# 2.2.8 Intermolecular Forces

**Bonding** --> refers to attraction that holds atoms ahd ions together

**Intermolecular forces** --> are forces that hold molecules together

* Molecular forces apply to molecular structure substances
* What forces keep water close together? Whats the fore that is overcome when the water molecules in ice break away and join the surrounding liquid water.
* Despite inter-molecular forces being weaker than chemical bonds --> affect physical properties such as Volatility, Solubility, and boiling point of molecular substances

<figure><img src="https://1345459981-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FU8XLVR7CYYLRlgazrP2A%2Fuploads%2FqAxnJcMgTlwIE5Mvudn7%2Fimage.png?alt=media&#x26;token=134f6057-2ed6-4890-ac54-723fdd53020f" alt=""><figcaption></figcaption></figure>

* When Molecular Substances boil, melt, or sublime --> intermolecular forces are overcome
  * Therefore melting and boiling points are indicators of intermolecular strength
* Note that covalent bonds do not break during phase changes

<figure><img src="https://1345459981-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FU8XLVR7CYYLRlgazrP2A%2Fuploads%2F5vaVpWOI58sqLTrqfmn6%2Fimage.png?alt=media&#x26;token=a26c0b7c-8cab-4be7-bafb-ee442fd9a9ff" alt=""><figcaption></figcaption></figure>

* Note that the ideal Gas law states assumes that there are no intermolecular forces in gases.  This is usually true for low pressure and high temperature gases, ( [Ideal gas equation ](https://knowmayus.gitbook.io/chemistry/structure/2.2-covalent-model/broken-reference)can be used )&#x20;

* But at lower temperatures it tends to deviate from it, and thus should be adjusted using a **real gas model**.  &#x20;

  * Take into account of actual volume of gas particles
    * Considered negligible in ideal gas model

*

```
<figure><img src="https://1345459981-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FU8XLVR7CYYLRlgazrP2A%2Fuploads%2Fr1lBz10BR0SGjTpC7CNZ%2Fimage.png?alt=media&#x26;token=6087520b-3fd9-404f-8ea3-7faed098609c" alt=""><figcaption></figcaption></figure>
```

**London ( dispersion ) Forces ( LDFs )**

* result from Instantaneous dipoles
  * Dipole when one side of molecule has partial charge and other the opposite charge
* Involves **induced** dipoles
  * One molecule causes / **induces** a  temporary dipole moment in another moelcule

    * Due to random movement of electrons around the molecule

    &#x20;

Take a simple non-polar example $$H\_2$$.  Note that the electron distribution is **on average equal**.  Due to the random movements of electrons, taking a picture of electrons in an atom would likely result in an unequal distribution of the electrons.

* This results in a slight negative charge ($$-\delta$$) on one side and a slight positive charge ( $$+\delta$$) in the other

  * Which results in a **dipole moment** in surrounding molecules

    * $$\because$$ the region of negative charge will repel other electrons, which creates a region of positive charge in the surrounding molecules, electrostatically attracting each other

      * These surrounding molecules then will have a partial negative charge on the other side and thus attract more surrounding molecules
      * Note this is temporary, and the next moment will have a different pattern of induced dipoles
      * This is the London ( Dispersion ) force

    <figure><img src="https://1345459981-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FU8XLVR7CYYLRlgazrP2A%2Fuploads%2F4xfQ2e3y0aDSaEel2Gtl%2Fimage.png?alt=media&#x26;token=56ca0638-d2f6-4ab1-aa6e-6b2931bc8f23" alt=""><figcaption></figcaption></figure>

    <figure><img src="https://1345459981-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FU8XLVR7CYYLRlgazrP2A%2Fuploads%2FNzhckefxrLGrCBxz1jRN%2Fimage.png?alt=media&#x26;token=f0d18b9e-68e6-4970-befa-7240bcf52a0f" alt=""><figcaption></figcaption></figure>

* Two factors affected LDFs

1. Number of Electrons
2. Molecular Shape

$$\because$$they affect the **polarizability** ( how easily the electron distribution is distorted by an electric field / Tendency to generate induced electric dipole moments ) of the molecule

* Greater the polariability--> stronger the LDF
  * $$P \propto LDF$$
* From the table, we see that when going down the group, more electrons --> Electron clouds become larger and less attracted to necleus --> More chance for electrons to get polarized and thus strength of LDF increases.

