In this chapter of A2/A-Level Chemistry, we will learn about predictions and its mechanism.
Predictions
The order of a reaction with respect to a reactant shows the number of molecules that the reactant is involved in with regard to the rate-determining step.
In A2/A-Level Chemistry, Rate determining step is the slowest step within a chemical reaction. The slowest step determines the rate of chemical reaction.The slowest step of a chemical reaction can be determined by setting up a reaction mechanisms. Many reactions do not occur in a single reaction but they happen in multiple elementary steps.
EXAMPLE: rate = k[X][Y]2. Here, one molecule of X and 2 molecules of Y will be involved in the rate determining step.
Chlorine free radicals in the ozone consist of 2 steps:
Cl•(g) + O3(g) —> ClO•(g) + O2(g) slow rate determining step
ClO•(g) + O•(g) —> Cl•(g) + O2(g) fast reaction
Therefore, Cl• and O3 must be in the rate equation as they are the reactants from the slowest step.
RATE = k[Cl•][O3]
In A2/A-Level Chemistry, the rate equation is derived by the slowest step in the reaction. When writing a rate equation you set up the equation by writing rate is equal to the rate constant of the slowest step times the concentrations of the reactant or reactants raised to there reaction order. Lets look at elementary step one.
elementary step one:
Here in this example rate=k1[NO2][F2].
Predicting Mechanisms
Once you know what the rate determining reactants are, you can think about what reaction mechanism it follows.
EXAMPLE:
If the rate equation is: rate = k[X][Y]
And the two different mechanisms are:
- X + Y —> Z
OR
- X —> Y + Z
In A2/A-Level Chemistry, from the rate equation, we know that X and Y MUST be in the rate determining step, therefore, it’s mechanism 1 which is the right one.
That's how predictions and its mechanism works. Good job!