So, from the cycle we get the calculations directly underneath it. The diagram is set up to provide two different routes between the thick lines. Now we can use Hess' Law and find two different routes around the diagram which we can equate. And finally, we have the positive and negative gaseous ions that we can convert into the solid sodium chloride using the lattice formation enthalpy.Remember that first electron affinities go from gaseous atoms to gaseous singly charged negative ions. The -349 is the first electron affinity of chlorine.Again, we have to produce gaseous atoms so that we can use the next stage in the cycle. The +122 is the atomization enthalpy of chlorine.Remember that first ionization energies go from gaseous atoms to gaseous singly charged positive ions. The +496 is the first ionization energy of sodium.We have to produce gaseous atoms so that we can use the next stage in the cycle. The +107 is the atomization enthalpy of sodium.The Born-Haber cycle now imagines this formation of sodium chloride as happening in a whole set of small changes, most of which we know the enthalpy changes for - except, of course, for the lattice enthalpy that we want to calculate. The arrow pointing down from this to the lower thick line represents the enthalpy change of formation of sodium chloride. Notice that we only need half a mole of chlorine gas in order to end up with 1 mole of NaCl. We are starting here with the elements sodium and chlorine in their standard states. If you wanted to draw it for lattice dissociation enthalpy, the red arrow would be reversed - pointing upwards.įocus to start with on the higher of the two thicker horizontal lines. You will see that I have arbitrarily decided to draw this for lattice formation enthalpy. Apart from catering students preparing for JEE Mains and NEET, PW also provides study material for each state board like Uttar Pradesh, Bihar, and others.\)Ĭonsider a Born-Haber cycle for sodium chloride, and then talk it through carefully afterwards.
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