Energy changes
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Hello
1 We notice that when we are dissolving NaOH in water the bottle will be hot (it is exothermic ) - why??Maybe due to formation of new bonds, but what is the source of this energy?
2 Hydrochoric acid dissolve in water - why? Maybe it forms dipole-dipole bonds, not hydrogen bonds?
3 Which one is the more soluble in water: ethanol or ethanoic acid?
Thank you very much.
Corrie writes ...
1. There are two energy factors to consider when an ionic substance, such as NaOH, dissolves in water. First of all, energy must be supplied to separate the ions in the crystal lattice. Where does this energy come from? Water molecules are little dipoles (one end slightly positive, the other end slightly negative) and are therefore attracted to + or - ions.
When an ion is surrounded by water molecules as it goes into solution, energy is released.
In the case of NaOH, the amount of energy released by this process (called hydration of the ions) is quite a bit more than the energy needed to separate the ions of the solid, so the 'excess' energy is released to the surroundings - the solution gets hot!
Other substances, e.g. ammonium nitrate, dissolve in water and cause a drop in temperature. This is because the energy needed here to separate the ions is more than the energy released when they are surrounded by water molecules - the solution gets cold.
2. Pure HCl (hydrogen chloride) is a gas, consisting of polar, covalent H-Cl molecules. When these molecules dissolve in water, they react with it, forming H+ and Cl- ions:
HCl + H2O -> H+(aq) + Cl-(aq)
This solution is called hydrochloric acid. The reaction goes completely over to the right - HCl is therefore a strong acid. The ions are strongly attracted by the polar water molecules. As a result HCl is very soluble in water.
3. Ethanol and ethanoic acid are both completely soluble in water in all proportions at room temperature. Both of them can from strong hydrogen bonds with water molecules, and their hydrophobic (water hating) hydrocarbon side chains are fairly short. Higher alcohols and acids become less and less soluble in water as the chain length increases.
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updated: 31 January 2007