<figure><img src="https://1345459981-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FU8XLVR7CYYLRlgazrP2A%2Fuploads%2FzMzLZLMMuZgEQQgtCdiZ%2Fimage.png?alt=media&#x26;token=e7c1f21b-aacc-43d2-8873-43fc90f7a99e" alt=""><figcaption></figcaption></figure>

* For the **Homologous series** of Organic Compounds **(** 3.2 **),** the boiling point increases for successive members
  * Each successive compound has one more $$CH\_2$$molecule than the one before.  Hence greater molecular size and larger number of electrons.  &#x20;

* Note that the molecular size can quantified in terms of mass, so its often refer to molecular mass when comparing the LDFs of molecules.&#x20;
  * Note that electrons have negligible mass so how does this work?
    * Well its not **causal ;** instead a larger amount of electrons are often accompanied by a greater atomic mass --> each proton an electron to stabilise.&#x20;

* Compare two isomers ( each of two or more compounds with the same formula but a different arrangement of atoms in the molecule and different properties ): pentate, $$CH\_3CH\_2CH\_2CH\_2CH\_3$$ and 2,2-dimethylpropane $$(CH\_3)\_4C$$
  * $$\because$$same chemical formula, and are non-polar, only differing factor is **shape**

* Pentane is long --> better SA to interact with others--> Higher boiling point --> Liquid at room temperature

* 2,2 dimethylpropane is gas room temperature

**Dipole - induced dipole forces**

* LDFs are forces of attraction between temporary, or **instanteous** dipole moments in the molecule
* Dipole-induced dipole forces occur between a Polar molecule and surrounding non-polar molecules
  * A permanent dipole in a molecule makes temporary charges on surrounding non-polar molecules
    * $$O\_2$$, a non-polar, attract to water molecules which are polar
      * This is weak as seen from the low aqueous solubility

<figure><img src="https://1345459981-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FU8XLVR7CYYLRlgazrP2A%2Fuploads%2FarjWEpCiAX4lPKKqcDJv%2Fimage.png?alt=media&#x26;token=b7403754-f956-4f22-8964-43c81c4d7681" alt=""><figcaption></figcaption></figure>

<figure><img src="https://1345459981-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FU8XLVR7CYYLRlgazrP2A%2Fuploads%2FIW8gqKKd0YpIOPW2Od4B%2Fimage.png?alt=media&#x26;token=695e9571-2d1d-4d55-a1ef-6f7c2e551841" alt=""><figcaption></figcaption></figure>

**Dipole-dipole forces**

* where as LDFs involved temporary dipoles, dipole-dipole forces involve permanent ones ( polar molecules )
  * $$HCl$$, hydrogen Chloride, and Flourine, $$F\_2$$have similar sizes and comparable molecular masses -->  Experience similar LDFs force&#x20;
    * However $$\because$$$$HCl$$molecules are polar, it experiences dipole-dipole forces in addition to LDFs, which makes the boiling forces higher than Flou]rine as a result of the stronger intermolecular force

<figure><img src="https://1345459981-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FU8XLVR7CYYLRlgazrP2A%2Fuploads%2FMLnmHsvmt5PEPdFytdND%2Fimage.png?alt=media&#x26;token=c6db2da3-adba-46a9-b438-f058871d65de" alt=""><figcaption></figcaption></figure>

<figure><img src="https://1345459981-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FU8XLVR7CYYLRlgazrP2A%2Fuploads%2F4jDY5KKaRm3Uh1ZHwepl%2Fimage.png?alt=media&#x26;token=c7b08156-4936-4134-9f7b-d03961277235" alt=""><figcaption></figcaption></figure>

**Hydrogen Bonding**&#x20;

* Strong inter-molecular forces with molecules that forms when a molecule containing a strong dipole involving hydrogen
  * When Hydrogen **covalently** with an atom with high EN (  I.e. Oxygen, Nitrogen, or Flourine), the electron distribution will lean towards the more electronegative atom, leaving a big $$\delta+$$ partial charge on the hydrogen &#x20;
    * This hydrogen atom forms a strong attraction ( Hydrogen bonds ), with electrons of another electronegative atom
* Usually found in different molecules but intramolecular hydrogen bonds exist as well

<figure><img src="https://1345459981-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FU8XLVR7CYYLRlgazrP2A%2Fuploads%2FFrR4LAsOJrXTzzrwrX18%2Fimage.png?alt=media&#x26;token=c345fdb5-a332-4e19-beb0-f10cba45e0ee" alt=""><figcaption></figcaption></figure>

* Hydrogen bonds occur in :

1. water Molecules
2. ammonia ( $$NH\_3$$) molecules
3. hydrogen fluoride ( $$HF$$) molecules
4. water molecules and dimethy either molecules $$(CH\_3)\_2O$$

<figure><img src="https://1345459981-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FU8XLVR7CYYLRlgazrP2A%2Fuploads%2F4w0H8tgohVgWzDsC8Yyp%2Fimage.png?alt=media&#x26;token=80a04f2e-80e0-4153-a3fc-baacf9e84763" alt=""><figcaption></figcaption></figure>

