![]() ![]() If the formula of the ionic compound has a 1:2 ratio between elements (eg MgCl 2 or Na 2 O), then you must double the relevant enthalpy changes in steps 1, 2 or 3. Note that, unlike the first electron affinity, it will be endothermic due to repulsion between the second electron and the negative ion. If the non-metal atom gains two electrons (eg O 2-), then add the second electron affinity to step 3. If the metal atom loses two electrons (eg Mg 2+), then add the second ionisation energy to step 2 after the first ionisation energy. The Born Haber cycle for the formation of calcium chloride from its constituent elements involves following steps.Ca(s) Ca(g) Ha°(Ca) 178 kj/mol.Ca(g) Ca+(g) + e- HIE° 590 kj/mol.½ Cl2(g) Cl(g) Ha° ½ HCl-Cl☁21 kj/mol.Cl(g) + e- Cl-(g) Hea° -364. ![]()
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