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The relationship between rate equation and mechanism of reaction

The decomposition of nitrogen(V) oxide in trichloroethane follows the stoichiometric equation

2N2O5 → 4NO2 + O2

The reaction is found by experiment to follow 1st order kinetics.

`text{Rate} = text{d} {text{[NO}_2text{]}}/text{dt} = text{k[N}_2text{O}_5text{]}`

Many reactions such as this take place in a number of steps. One of these steps will often be slower than the other(s). This slow step is known as the 'rate determining step'. The rate equation indicates that N2O5 takes part in this rate determining step.

A possible mechanism could therefore be-:

  1. `"N"_2"O"_5 stackrel "slow" rarr "O"_2 + "N"_2"O"_3`
  2. `"N"_2"O"_3 + "N"_2"O"_5 stackrel "fast" rarr 4"NO"_2`

Adding (a) and (b) gives 2N2O5 → 4NO2 + O2

Another possible sequence is

  1. `"N"_2"O"_5 stackrel "slow" rarr "NO"_2 + "NO"_3`
  2. `"NO"_3 + "N"_2"O"_5 stackrel "fast" rarr 3"NO"_2 + "O"_2`

Adding (c) and (d) gives the same overall equation.

The reaction between nitrogen(II) oxide and hydrogen is found to follow 3rd order kinetics-:

Molecular equation -: `2"H"_"2(g)" + 2"NO"_"(g)" rarr 2"H"_2"O"_"(g)" + "N"_"2(g)"`

Rate equation-: `"Rate" = "k[NO"_"(g)""]"^2"[H"_"2(g)""]"`

If the overall reaction takes place in two steps, suggest a possible mechanism which fits this information, by choosing from the following pairs of equations.

Click on the correct pair of equations to reveal the possible mechanism.

1. NO + 2H2 → H2O + NH2
2. NO + NH2 → H2O + N2

3. NO + H2 → H2O + ½N2
4. NO + H2 → H2O + ½N2

5. 2NO + H2 → H2O + N2O
6. N2O + H2 → H2O + N2

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