* Stronger than other types of intermolecular forces, but much weaker than covalent&#x20;
  * Not actually a chemical bond&#x20;

$$LDFs < DiD < DD < HB$$

<figure><img src="https://1345459981-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FU8XLVR7CYYLRlgazrP2A%2Fuploads%2FLxPvwOvyWxOjtJfX1f6j%2Fimage.png?alt=media&#x26;token=1bae81fa-87c9-403b-8bdb-ba402f71fa0e" alt=""><figcaption></figcaption></figure>

* A way to remember this is to note the boiling of water
  * At $$100\degree$$C it is sufficient to remove the hydrogen bonds but not the covalent bonds between oxygen and hydrogen atoms

<figure><img src="https://1345459981-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FU8XLVR7CYYLRlgazrP2A%2Fuploads%2FXwB8DEuCJYMwsqV0npPz%2Fimage.png?alt=media&#x26;token=287806e8-8e36-44dc-8b6d-e6afe4ef0643" alt=""><figcaption></figcaption></figure>

* Looking at the group 14 Hydrides ( $$CH\_4, SiH\_4, GeH\_4, SnH\_4$$)
  * The molecular mass increases $$\therefore$$ more electrons and a strong polarization and $$\therefore$$ a higher boiling point
    * Seen in curve

* Looking at group 15 hydrides ( $$NH\_3, PH\_3, AsH\_3, SbH\_3$$
  * For the most part the latter pattern works except for Ammonia, $$NH\_3$$
    * This is due to hydrogen bonds
  * Similar trends with group 16 and group 17 hydrides with the outliers of high boiling points:  $$H\_2O$$and $$HF$$respectively

* A single water molecules can form up to 4 hydrogen bonds and therefore its unexpectedly higher than the Flourine hydride!

<figure><img src="https://1345459981-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FU8XLVR7CYYLRlgazrP2A%2Fuploads%2FaXgee1f7BS1nBAWNfPAv%2Fimage.png?alt=media&#x26;token=84d0a30a-8219-4174-93df-ecbba134cede" alt=""><figcaption></figcaption></figure>

* Hydrogen bonds give water its notable properties
  * Normally the solid form should be more dense than the liquid form as a liquid is more free-moving

* But Ice can float on water

  * Due to the excessive hydrogen bonding the ice contains well-ordered spaced out open-cavity network of molecules
    * This is less dense than the liquid water, as liquid water's free-moving property makes the molecule locations much more random and $$\therefore$$ more dense

* Note that there are many important applications of this unusual property --> for the formation of life
  * Due to ice being less dense than water, it floats up when water freezes
    * This ice acts like an insulator which keeps the liquid water underneath liquid..&#x20;
      * allowing aquatic ecosystems to survive

<figure><img src="https://1345459981-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FU8XLVR7CYYLRlgazrP2A%2Fuploads%2FU6re7kHmFESF9IFma2K0%2Fimage.png?alt=media&#x26;token=91652c9a-2596-41ba-9538-8f71b66a828a" alt=""><figcaption></figcaption></figure>

<figure><img src="https://1345459981-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FU8XLVR7CYYLRlgazrP2A%2Fuploads%2F6KBsytUKsuszTU0QJQX8%2Fimage.png?alt=media&#x26;token=57e4f143-58c4-4853-822a-635cb392df9d" alt=""><figcaption></figcaption></figure>

**Summary**

<figure><img src="https://1345459981-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FU8XLVR7CYYLRlgazrP2A%2Fuploads%2F49QENXHtdGR8Xst1yEFP%2Fimage.png?alt=media&#x26;token=fb351dc7-c2a5-4bc9-8908-d16cc40c3cb1" alt=""><figcaption></figcaption></figure>

<figure><img src="https://1345459981-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FU8XLVR7CYYLRlgazrP2A%2Fuploads%2F4TMmgAFRu0KR2JvZQG4P%2Fimage.png?alt=media&#x26;token=ec0079a7-a130-4da3-8456-1595bf5b5fa3" alt=""><figcaption></figcaption></figure>

**TOK**

* &#x20;DNA ( deoxyribonucleic acid ) molecules store genetic information
  * A key feature for DNA has to be an accurate genetic copying mechanism to pass on to future generations
* DNA has a double-helix structure that contains two strands of organic molecules held together by hydrogen bonds. &#x20;
  * When DNA needs copying, the hydrogen bonds are broken or "unzipped" to allow copies to be made

> *What other information storage system exists in the natural world? In Science? In other areas of knowledge??*
