The gas phase decomposition of acetaldehyde.
CH3CHO(g)   CH4(g) + CO(g)

at 680 K is observed to follow the rate expression.

Rate-dCH3CHOdt = kCH3CHO3/2

If the rate of decomposition is followed by monitoring the partial pressure of the acetaldehyde we can express the rate as

-dPCH3CHOdt = kPCH3 CHO3/2

If the pressure is measured in atmosphere and time in minutes, then
(a) What are the units of the rate of reaction?
(b) What are the units of rate constant k?

Rate=-dPCH3CHOdt = kPCH3 CHO3/2

(i) Unit of rate of reaction

                 = AtmosphereUnit of time = atm min-1                              (If time is in minutes)


(ii) 
            k = Rate(PCH3CHO)3/2 = atm min-1(atm)3/2    = atm-1/2 min-1.
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(a) Show graphically how the rate of first order reaction vary with only one reactant depends concentration of reactant.
(b) Give one example of first order reaction.


(a)

(a)(b) Example of first order reaction is

(b) Example of first order reaction is
straight N subscript 2 straight O subscript 5 space rightwards arrow space space straight N subscript 2 straight O subscript 4 plus 1 half straight O subscript 2

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For the reaction:
   3H2(g) + N2(g)  2NH3(g)
express the rate of the reaction in terms of
    -dH2/dt,  -dN2/dt     and    dNH3dt.

The rate of reaction can be express by Dividing the individual rate expression by the coefficients in the balanced chemical equation.

   =-13dH2dt=-dN2dt=+12dNH3dt

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What is the initial rate method for the determination of a reaction rate when more than one reactant is present?

R1 = dxdt=kA1a B1b         ...(i)

R2 = k[A2]a B1b                     ...(ii)

Dividing (ii) by (i), we get

         R2R1 = A2A1a

From this a can be calculated

          R3 = kA1a B2b        ...(iii)

Dividing (iii) by (i)

           R3R11 =B2B1bThus b=1

From this we can calculate b.

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For the reaction:
NO2(g) + CO (g) → CO2(g) + NO(g)
the experimentally determined rate expression between 440 K is rate = k[NO2]2. What mechanism can be proposed for the above reaction?


NO2(g) + CO(g) → CO2(g) + NO(g)
The mechanism proposed for the above reaction involves two steps:

Step. 1.
 
 NO2(g) + NO2(g) slow NO(g) + NO3(g)


Step. 2.
NO3(g) + CO(g) fast  CO2(g) + NO2(g)


Overall reaction:
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NO2(g) + CO(g)     CO2(g) + NO(g)

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As we have given that  rate = k[NO2]2
 
Since concentration of CO(g) is not involved in the rate expression, so the overall rate of reaction is determined by the intermediate formation of NO3(g) i.e., Step 1.
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