Note: If you aren't happy about enthalpy changes, you might want to explore the energetics section of Chemguide, or my chemistry calculations book. I can't find a value for the radius of a carbonate ion, and so can't use real figures. The lattice enthalpies of both carbonates and oxides fall as you go down the Group because the positive ions are getting bigger. If you worked out the structure of a carbonate ion using "dots-and-crosses" or some similar method, you would probably come up with: This shows two single carbon-oxygen bonds and one double one, with two of the oxygens each carrying a negative charge. What factors affect this trend? The ones lower down have to be heated more strongly than those at the top before they will decompose. Exactly the same arguments apply to the nitrates. The thermal stability/reducibility of metal nitrates in an hydrogen atmosphere has also been studied by temperature-programmed reduction (TPR). GROUP 2: THERMAL STABILITY OF THE CARBONATES AND NITRATES 1. a) Both barium carbonate and barium oxide (the product) are white. The lattice enthalpy of the oxide will again fall faster than the nitrate. It describes and explains how the thermal stability of the compounds changes as you go down the Group. Don't waste your time looking at it. The small positive ions at the top of the Group polarise the nitrate ions more than the larger positive ions at the bottom. GROUP 2: THERMAL STABILITY OF THE CARBONATES AND NITRATES 1. a) Both barium carbonate and barium oxide (the product) are white. The shading is intended to show that there is a greater chance of finding them around the oxygen atoms than near the carbon. Explain why the two nitrates have different stability to heat. As the positive ions get bigger as you go down the Group, they have less effect on the carbonate ions near them. Note: If you are working towards a UK-based exam (A-level or its equivalent) and haven't got copies of your syllabus and past papers follow this link to find out how to get hold of them. That implies that the reactions are likely to have to be heated constantly to make them happen. You need to find out which of these your examiners are likely to expect from you so that you don't get involved in more difficult things than you actually need. This decreases the charge density and the ability of the cation to polarize the anion. Thermal stability increases down the group because the size of the cation (positive ion) increases, so the lattice energy of the carbonate decreases, but the lattice energy of the oxide decreases faster. That implies that the reactions are likely to have to be heated constantly to make them happen. down the group as electro positive character increases down the group. All the nitrates in this Group undergo thermal decomposition to give the metal oxide, nitrogen dioxide and oxygen. In a Unit 2 question it asks: Calcium nitrate decomposes in a similar way to magnesium nitrate, but at ahigher temperature. The rates at which the two lattice energies fall as you go down the Group depends on the percentage change as you go from one compound to the next. The thermal stability of ring-substituted arylammonium nitrates has been investigated using thermal methods of analysis. I know stability increases as you go down group 2, please explain why in language a good A level student can understand. This is a rather more complicated version of the bonding you might have come across in benzene or in ions like ethanoate. The nitrates are white solids, and the oxides produced are also white solids. Since the ionic radius of the metal ion increases, this will reduce the distortion to the NO3^ - electron cloud. Forces of attraction are greatest if the distances between the ions are small. The argument is exactly the same here. 3. A bigger 2+ ion has the same charge spread over a larger volume of space. You can dig around to find the underlying causes of the increasingly endothermic changes as you go down the Group by drawing an enthalpy cycle involving the lattice enthalpies of the metal carbonates and the metal oxides. The effect of heat on the Group 2 nitrates All the nitrates in this Group undergo thermal decomposition to give the metal oxide, nitrogen dioxide and oxygen. That's entirely what you would expect as the carbonates become more thermally stable. If this is heated, the carbon dioxide breaks free to leave the metal oxide. All of these carbonates are white solids, and the oxides that are produced are also white solids. Unfortunately, in real carbonate ions all the bonds are identical, and the charges are spread out over the whole ion - although concentrated on the oxygen atoms. The nitrates are white solids, and the oxides produced are also white solids. A bigger 2+ ion has the same charge spread over a larger volume of space. You have to supply increasing amounts of heat energy to make them decompose. In other words, as you go down the Group, the carbonates become more thermally stable. The effect of heat on the Group 2 nitrates. Explaining the trend in terms of the polarising ability of the positive ion. \end{gathered}. Remember that the reaction we are talking about is: You can see that the reactions become more endothermic as you go down the Group. The nitrates are white solids, and the oxides produced are also white solids. In order to make the argument mathematically simpler, during the rest of this page I am going to use the less common version (as far as UK A level syllabuses are concerned): Lattice enthalpy is the heat needed to split one mole of crystal in its standard state into its separate gaseous ions. Therefore they are 2 2 If you think carefully about what happens to the value of the overall enthalpy change of the decomposition reaction, you will see that it gradually becomes more positive as you go down the Group. The effect of heat on the Group 2 carbonates. The explanation for change in thermal stability is the same as for carbonates Magnesium nitrate decomposes the easiest because the Mg 2+ ion is smallest and has the greater charge density. If this is the first set of questions you have done, please read the introductory page before you start. The nitrates are white solids, and the oxides produced are also white solids. See the oxygen also produced ) ca n't find a value for radius... Be heated more strongly than those at the problem if this is heated, ionic... Gas is given off together with oxygen question it asks: Calcium nitrate decomposes in fairly... - and -substituted derivatives is found to be linearly related to the Hammett substituent constant.... 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