Collision Theory & Reaction Mechanisms. How molecules actually react/interact with one another in a chemical reaction is explained through Collision Theory . Collision Theory. Particles are in constant motion A chemical reaction involves the effective collisions of particles
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How molecules actually react/interact with one another in a chemical reaction is explained through Collision Theory.
ie. Sufficient energy with correct orientation
(alignment of molecules so bonds can be broken or formed)
In order for reactions to occur, molecules must collide with the right geometry (in order for the nuclei to bond) and with the right force of collision (activation energy).
Activation Energy is this “right amount of energy” needed for reaction to occur.
A slow reaction has a high
activation energy while a fast
reaction has a low activation energy.
Chemical reactions are not usually carried out in a single step process, as seen in a balanced chemical equation.
Eg. 2C8H18(l) + 25O2(g) 16CO2(g) + 18H2O(g)
In fact, many simple steps must be completed before the overall reaction can proceed. This series of steps (called elementary steps) in order for a reaction to occur is known as the reaction mechanism.
For example, nitrous oxide (laughing gas) decomposes into nitrogen and oxygen gases according to the balanced chemical equation:
2N2O(g) 2N2(g) + O2(g)
This implies that 2 molecules of N2O must collide with one another for reaction to occur (suggests 2nd order).
However, experimental data indicates that this reaction is 1st order in N2O (dependent on only 1 molecule of N2O). This means that the slowest step in the series of possible steps must be the decomposition of just 1 N2O molecule.
Proposed Mechanism for this reaction consists of 2 elementary steps:
1. N2O(g) N2(g) + O
2. N2O(g) + O N2(g) + O2(g)
Overall reaction:2N2O(g) 2N2(g) + O2(g)
rate = k[N2O]
and must be the rate determining step (slowest step)
Eg. Predict a reaction mechanism for the reaction:
X + 2Y + 2Z XY2Z2
Given the rate law expression: rate =k[Y]2[Z]
Y2Z and XY2Z are known as: