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I have 3 main questions that I am unsure of:

  1. Are intermolecular forces acting simultaneously causing attraction between particles in a sample of a substance? Example: Hydrogen bonding and dipole-dipole attraction present in a substance both influencing the attraction between particles.

  2. If there are multiple intermolecular forces present, would they be overcome in an order?

  3. What actually happens to the intermolecular forces between particles as a non-metallic molecule is heated?

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  • $\begingroup$ I'll try to answer this using $\ce{H2O}$ as example. 1) I didn't clearly understand what kinda overlap but in general sense will they overlap? Yes, a H-bond and any other intermolecular forces could overlap. 2) Yes, induced ones should be overcome easily. I'm not confident about the order so I'll not comment about that. 3) The forces become negligible because at higher temperatures the interaction becomes negligible due to high speeds. $\endgroup$
    – Desai
    Apr 24, 2021 at 11:44
  • $\begingroup$ Yup, just rewrote the question. Thanks for the answer. $\endgroup$
    – pothas
    Apr 24, 2021 at 11:49
  • $\begingroup$ From your first question I assume you're asking if the intermolecular forces are causing attraction between particles or can there be repulsion as well? For this you can find info on Wikipedia. There are attractive as well as repulsive intermolecular forces. But if as you mentioned the two simultaneous forces are only attractive, then yes only attraction is caused. But repulsive forces due to electron clouds always are in existence, we just neglect them for simplicity. $\endgroup$
    – Desai
    Apr 24, 2021 at 11:54
  • $\begingroup$ @Desay better speak of kinetic energy rather than speed, as it is the quantity of relevance. $\endgroup$
    – Alchimista
    Apr 24, 2021 at 15:26

2 Answers 2

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First of all, multiple intermolecular forces could be present between molecules, and, yes, all forces influence attraction, binding and etc. However, if you study any chemical system, you will see that is usually dynamic.

It all depends on conditions like temperature, pH and especially compound properties, etc. For example, the drug captopril (an antihypertensive drug) could form multiple bonds with the protein angiotensin-converting enzyme (ACE).:

Captopril

How about one specific atom of a molecule? In theory, one atom could form multiple intermolecular forces at a specific time (let's say picosecond), however, it is not practical - energetically unfavorable. Imagine that atom should be in such a position, an electronic configuration that reduces its potential energy and also form multiple intermolecular bonds. It is possible that such systems exist, but they are not common.

Second answer:

Yes, intermolecular forces have specific values of bonding energy: enter image description here

For example, it takes less energy to break London dispersion forces than to break hydrogen bonds.

Third answer:

It all depends on the level you want to know... Basically, when you heat molecules (give them more energy) they absorb it and move faster. Vigorous movements make bonds break. For example, water molecules at 100 °C start to move so fast that almost all its intermolecular forces are broken. Fun fact: you can smell lemons at room temperature because it has volatile molecules (molecules that form very few intermolecular bonds) called Limonene.

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The forces among molecules don't change with temperature; the kinetic energy of the particles is what changes

All the intermolecular forces acting on molecules contribute to the net forces making a liquid hold together. They vary in strength but they all matter in the aggregate liquid.

But the reason liquids boil isn't because the forces get weaker or change with temperature: it is because, at higher temperatures, the average molecule has more kinetic energy (this is essentially the definition of temperature). At any given temperature the momentum or kinetic energy of the molecules shows a distribution of values (some move very fast; some slow). There are a very large number of molecular interactions happening all the time that redistribute that energy among the molecules to give a particular statistical distribution of speeds. Evaporation and, eventually, boiling occur because a proportion of molecules have enough speed to escape the liquid and that proportion depends on the temperature. The intermolecular forces don't need to change at all for this to happen.

So, to answer the questions in order:

  1. Yes. All the forces contribute to the attraction between molecules
  2. No. The strength order doesn't matter, just the net forces. And they are not "overcome" during heating it is just that a larger proportion of the molecules have enough speed to escape from the liquid.
  3. Nothing. The net forces remain the same on heating. It is just the average kinetic energy that changes (and therefore the proportion of molecules in the liquid having enough energy to escape the net forces holding the liquid together.)
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