© Chemistry in a Cup (some images I don’t own copyright.)© Chemistry in a Cup ‘David Dewar’ (some images I don’t own copyright.) Intermolecular Forces Unit 2.5

© Chemistry in a Cup (some images I don’t own copyright.) Intermolecular Forces Unit 2.5 London Forces a.k.a (Van Der Waals) Electrons in atoms or molecules are always whizzing around. Though they are not or equally situated around the atom or molecule. They’re often to one side. So sometimes dipoles are created. A positive and negative end. If you look at the diagram opposite. Dipoles are found around the molecule of hydrogen. When one of the molecules of hydrogen form a dipole, it effects other hydrogen molecules around then making them form dipoles. When two hydrogen have formed dipoles then an attraction is created between the two molecules forming a temporary bond. ( A London Bond ) With all these dipoles being created then more and more dipoles in more and more hydrogen molecules are then attracting one another. This could be seen as a domino effect. If one falls down the one in front falls down or in this case.... When 1 dipole is formed, one of the surrounding molecules then forms a dipole, and then attract each other..

© Chemistry in a Cup (some images I don’t own copyright.) Intermolecular Forces Unit 2.5 London Forces a.k.a (Van Der Waals) Name of GasBoiling Point °C Hydrogen-296 Neon-246 Argon-186 Krypton-152 Xenon-108 Radon-68 London Forces can effect the Boiling Point of Gasses too. The table shows the Boiling Point of all the Nobel gasses. As you can see as the size of the atoms getting bigger, so does the Boiling Point. But why? As you go down the group the number of electrons increase and so does the radius of the atom. The more electrons and the more distance to which they can move in, the bigger the possible temporary dipoles so bigger the London Forces. As the strength of the London Forces gets bigger, the more energy is needed to overcome the force so more energy is needed to break the bond.

© Chemistry in a Cup (some images I don’t own copyright.) Intermolecular Forces Unit 2.5 Dipole-Dipole Bonds From the last Topic ‘Unit 2.4’ you will know about electronegative atoms. δ+δ+ δ-δ- Due to the electronegativity of chlorine, the chlorine has the greater share of electrons do becomes slightly negative, and hydrogen slightly positive. δ+δ+δ+δ+δ-δ-δ-δ-δ-δ-δ+δ+ You should know that if you have a magnet, negative will want to come into contact with a positively charged object and ditto for the positive end. This is much the same with Dipole-Dipole Bonds. The positive end (the hydrogen) want to come into contact with a negative object (Chlorine) so then they are attracted to one another and form a temporary bond. Though these bonds aren’t always strong enough to keep the bond together. Sometimes the can stay together such as the think were going to look at next.

© Chemistry in a Cup (some images I don’t own copyright.) Intermolecular Forces Unit 2.5 Dipole-Dipole Bonds Dipole-Dipole Bonds are very important to use. Are DNA is made from lots of dipole bonds. What so good about Dipole Bond is that they are strong enough to keep all of our DNA together. But is weak enough to allow our DNA to split during Mitosis. (Replication of cells.) This shows that not all Dipole bonds are temporary but can properly bond lots of other molecules. And like London Forces.... Dipole-Dipole bonds also effects boiling and melting points. Though bet you guessed that already. δδ +

© Chemistry in a Cup (some images I don’t own copyright.) Intermolecular Forces Unit 2.5 Hydrogen Bonds Just looking at the title I but you can guess what this involves. Hydrogen... But involves a few more things. Hydrogen Bonding can ONLY happen when hydrogen covalently bonds with Fluorine, Nitrogen or Oxygen. In water you can see that there’s oxygen and hydrogen involved. One of the ‘F O N’ group and a hydrogen are the key ingredients for hydrogen bonds. In a hydrogen bond, the hydrogen has a +ve dipole and oxygen has a –ve dipole. This forms an attraction. Because Oxygen is so greedy for electrons when the 2 elements come together a bond is formed. Hydrogen Bonds are the Strongest Intermolecular Force. It nearly always makes a permanent bond.

© Chemistry in a Cup (some images I don’t own copyright.) Intermolecular Forces Unit 2.5 Hydrogen Bonds You can see hydrogen bonding in everyday life! Hydrogen Bonds are fairly long bonds in water. But when water turns solid to form Ice its completely different to any other compounds solid state. Often what a substance turns solid, its density increases because all the atoms are closely packed together. But Ice is different. When Water turns solid, more hydrogen bonds are formed. Because hydrogen bonds are fairly long. A lot of space between molecules is left, making Ice less dense than water so can float. Most solid states sink in their liquid forms. If water didn’t have hydrogen bonds all the ice would sink, resulting in our planet getting colder as the sun can heat up the ice on the seabed. So hydrogen bonds have helped us be alive on this planet today.