The change in enthalpy for this reaction is -822kJ.
Here the negative sign indicates that the reaction is an exothermic and heat is to be released for the reaction to take place.
2 moles of Na(s) reacts with 1 mole of Cl2(g) to give 2 moles of NaCl(s) by releasing 822 kJ of energy.
a)
In this reaction 1 mole of NaCl(s) decomposes to 1 mole of Na(s) and 1/2 moles of Cl2(g).
If formation of NaCl is a exothermic reaction , i.e., energy is released, then decomposition of NaCl will be an endothermic reaction, i.e., energy is absorbed.
For endothermic reaction the change in enthalpy sign is positive.
If for 2 moles the change in enthalpy value is 822 kJ then for one mole the value will be:
Since it is an endothermic reaction the sign is positive.
Therefore, the change in enthalpy is +411kJ
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