Step 1. H2O2 + H+ H3O2+ Step 2. H3O2+ + I- H2O + HOI Step 3. HOI + I- OH- + I2 S

Question

Step 1. H2O2 + H+ H3O2+

Step 2. H3O2+ + I- H2O + HOI

Step 3. HOI + I- OH- + I2

Step 4. OH- + H+ H2O

Step 5. I2 + I- I3-

How many of the following statements are correct based on the information given?

1) Correct: If step 2 is the rate determining step, and step 1 is a fast equilibrium step, this reaction would be third order overall

2) Incorrect: The reaction is first order with respect to H2O2

3) Incorrect: An energy diagram of this reaction would have 5 peaks and the starting materials would be higher in energy than the products

4) Correct: If step 1 were the rate determining step the order of the overall reaction would be second order.

These are all the correct answers already. I'm just having trouble figuring out why #3 is incorrect. I know there is 5 peaks, but wouldn't the reactants also have more energy than the products since reactions usually are spountaneous thus lower in product energy than reactant energy???

HELP

Explanation / Answer

Looking at the elementary reaction step one can't predict the energy of the reactant and product.

You are correct, if the reaction is spontaneous then product will have lower energy than reactant energy.

But these are 5 step reaction. If all the step is exothermic then, product will have lower energy than reactant energy.

In some intermediate step, the reaction is endothermic.

So, without looking at the thermodynamic data, its not possible to comment.

In this case some step, ( 1 to 5) is endothermic step, which requires energy and is greater than the sum of the rest steps which makes the overall reaction endothermic and makes energy of the product higher.

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