Question

The decomposition of hydrogen peroxide is thought to occur in three steps:

step 1: H2O2(g) → 2 OH(g)

step 2: H2O2(g) + OH(g) → H2O(g) + HO2(g)

step 3: HO2(g) + OH(g) → H2O(g) + O2(g)

a- Write the overall reaction:

b- List any intermediates and/or catalysts and label which one they are

c- What is the molecularity of each step?

d- If the rate law is first order with respect to H2O2, which step is the rate-determining step?

When answering please show work (if possible) so I can have a better understanding. Thank you!

Answer 1

a- Write the overall reaction

For the overall reaciton, just add ALL the reacitons involved

H2O2(g) → 2 OH(g)

H2O2(g) + OH(g) → H2O(g) + HO2(g)

HO2(g) + OH(g) → H2O(g) + O2(g)

In the same side REACTANTS:

H2O2(g)+H2O2(g) + OH(g)+HO2(g) + OH(g) --> 2OH(g) + H2O(g) + HO2(g)+H2O(g) + O2(g)

cancel common terms

2H2O2(g) + OH(g)+HO2(g) + OH(g) --> 2OH(g) + H2O(g) + HO2(g)+H2O(g) + O2(g)

2H2O2(g) = 2H2O(g) + O2(g)

b- List any intermediates and/or catalysts and label which one they are

intermediates = will be consumed

catalyst= will not be consumed

H2O2(g) → 2 OH(g)

H2O2(g) + OH(g) → H2O(g) + HO2(g)

HO2(g) + OH(g) → H2O(g) + O2(g)

Note that OH is formed early, then consumes in the following reactions, these are the INTERMEDIATES

Note that HO2 is formed, and then appears once again, therefore this i s part of the CATALYST

c- What is the molecularity of each step?

Molecularity = number of molecules involded in each step

step 1:

only H2O2 is present = unimolecular

step 2:

H2O2 and OH are present, this is bimolecular ( 2molecuels are present)

step 3:

once again;

HO2 and OH are present, so it must be bimolecular

d- If the rate law is first order with respect to H2O2, which step is the rate-determining step?

if it is 1st order with respect to H2O2

then, the rate law must be that of the SLOWEST /DETERMINING step

from the reaction; it is most likely

Steps (elemental rate law)

step 1)

Rate = k*[H2O2]

step2)

Rate = k*{H2O2][OH]

step 3)

Rate = k *[HO2][OH]

Therefore, the best rate law is that ofr STEP 1