Covalent structures and bonding
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We have been doing bonding between atoms in class. Is there any information I can find about the types of bonds in nitric acid and hydrochloric acid (e.g. hydrogen bonds or intermolecular forces present?) which affect the way the acids react with metals, like magnesium and zinc? What bonds hold the acid atoms together, and would it be right to say that the metal breaks those bonds to release hydrogen gas?
Igloo writes ...
You need to distinguish hydrogen chloride (the molecular gas) from hydrochloric acid. Also between concentrated nitric acid which is the (more or less) pure compound and dilute nitric acid which is a solution in which the acid is fully ionised. If you are exploring the reactions with metals you are likely to be working with quite dilute aqueous solutions of these strong (fully ionised) acids.
Have a look at the bonding in hydrogen chloride gas and in concentrated (pure) nitric acid. You appear to be studying Topic 7 of the Nuffield Advanced Chemistry course at present; you will find information about the bonding of hydrogen chloride on page 141 and of nitric acid on page 147 of the Nuffield Chemistry Students’ Book.
The molecule of hydrogen chloride consists of a hydrogen atom linked to a chlorine atom by a single covalent bond. Since the molecule is polar there will be strong dipole-dipole intermolecular attractions between these molecules.
The molecule of nitric acid is more complex. The diagram on page 147 shows that it appears to consist of a hydrogen atom, a nitrogen atom and three oxygen atoms all linked together by covalent bonds and a dative covalent bond. However, as the statement underneath the diagram indicates, “delocalization” occurs, whereby the electrons are evenly spread between the nitrogen atom and the two individual oxygen atoms (i.e. not the one linked to the hydrogen atom as well). If you are unsure what is meant by delocalization, you should not worry about it at this stage, because it will be explained more fully when you encounter benzene in Topic 12. With concentrated nitric acid there will be hydrogen bonding as well as dipole-dipole attractions. This is because the hydrogen atom is linked to an oxygen atom in the molecule. This intermolecular bonding is stronger than in hydrogen chloride, and explains why nitric acid is a liquid whereas hydrogen chloride is gaseous at room temperature.
When each of these pure compounds is brought into contact with water, IONS are formed – hydrogen ions and chloride ions, in the case of hydrogen chloride, and hydrogen ions and nitrate ions, in the case of nitric acid – and solutions of hydrogen chloride (better known as hydrochloric acid) and nitric acid are formed. It is at this “dissolving” stage that some covalent bonds are broken and others re-formed. Not surprisingly this process involves large changes of energy, and in both cases the enthalpy change is exothermic (negative).
i.e. HCl (covalent gas) with water -> H+(aq) + Cl–(aq)
HNO3 (covalent liquid) with water -> H+(aq) + NO3–(aq)
If a metal such as magnesium is dropped into (dilute) hydrochloric acid, hydrogen is formed when the mobile hydrogen ions exchange electrons with magnesium atoms (REDOX):
Mg(s) + 2H+(aq) -> Mg2+(aq) + H2(g)
Unfortunately, in the case of nitric acid, there is the added complication that the nitrate ions also take part in the process, oxidizing the hydrogen to water. In this process they themselves are reduced to oxides of nitrogen, such as nitrogen dioxide:
Mg(s) + 4H+(aq) + 2NO3–(aq) --> Mg2+(aq) + 2H2O(l) + 2NO2(g)
The point I am trying to make, though, is that in both cases the reaction between metals and acids involves an interaction between the magnesium atoms in the strip of metal and the mobile ions in the solution (not molecules of the “original” acids). The intermolecular forces between the molecules of the “pure” acids are of no relevance here.
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updated: 26 February 2004
