LabMouse Chemistry A2 | Kinetics AQA 3.4.1 | The relationship between rate equation and mechanism of reaction

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Labmouse Chemistry AS

LabMouse Chemistry A2

Kinetics AQA 3.4.1

Homogeneous and heterogeneous catalysis

Example of heterogeneous catalysis

Examples of Homogeneous Catalysis

Rate equations

The relationship between rate equation and mechanism of reaction

Practical: The iodination of propanone

Determination of the activation energy for a reaction

Review questions

Equilibria AQA 3.4.2

Acids and Bases AQA 3.4.3

Nomenclature and Isomerism AQA 3.4.4

Compounds containing the carbonyl group AQA 3.4.5

Aromatic Chemistry AQA3.4.6

Amines AQA 3.4.7

Organic Synthesis and Analysis AQA 3.4.10

Structure Determination AQA 3.4.11

Thermodynamics AQA 3.5.1

Redox Equilibria AQA 3.5.3

Redox Calculations AQA 3.5.4

LabMouse Physics AS

LabMouse Physics A2

LabMouse Reactions

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

2N_{2}O_{5} → 4NO_{2} + O_{2}

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 N_{2}O_{5} takes part in this rate determining step.

A possible mechanism could therefore be-:

Adding (a) and (b) gives 2N_{2}O_{5} → 4NO_{2} + O_{2}

Another possible sequence is

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 + 2H_{2} → H_{2}O + NH_{2} 2. NO + NH_{2} → H_{2}O + N_{2}

3rd order type slow step, but not in agreement with the rate equation.

3. NO + H_{2} → H_{2}O + ½N_{2} 4. NO + H_{2} → H_{2}O + ½N_{2}

Cannot have the same slow step and fast step!

5. 2NO + H_{2} → H_{2}O + N_{2}O 6. N_{2}O + H_{2} → H_{2}O + N_{2}

Slow step: 5. 2NO + H_{2} → H_{2}O + N_{2}O Fast step: 6. N_{2}O + H_{2} → H_{2}O + N_{2}

Slow step follows kinetics and overall both steps follow stoichiometry.