chemistry help!

[alay]

could someone pls explain no.13 of http://www.xtremepapers.net/CIE/Cambrid ... 5_qp_1.pdf

 

[hana217]

The correct ans is A.Activation energy is the minimum energy needed to start a reaction.Since it is endothermic reaction, Activation energy is between H3 and H1 so to get the value we need to minus H3 and H1.But for exothermic,we need to minus H3 and H2.
sorry don't know how to explain the part c and d..hope my explanation helps..

 

[alay]

thanks..but im wondering y isnt it C..

 

[carolinetcw]

thanks..but im wondering y isnt it C..

because the reaction is endothermic. H1 - H3 will give you a negative ethalphy change which would mean exothermic

 

[Silent Hunter]

its A .