When 0.813 g of sodium metal is added to an excess of hydrochloric acid, 8450 J of heat are produced. What is the enthalpy of the reaction as written? 2Na(s)+2HCl(aq)⟶2NaCl(aq)+H2(g)

Answer 1

Hello, Adoria,

The heat was produced by the reaction, so the value must be negative, representing an exothermic reaction. (The energy is removed from the reaction system). This is the amount of energy released by only 0.81 grams. We need to adjust the heat measure to account for how much sodium was reacted. We want an enthalpy that is expressed in J/g or J/mole. Joules/mole is the most useful expression, so calculate the number of moles of Na in this reaction (0.0353 moles). Then divide the -8450J of energy by this amount to yield -239,053 J/mole. Impressive! Stand back.

The next step is to make this kJ/mole, to better express this number (-239.05 kJ/mole).

I hope this helps,

Bob