GC question

[299678]

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I am confusing for the part of negative sign for this question
Q. if the activation E of a RXN is 50 KJ and delta H is 15KJ. what is the activation of energy of the reverse reaction?

The answer is positive 35KJ

Could you explain me why it is positive 35, not -35? Thank you

 

[UndergradGuy7]

I am confusing for the part of negative sign for this question
Q. if the activation E of a RXN is 50 KJ and delta H is 15KJ. what is the activation of energy of the reverse reaction?

The answer is positive 35KJ

Could you explain me why it is positive 35, not -35? Thank you

I guess because you always need to add energy to overcome the barrier. This is what activation energy is, the energy needed to be added to overcome the hill. So a + sign, no matter if going forward or backwards. H, enthalpy, however changes. It is released or absorbed depending on whether u r going backwards or forwards. It shows that the products are lower or higher in energy than the reactants.

Also http://images2.wikia.nocookie.net/__cb20090211212307/gcse/images,/4/4f/Activation_energy.gif look at this diagram. The activation energy is the energy to go from the reactants to the top of the hill at the activated complex. So 50 kj in your problem. If the distance from the reactants to products is 15 kj, what is the distance from the products to the top of the hill? 50-15 = 35 kj. You need to add energy, activation energy, in both directions to overcome the hill. So a + sign.