8. When solid NH4NO3 is placed in water, it dissolves:chemical AHr (kJ/mol) 0 39

Question

8. When solid NH4NO3 is placed in water, it dissolves:chemical AHr (kJ/mol) 0 393.5 0 -208.4 824.2 167.2 0 -241.8 285.8 0 80.3 365.6 132.5 -205.0 NH4NO3 (s)NH4 (aq) + NO3 (aq) CO2 Fe(s) C8H180) Fe203(S HCI(a Calculate the heat of the dissolution reaction from the standard heats of formation, tabulated at right. Is this reaction endothermic or exothermic? How do you know? The temperature of the water used for the dissolution reaction was 22°C before the solid was added. As the solid dissolves, would you expect to see the temperature decrease or increase? Explain what information you used to make this decision and how you used the information. a. b. H20 H2000 NH3 NH4NO3s NH4 (a NO3 (a

Explanation / Answer

Enthalpy of dissolution = Enthalpy of products - Enthalpy of reactants

If this is negative, then the reaction is exothermic. If it is positive, then the reaction is endothermic.

Here, enthalpy of products = sum of enthalpy of formation of NH4+ and NO3 -

Therefore, Enthalpy of products = -132.5 + (-205.0) kJ/mol

= -337.5 kJ/mol

Hence, enthalpy of dissolution = -337.5 - (-365.6) kJ/mol

= 28.1 kJ/mol

As enthalpy of dissolution is positive, it is an endothermic reaction, i.e., energy is absorbed.

Due to this, the temperature of water would reduce from 22°C.

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