group 2 carbonates nitrates and hydroxides all decompose if we heat them strongly enough the metal carbonates decompose to form carbon dioxide and the metal oxide so we have here for example magnesium carbonate decomposing to form magnesium oxide and CO2 the metal nitrate is decomposed to form the metal oxide nitrogen oxide and oxygen so here we have calcium nitrate breaking down to form calcium oxide nitrogen oxide and oxygen and finally the metal hydroxides they break down to form the metal oxide and water in this case strontium oxide from strontium hydroxide and H2O we can see here that regardless of whether we're looking at carbonates nitrates or hydroxides as we go down the group we are having to heat our compounds to higher and higher temperatures in order to force them to decompose as we go down the group these compounds are becoming more thermally stable so barium carbonate for example takes considerably more energy to force it to decompose compared with say magnesium carbonate the fact that each of these reactions requires an input of heat energy tells us that the overall enthalpy change of reaction is endothermic so the enthalpy change for this reaction is endothermic is going to be a positive value and logically if we're having to put more heat in to get our compounds to decompose as we go down the group these reactions must be coming more endothermic so Delta RH plus plus plus positive for the thermal decomposition of barium carbonate and this is what we're really trying to explain we want to figure out why these thermal decomposition reactions are becoming more endothermic as we go down the group there are essentially two parts to these decomposition reactions firstly we need to break up the metal carbonate or nitrate or hydroxide lattice so that we can form our metal oxide in each case part two is about decomposition of the carbonate ion to form an oxide ion and carbon dioxide in the case of the decomposition of carbonate in the case of the metal nitrates our nitrate ions are breaking down to form oxide ions and nitrogen oxide and oxygen and if we're talking about the metal hydroxides then again we're breaking down to form an oxide ion and water let's start by considering what's happening to the lattices the enthalpy change when we break up an ionic lattice into its ions is called the lattice enthalpy and theoretically the reaction that it refers to is well if we take magnesium carbonate for example breaking down to form magnesium ions in the gaseous state and carbonate ions also in the gaseous state we can see here the ionic lattice of magnesium carbonate we've got the Magnesium ions and we've got the carbonate ions now in the lattice we have got millions of strong electrostatic attractions between the two and if I want to break up this lattice then clearly I'm going to have to put a lot of energy in it's a highly endothermic process in fact in the case of breaking up magnesium carbonate we would actually need to put in 3123 kilojoules per mole now obviously if I'm forming a lattice from gaseous ions so my reaction is going from ions to lattice then that's going to be an exothermic process now it turns out that the lattice ends or piece of the metal oxides so here we've got magnesium oxide are always more endothermic than those of the carbonate here's our lattice for magnesium carbonate and the reason is simply that the oxide ion is so much smaller than the carbonate ion so the ions can pack more closely together into the lattice and as a result we have got stronger electrostatic attractions stronger ionic bonding in other words we can think of magnesium oxide as being more stable than magnesium carbonate if I want to break up magnesium carbonate as we've just seen the lattice enthalpy is plus three one two three kilojoules per mole if I want to break up magnesium oxide then the lattice enthalpy is plus three eight eight nine kilojoules per mole during a thermal decomposition reaction we are breaking up the magnesium carbonate atus but we are making a magnesium oxide lattice which means we need to reverse our lattice enthalpy we don't need to put energy in to this side of the equation essentially it's exothermic we're going to release 3889 kilojoules per mole when we make the magnesium oxide lattice from its ions in the gaseous State remember that this is mainly theoretical however it illustrates the purpose the difference between these two so enthalpy of reaction simply from going from the metal carbonate to the metal oxide in this case of magnesium is minus 766 kilojoules per mole if we do a similar calculation for the other group two metals I.E we figure out the difference in lattice enthalpies for breaking up the carbonate and making the metal oxide we find that it gets smaller as we go down the group the difference gets smaller so we've just seen for magnesium carbonate it was minus 766 kilojoules per mole but for barium carbonate turning to barium oxide it's around about minus 596 kilojoules per mole now it gets a little confusing because on the face of it it would seem that this reaction would be spontaneous at room temperature after all these are exothermic values metal carbonates shouldn't be stable at all but they are but of course we haven't yet considered the energy it takes to break up the anion whether it be the carbonate the nitrate or the hydroxide ion now we can s edimate the enthalpy change for each of these decomposition reactions in a couple of ways it always comes out endothermic we need to put in energy to break up our anion and this seems entirely reasonable we might think that this process is independent of the cation in the metal carbonate lattice I.E that whatever these values are whether the carbonate nitrate hydroxide answer when elatis is magnesium or barium why would that make a difference well as we go down group two the charge density of the cations gets less magnesium ions have a very high charge density that two plus charge is packed into a very small ionic radius this means that magnesium ions polarize the electron density of anions they're in a lattice with in this particular animation the carbonate ion this place is a strain on the carbonate iron which means it takes a little extra energy to break it down to oxide and carbon dioxide that might be expected barium ions which do not have a high charge density don't have the same effect let's finish by putting all this together in order to go from the metal carbonate to the metal oxide lattice and just that little bit there is exothermic but as we go down the group it becomes less exothermic breaking up the carbonate ion or the anion into an oxidan and carbon dioxide is endothermic now because overall each of these reactions is endothermic that tells me that this value here is greater than the amount of energy that's released when we move from the carbonate or the nitrate or the hydroxide lattice to the oxide lattice and we also know that in the case of magnesium carbonate because magnesium ants have a high charge density the carbonate ion is polarized so moving from carbonate iron to oxide and carbon dioxide takes less energy than we might expect the overall result is that as we go down the group these reactions are becoming more endothermic the thermal stability increases magnesium compounds are the least thermally stable and barium compounds are the most there's a link in the below below to the crunch chemistry website where you can find out more about the reactions of properties of group two metals and if this has been useful then please like share subscribe it makes a huge difference to a small Channel like us I look forward to seeing you next time