Question

1.In Mackenzie and Garrels’ normative mass balance, it was assumed the input of chloride ions into the ocean was balanced by the precipitation of halite (i.e. sodium chloride) according to:

Na⁺+Cl^-<->NaCl

Calculate the free energy of reaction for the formation of halite in seawater with a salinity of 35 ppt. For thermodynamic info you may refer to: http://bilbo.chm.uri.edu/CHM112/tables/thermtable.htm

Mackenzie & