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# Kinetics question watch

1. The question is "Give one reason why most collisions between gas phase reactants do not lead to a reaction."
Surely in a gas particles move faster so collisions are more likely to lead to a successfull collision than reactants in liquid or solid phase??
Thanks
2. Nonetheless the answer is "most collisions have energy less than the activation energy of the reaction and thus the reaction cannot proceed". You are right in what you say about gaseous vs liquid/solid phase reactants but nonetheless gaseous particles mostly do not have nearly enough energy.
3. (Original post by MEPS1996)
The question is "Give one reason why most collisions between gas phase reactants do not lead to a reaction."
Surely in a gas particles move faster so collisions are more likely to lead to a successfull collision than reactants in liquid or solid phase??
Thanks

(Original post by Big-Daddy)
Nonetheless the answer is "most collisions have energy less than the activation energy of the reaction and thus the reaction cannot proceed". You are right in what you say about gaseous vs liquid/solid phase reactants but nonetheless gaseous particles mostly do not have nearly enough energy.
Angle of approach is also important, this leads to what is called the steric factor in simple collision theory.
4. Ok so even though gas particles move fast the activation energy for reactions in the gas phase is unusually high?
cheers
5. (Original post by Big-Daddy)
Nonetheless the answer is "most collisions have energy less than the activation energy of the reaction and thus the reaction cannot proceed". You are right in what you say about gaseous vs liquid/solid phase reactants but nonetheless gaseous particles mostly do not have nearly enough energy.
so do gases usually not have enough energy to react not because the gas particles themselves do not have much energy, but because the activation energy for reactions in the gas phase is usually very high?
Sorry for late reply I am revising and have got confused.
Thanks a lot.
6. (Original post by MEPS1996)
so do gases usually not have enough energy to react not because the gas particles themselves do not have much energy, but because the activation energy for reactions in the gas phase is usually very high?
Sorry for late reply I am revising and have got confused.
Thanks a lot.
The way I understand it, by the time you reach a temperature so high you have pushed the Maxwell-Boltzmann curve far enough to the right for most gaseous particles to have energy > activation energy, you'll have lost the potential for chemical reactions at all because your matter will be highly ionized (plasma?).
7. (Original post by Big-Daddy)
The way I understand it, by the time you reach a temperature so high you have pushed the Maxwell-Boltzmann curve far enough to the right for most gaseous particles to have energy > activation energy, you'll have lost the potential for chemical reactions at all because your matter will be highly ionized (plasma?).
OK thanks i think plasma is beyond AS AQA syllabus but thanks anyway. The key word is not 'gas phase' but 'most' i think
8. (Original post by MEPS1996)
OK thanks i think plasma is beyond AS AQA syllabus but thanks anyway. The key word is not 'gas phase' but 'most' i think
I can't think why plasma would be the domain of chemistry at all, much less the AS syllabus Completely ionized atoms won't be able to react ...
9. (Original post by Big-Daddy)
I can't think why plasma would be the domain of chemistry at all, much less the AS syllabus Completely ionized atoms won't be able to react ...
Completely discarding the complex field of astro chemistry there! Some very important chemistry takes place... Think of atmospheric chem in the high atmosphere.
(Original post by Big-Daddy)
The way I understand it, by the time you reach a temperature so high you have pushed the Maxwell-Boltzmann curve far enough to the right for most gaseous particles to have energy > activation energy, you'll have lost the potential for chemical reactions at all because your matter will be highly ionized (plasma?).
You don't need a large proportion of particles over the activation energy in order to get a good rate. Collisions constantly exchange the energies of the particles over time so all the particles will at some point have enough energy to react if they collide with another high energy particle.
(Original post by MEPS1996)
so do gases usually not have enough energy to react not because the gas particles themselves do not have much energy, but because the activation energy for reactions in the gas phase is usually very high?
Sorry for late reply I am revising and have got confused.
Thanks a lot.
Barriers are all relative. It is x joules above the ground state energy so doesn't matter what thet absolute energy is. Gases are in general high energy species (especially in terms of kinetic energy).

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10. (Original post by JMaydom)
Barriers are all relative. It is x joules above the ground state energy so doesn't matter what thet absolute energy is. Gases are in general high energy species (especially in terms of kinetic energy).

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The activation energy barrier increases as the average energy of the particles increases?! But I was taught that if you increased the temperature then a greater number of particles have energy > activation energy?
11. (Original post by MEPS1996)
The question is "Give one reason why most collisions between gas phase reactants do not lead to a reaction."
Surely in a gas particles move faster so collisions are more likely to lead to a successfull collision than reactants in liquid or solid phase??
Thanks
also bear in mind that atoms aren't ball shaped and have to collide at the correct orientation for a reaction to happen as well (actually if you delve into stat mech a bit you find that the probabilty of a collision occuring at the correct orientation decreaases as you increase the energy but the number of collisions increases which far outweighs this factor )
12. (Original post by Big-Daddy)
The activation energy barrier increases as the average energy of the particles increases?! But I was taught that if you increased the temperature then a greater number of particles have energy > activation energy?
What? Is that something you've been taught, or did you think I said that??? Activation energy is determined by the energy of the transition state relative to the ground state. To a reasonable approximation this should be constant with temperature.
To say that in general the energy barrier increases as the average energy of the particles increases seems very dodgy, if anything i would say the other way round. You do get what I mean by absolute energy right?

Yes at higher T more particles have the required energy, but you're thinking solely in terms of simple collision theory. Have a look at the eyring equation in contrast to SCT and you'll see just how basic the model you know is.

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