delta H= -90-80=-170kJ/mol of forward reaction => reverse reaction is 170kJ/mol
Ea of reverse reaction = 120+170=290kJ/mol

[darkmaster25]

wait i don't understand. isn't the activation the number of kJ from the enthlapy of the reactants to the maximum which is at 120kJ, hence isn't the activation energy from -90 to 120. Why are you adding 170? I think I may be a little confused whith the concept, can you explain further.
Thanks

[Jenny_2108]

see the pic on this link
https://picasaweb.google.com/110812625607666851999/July42012#slideshow/5761257331791377842

[darkmaster25]

it says "Sorry, that page was not found."

[Jenny_2108]

Okay, the enthalpy of the reactants is 80kJ/mol and Ea is 120kJ/mol, so the maximum point of the graph is 80+120=200kJ/mol
Thus, the activation energy of inverse reaction is from -90 to 200, not 120. See the graph I draw, maybe it helps you somehow



P/S: Thanks Vegemite for helping me to post this pic LOL

[darkmaster25]

Okay thanks I get it, so we have to assume that the activation energy is 120kJ more than the total reactant energy?

[Jenny_2108]

Okay thanks I get it, so we have to assume that the activation energy is 120kJ more than the total reactant energy?

Yep, thats true. The Ea is from the point of reactants enthalpy to the highest point of the graph.
Glad that you got it