potential and kinetic energy

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WWEKANE
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#1
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I know that electrons when about to react. Orbitals become hyberdized and this is closer to the nuclei and therefore at a Lower energy level then orbitals the electrons were previously at.

But why do we say there is a decrease in potential energy and increase in kinetic energy.

What is potential energy and kinetic energy in terms of the atoms reacting.

Im confused with this concept. Thanks
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Kian Stevens
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#2
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Electrons have both potential and kinetic energy; this is due to the Bohr model, i.e. electrons orbit nuclei at different radii.
An analogy for this is in every day life: imagine you're climbing to the top of a slide; you gain potential energy, as your radius from the earth's centre increases. As you go down the slide, you lose the potential energy as your radius decreases, and due to the first law of thermodynamics it's all converted into other form(s) of energy.

The same thing happens in the case of an electron in orbit.
As it lowers in energy levels, its potential energy decreases as it goes down some imaginary quantum slide (that was figurative ), and the energy from this is converted into electromagnetic radiation; this consists of heat, and even light.
However, the kinetic energy of an electron is entirely dependent on its radius in orbit, as smaller orbit radii correlate to larger angular velocities (and so a larger kinetic energy). Hence, as the electrons lower in energy levels and their orbit radii decrease, their velocities increase and so they 'gain' kinetic energy... I say 'gain', as they don't exactly 'gain' it, they just do have a higher kinetic energy due to their position in orbit causing them to have a larger angular velocity.

In terms of atoms reacting, it's simply just a case of electrons lowering in energy levels during hybridisation. This reduction in potential energy is what causes a system undergoing a reaction to release energy.
You may be able to see that with increasingly exothermic reactions, there's a larger overall potential energy being lost from electrons in atoms.
In terms of endothermic reactions though, more energy is absorbed by a system than what's released. For example, consider A \rightarrow B being endothermic. This must mean that the bonds in A are stronger and more stable than those in B. This is because more energy is absorbed by A to break its strong stable bonds, than what's released by B when the electrons reduce in potential energy and consequently form weaker, less stable bonds. Furthermore, this means that endothermic reactions are generally unfavourable, as you're converting a stable system into a less stable one.

Don't forget, hybridisation and all of this only applies to covalent systems.
With ionic systems, it's all different, as different energy changes happen for different reasons.
Last edited by Kian Stevens; 3 years ago
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WWEKANE
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#3
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#3
Thank you again sir. I would rep but I can't as I have recently done so.

When an atom gains energy eg by heat. Isnt this a form of kinetic energy. And if so why does this cause electrons to move up to a higher orbital.

Is KE converted into potential energy and for a molecule

Do the hyberized bonds when they gain energy change shape to form a new orbital shape at a higher energy level? I much appreciate all your help
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Kian Stevens
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(Original post by WWEKANE)
Thank you again sir. I would rep but I can't as I have recently done so.

When an atom gains energy eg by heat. Isnt this a form of kinetic energy. And if so why does this cause electrons to move up to a higher orbital.

Is KE converted into potential energy and for a molecule

Do the hyberized bonds when they gain energy change shape to form a new orbital shape at a higher energy level? I much appreciate all your help
If an atom is heated, then the atom itself gains kinetic energy and simply moves faster due to it having a greater velocity.
However, if an atom gains energy from say a gamma photon, then the whole atom absorbs the momentum of the photon, and consequently the atom transitions to a higher-energy 'excited' state.
Don't forget that orbitals are purely mathematical byproducts of solving the Schrödinger equation, and so they don't actually exist. They just give the probabilities of finding electrons at different orbit radii, and so if an atom is excited to a higher energy state, the electrons will be too; this means that they'll have a greater potential energy due to them orbiting in different radii, and so they'll be in a new orbital as they'll have a new range of probabilities for these new radii in this new quantum state.

There are only two possible shapes for the orbitals in an atom: when the atom's hybridised and when it's not.
When it's not hybridised and the atom's in its ground state, the orbitals exhibit standard orbital shapes, i.e. the s-orbitals look like s-orbitals, p-orbitals look like p-orbitals, etc.
However, when it hybridises, the orbitals merge to form new hybrid orbitals, which have their own set shapes with each hybridisation mode. They don't just form random shapes; they form the same shapes every time due to the LCAO (Linear Combination of Atomic Orbitals) theory. This is where orbital wavefunctions combine to form new hybrid orbital wavefunctions, so if the same orbital wavefunctions are combining each time in a certain hybridisation mode, then the same hybrid orbital wavefunctions will be formed each time too, resulting in the same hybrid orbital shapes.

Their geometries, i.e. angles between each hybrid orbital, may be different however.
For example, if you were to have a covalent bond -- which by default has already arisen due to hybridisation -- and you broke it (increasing in energy levels), the atom(s) would simply just change their hybridisation mode. The same thing happens if you were to form covalent bonds too; for example, if you were to have an sp2 hybridised carbocation and formed a new covalent bond (lowering energy levels), the hybridisation of that carbocation would change to sp3 in order to accommodate the new tetrahedral molecular geometry. Hence, the hybrid orbitals' shapes remain the same regardless of energy, but their geometries may be different.
Last edited by Kian Stevens; 3 years ago
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WWEKANE
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#5
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Thank you once again. I'm in year 12 so I'm trying to understand why these things happen rather just learning a fact hence why Im asking a lot. If you don't mind me asking when do you learn all this? Year 13 or university?
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Kian Stevens
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#6
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(Original post by WWEKANE)
Thank you once again. I'm in year 12 so I'm trying to understand why these things happen rather just learning a fact hence why Im asking a lot. If you don't mind me asking when do you learn all this? Year 13 or university?
If you're in year 12 then you'll have only learned about orbital basics, and so you won't need to know about hybridisation or wavefunctions or any other things I've mentioned. It's not even covered in year 13, so these things simply aren't on Chemistry specifications.
It's good that you're searching for answers though! You only learn by asking questions

All the stuff I've discussed such as hybridisation, as well as exploring orbitals in more detail and even learning about new types of orbital, is covered in the first year of university. It's all really interesting, but is fundamental knowledge.
Last edited by Kian Stevens; 3 years ago
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