These vary depending on the size and shape of the molecule, but will always be far weaker than ionic bonds. It is essential to know what your examiners expect. In the first, with cold water, phosphorus oxychloride, POCl3, is produced along with HCl. Finally we can put this all together to compare the polarising power and pH trends of the Period 3 metal chlorides. Solid phosphorus(V) chloride contains ions - which is why it is a solid at room temperature. The most purely ionic of the magnesium halides is magnesium fluoride, because that has the greatest electronegativity difference between the magnesium and the halogen. Unfortunately, phosphorus(V) chloride is structurally more complicated. The coordination of the aluminium changes at these temperatures. Molten aluminium chloride (only possible at increased pressures) doesn't conduct electricity because there aren't any ions any more. Check out other A Level Chemistry Video Lessons here! Found this A Level Chemistry video useful? In the first stage, with cold water, phosphorus oxychloride, POCl3, is produced Chemistry Guru | Making Chemistry Simpler Since 2010 | A Level Chemistry Tuition | Registered with MOE | 2010 - 2019, pH of Period 3 Metal Chlorides in Aqueous Medium, As we move across Period 3, there is an increase in charge from Na, Polarising Power and Distortion of Electron Cloud, This makes it easier to break the O-H bonds and release more H, Chemistry Guru | Making Chemistry Simpler Since 2010 |. In this discussion we want to focus on the pH of Period 3 metal chlorides. They change their structure from ionic to covalent when the solid turns to a liquid or vapour. Before you go on, you should find and read the statements in your copy of the syllabus. Join my 1000+ subscribers on my YouTube Channel for new A Level Chemistry video lessons every week. At temperatures around 180 - 190°C (depending on the pressure), aluminium chloride coverts to a molecular form, Al2Cl6. It becomes 4-coordinated - each aluminium now being surrounded by 4 chlorines rather than 6. These equilibria (whichever you choose to write) lie further to the right, and so the solution formed is more acidic - there are more hydroxonium ions in it. In a large excess of water, the hydrogen chloride will, of course, dissolve to give a strongly acidic solution containing hydrochloric acid. Sodium chloride simply dissolves in water to give a neutral solution. Magnesium chloride dissolves in water to give a faintly acidic solution (pH = approximately 6). The fact that there aren't many of them formed (the position of equilibrium lies well to the left), means that the solution is only weakly acidic. In this case the electron cloud of water molecules will be distorted to a greater extent which weakens the O-H bonds in water. All the others react violently with water in a strongly exothermic reaction to give steamy fumes of hydrogen chloride gas, and produce acidic solutions. Some examiners simply say that magnesium chloride just dissolves in water. The hydrogens are made rather more positive than they would otherwise be, and more easily pulled off by a base. Sodium chloride The phosphorus chlorides Period 3 chlorides and water Silicon tetrachloride There are two phosphorus chlorides - phosphorus (III) chloride, PCl3, and phosphorus (V) chloride, PCl5. Before you go on, you should find and read the statements in your copy of the syllabus. This causes it to melt or vaporise because there are now only comparatively weak intermolecular attractions. There is much more about this later on this page. © Jim Clark 2010 (last modified January 2018). The reaction happens in two stages. If you add solid aluminium chloride to an excess of water, it still splutters, but instead of hydrogen chloride gas being given off, you get an acidic solution formed. Magnesium chloride is also ionic, but with a more complicated arrangement of the ions to allow for having twice as many chloride ions as magnesium ions. Sodium chloride is a simple ionic compound consisting of a giant array of sodium and chloride ions. Phosphorus(V) chloride is a white solid which sublimes at 163°C. Leaving aside the aluminium chloride and phosphorus(V) chloride cases where the situation is quite complicated, the attractions in the others will be much weaker intermolecular forces such as van der Waals dispersion forces. As an approximation, the simple ionic chlorides (sodium and magnesium chloride) just dissolve in water. The other chlorides are all simple molecular compounds involving covalent bonding. To return to the list of learning outcomes in Section 9, To return to the list of all the CIE sections. The others are simple covalent molecules. You may also find the last equation in a simplified form: Hydrogen ions in solution are hydroxonium ions. The reaction with water is known as hydrolysis. Reaction of the elements with water. Because there are only van der Waals dispersion forces between these, it then vaporises. As you will see later, aluminium chloride exists in some circumstances as a dimer, Al2Cl6. They suggest comparatively weak attractions between molecules - not strong attractions between ions. As with the other covalent chlorides, if there is enough water present, these will dissolve to give a solution containing hydrochloric acid. Sodium and magnesium chlorides are simple ionic chlorides with giant ionic structures. Need an experienced tutor to make Chemistry simpler for you? Phosphorus(III) chloride reacts violently with water. Hence charge density and polarising power increase across Period 3. That extra charge pulls electrons from the water molecules quite strongly towards the aluminium. You will see that this is very similar to the magnesium chloride equation given above - the only real difference is the charge on the ion. The reaction of aluminium chloride with water is dramatic. A small representative bit of a sodium chloride lattice looks like this: This is normally drawn in an exploded form as: The strong attractions between the positive and negative ions need a lot of heat energy to break, and so sodium chloride has high melting and boiling points. A chloride ion transfers from one of the original molecules to the other, leaving a positive ion, [PCl4]+, and a negative ion, [PCl6]-. You should obviously check your syllabus, but you also need to look at past papers and mark schemes so that you know what your examiners are actually asking.
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