overall equation is 3H2 + CO = CH4 + H2O

We are given the following LHHW-type mechanism:

1. CO + 2S C.S + O.S

2. H2 + 2S 2H.S

3. C.S + 3H.S CH3.S + 3S (rate-determining step)

4. CH3.S + H.S CH4 + 2S

5. 2H.S + O.S H2O + 3S

Solution given by expert

Now, we can write the rate expressions for each elementary step:

1. k1: CO + 2S C.S + O.S r1 = k1[CO][S]^2 - k1_reverse[_C][_O]

2. k2: H2 + 2S 2H.S r2 = k2[H2][S]^2 - k2_reverse[_HS]^2

3.k3: C.S + 3H.S CH3.S + 3S (rate-determining step) r3 = k3[_C][_HS]^3

MY QUESTION IS WHY STEPS 4 AND 5 FROM MECHANISM NOT USED DURING THE DERIVATION OF RATE EPRESSION FOR THE DISAPPEARANCE OF CO if step 3 is the rate determining step. You can write your answer using a pen or post the answer in form of a pic