Sapling Week 9/10 #17

[Scot Widjaja Dis 1J]

Can someone walk me through the steps of how to solve this problem? I'm confused with how to find the energy of the transition state.

A certain reaction has an enthalpy of delta H = 24 kJ and an activation energy of Ea = 41 kJ.
What is the activation energy of the reverse reaction?

Thanks!

[Faaizah Arshad 1H]

For this problem, I would suggest drawing a reaction profile or visualizing one.

First, the problem states that delta H = + 24kJ. This means the reaction is endothermic and would be represented as the distance from the energy of the reactants to the energy of the products on a reaction profile.

Next, the problem states that the activation energy for the forward reaction is 41 kJ. This is the distance from the energy of the reactants to to the peak (transition state) on a reaction profile.

If you draw this, you'll see that the activation energy for the forward reaction of this endothermic reaction is equal to the delta H plus the activation energy for the reverse reaction.

Ea (forward) = delta H + Ea (reverse)
41 = 24 + Ea(reverse)
Ea (reverse) = 17 kJ

[Heather Costa 2C]

the easiest way to think of this problem is Ea(reverse) = Ea(forward)- delta H which would give you the answer in kj

[Faith St Amant 3D]

Exactly what I did^^ Ea(reverse) = Ea(forward)-deltaH so in this case that would be Ea(reverse) = (41kJ) - (24kJ), which should give you the correct answer. Drawing a reaction profile helps to figure out this relationship conceptually.

[Nathan Tong 3G]

Right, for this problem all you need to do is subtract the enthalpy from the activation energy, since this is an endothermic reaction. This will give the activation energy of the exothermic reverse reaction.

[Valerie Doan 3I]

Here is an image that may help you visualize the problem. From looking at the pic, you can get the equation Ea(reverse)=Ea(forward) + delta H.