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Chapter 5.7
Shapes of molecules & Intermolecular forces
| Question | Answer |
|---|---|
| What does VSEPR theory stand for | valence-shell electron-pair repulsion theory. |
| What do you use VSEPR theory for | We use a theory called VSEPR (pronounced ‘vesper’) theory to predict the shape of a molecule. |
| How does VSEPR theory work | As we know, atoms share electrons to form molecules. The VSEPR theory is based on the fact that electron pairs in the valence shell (the outermost energy level) repel each other. Since electron pairs repel each other, they’ll arrange themselves as far away from each other as possible. For instance, if a molecule has two electron pairs around its central atom, the electron pairs will position themselves as far apart as possible, resulting in a linear shape, with a bond angle of 180. |
| What are lone pairs and bond pairs of electrons | When two electrons are shared between atoms, they’re a bond pair of electrons. When two electrons aren’t shared and aren’t involved in a bond (i.e. belong to one atom only), they’re a lone pair of electrons. |
| what is the difference between lone pairs and bond pairs of electrons | Lone pairs have extra repulsion power – i.e. they repel other electron pairs to a greater extent than bond pairs do. Since lone pairs repel electron pairs to a greater extent, they’ll change the shape of the molecule. For example, if one molecule has four bond pairs around the central atom and another molecule has three bond pairs and one lone pair, then the two molecules will be different shapes (even though both molecules have four electron pairs around the central atom). |
| What are all the molecule shapes and how many electron pairs are there and what angle do they create | Linear→ Two electron pairs | Two bond pairs | Zero lone pairs | 180° Trigonal Planar→ Three electron pairs | Three bond pairs | Zero lone pairs | 120° Tetrahedral→ Four electron pairs | Four bond pairs | Zero lone pairs | 109.5° Trigonal Pyramidal → Four electron pairs | Three bond pairs | One lone pair | 107° Bent (V-shaped) → Four electron pairs | Two bond pairs | Two lone pairs | 104.5° |
| Are molecules actually 3d or 2d | 3d |
| what does polar mean | Polar indicates that a molecule has two poles of opposite charges. Electron density is focused at one end of the molecule, making it partially negative (δ−) and leaving the other side partially positive (δ+). |
| can a molecule be non polar even if it has polar covalent bonds | If a molecule has polar covalent bonds, it may be polar or non-polar. |
| What two characteristics must a polar molecule have | A polar molecule must have two characteristics: 1. It must contain polar covalent bonds. 2. The centres of positive and negative charge must not coincide (the molecule must not be symmetrical). Let’s have a look at some examples to explain this |
| What two characteristics must a non polar molecule have | A molecule will be non-polar if: 1. It contains only non-polar covalent bonds. 2. It contains polar covalent bonds but the centres of negative and positive charge coincide (the molecule is symmetrical). |
| What two shapes of molecules will always be polar | Molecules containing polar covalent bonds that are pyramidal or V-shaped will always be polar |
| What is the difference between intermolecular forces and intramolecular bonding | Intramolecular bonding refers to bonds inside a molecule (e.g. polar covalent bonds). Intermolecular forces are the forces between molecules. It’s important that this is very clear in your mind. |
| What are the 3 intermolecular forces we have studied | There are different types of intermolecular forces between molecules depending on the type of compound. The forces you will study are: 1. Permanent dipole-dipole forces (including hydrogen bonding) 2. London dispersion forces 3. Ion–dipole forces |
| What is a dipole | two poles of charge in the molecule: one partially positive, one partially negative. |
| What is permanent dipole–dipole force. | The slightly positively charged end of one molecule is attracted to the slightly negatively charged end of the other. This force of attraction between molecules is called a permanent dipole–dipole force. Since the δ+ and δ− charged poles are permanent (due to electronegativity differences between the atoms), these intermolecular forces are also permanent. Permanent dipole–dipole forces exist between the negative pole of one polar molecule and the positive pole of another polar molecule. |
| What is Hydrogen bonding | Hydrogen bonds are a special type of permanent dipole-dipole intermolecular force that occurs between the hydrogen of one molecule and the nitrogen, oxygen or fluorine of another. Atoms of nitrogen, oxygen and fluorine are all very small and highly electronegative. The large difference in electronegativity between H and N, O or F causes a very strong dipole. Essentially, hydrogen bonds are a stronger type of permanent dipole–dipole force. |
| Hydrogen bonds official meaning | Hydrogen bonds are a specific type of permanent dipole–dipole attraction between a hydrogen atom of one molecule and a small, highly electronegative atom (nitrogen, oxygen or fluorine) of a neighbouring molecule. |
| What are London-dispersion forces | London dispersion forces are weak, temporary forces of attraction between neighbouring molecules as a result of temporary dipoles (caused by the movement of electrons within a molecule). They’re still a force of attraction between the partially positive end of a molecule and the partially negative end of a neighbouring molecule, except that those partial charges are very short-lived, and therefore the attraction between them is also short-lived. |
| Which force is stronger between dipole dipole forces and London-dispersion forces | London dispersion forces are weaker than permanent dipole-dipole forces (when compared to comparable substances with the same or a similar number of electrons). This is because they’re temporary. |
| what are ion dipole forces | Ion-dipole forces are forces of attraction between an ion and a molecule with a dipole. |
| What type of ions will be attracted to positively and negatively charged ends of molecules | Opposites attract – cations will be attracted to the partially negative end of a polar molecule, while anions will be attracted to the partially positive end of a polar molecule. This force of attraction is an ion-dipole force. |
| What do ion dipole forces play a key part in | Ion-dipole forces play a key role in dissolving ionic compounds in polar solvents – e.g. a solution of sodium chloride (NaCl) in water. |
| What does the strength of ion-dipole forces depend on | The strength of ion-dipole attractions depends on the size of the charge on the ion; the strength of the dipole on the molecule; the distance between the ion and the dipole. |
| What effect do intermolecular forces have on melting and boiling points | If the particles have strong attractive forces between them – i.e. strong intermolecular forces – then molecules must gain a significant amount of heat energy in order to weaken those forces and change state. This means the compound would have high melting and boiling points. If the particles have weak intermolecular forces between them, a smaller amount of energy is required. Therefore, these substances would have lower melting and boiling points. |
| What two factors have an effect on the strength of intermolecular forces | 1. The type of intermolecular force 2. The molecular mass |
| List the intermolecular forces from weakest to strongest | London dispersion forces, Dipole-dipole forces, Hydrogen bonds, Ionic |
| How does molecular mass affect intermolecular force strength | The strength of intermolecular forces increases as the size of the molecule increases. This is because larger molecules have more electrons and can form stronger dipoles (dipoles with a greater imbalance of electrons). Stronger dipoles can induce stronger dipoles in neighbouring molecules. This increases the attraction between molecules – i.e. increases the strength of the intermolecular forces. |
| How do intermolecular forces affect solubility | Like dissolves like and Ionic in polar Ionic compounds like sodium chloride dissolve in polar solvents, e.g. salt in water, because the ionic bonding in NaCl is overcome by the strong attraction between the ions and the polar water molecules – i.e. ion-dipole forces. |
Created by:
21JulianM
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