# OCR - Module 3 Topic Test Q14

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Thread starter 2 years ago
#1
Ahhhh !!!! I've been trying to answer his question and it's only 1 mark - I feel like I'm over complicating it ... somebody please HELP ME PLEASE😓😇 I've done the first part - I'm fine wth that . Bt how do you find the Ea for the reverse reaction with jus the Ea for the forward reaction ????

Hydrogen iodide, HI, is a colourless gas that can be made from the reaction of hydrogen, H2, and iodine, I2.
​This reversible reaction is shown in equilibrium 3.1 below.
​​H2(g) + I2(g) ⇌ 2HI(g)
​The activation energy for the forward reaction is 173 kJ mol–1.
​(a) ​Complete the enthalpy profile diagram below for the forward reaction in equilibrium 3.1.
​​​• ​Label the activation energy, Ea
​​​• ​Label the enthalpy change of reaction, DH
​​​• ​Include the formulae of the reactants and products.
​ [2]

​(b) ​Calculate the activation energy, Ea, for the reverse reaction.

​​​ Ea (reverse reaction) = ................................ ............ kJ mol−1 [1]
0
2 years ago
#2
(Original post by kavi1403)
Ahhhh !!!! I've been trying to answer his question and it's only 1 mark - I feel like I'm over complicating it ... somebody please HELP ME PLEASE😓😇 I've done the first part - I'm fine wth that . Bt how do you find the Ea for the reverse reaction with jus the Ea for the forward reaction ????

Hydrogen iodide, HI, is a colourless gas that can be made from the reaction of hydrogen, H2, and iodine, I2.
​This reversible reaction is shown in equilibrium 3.1 below.
​​H2(g) + I2(g) ⇌ 2HI(g)
​The activation energy for the forward reaction is 173 kJ mol–1.
​(a) ​Complete the enthalpy profile diagram below for the forward reaction in equilibrium 3.1.
​​​• ​Label the activation energy, Ea
​​​• ​Label the enthalpy change of reaction, DH
​​​• ​Include the formulae of the reactants and products.
​ [2]

​(b) ​Calculate the activation energy, Ea, for the reverse reaction.

​​​ Ea (reverse reaction) = ................................ ............ kJ mol−1 [1]
The activation energy for the reverse reaction is the activation energy for the forward reaction - the enthalpy change (forward)
0
Thread starter 2 years ago
#3
(Original post by charco)
The activation energy for the reverse reaction is the activation energy for the forward reaction - the enthalpy change (forward)
Thank you for replying. Oh but on the mark scheme the answer is given as 182 but here in the question the activation energy for the forward reaction is 173 . 😞
0
2 years ago
#4
(Original post by kavi1403)
Thank you for replying. Oh but on the mark scheme the answer is given as 182 but here in the question the activation energy for the forward reaction is 173 . 😞
Just look at the enthalpy profile diagrams for exo and endothermic reactions:

The enthalpy change is the difference between enthalpy of reactants and products, in this case negative as it is an exothermic reaction (forwards)

Ea(forward) = difference between reactants enthalpy and the transition state energy (top of the hump)

Ea(back) = Ea(fwd) - enthalpy change
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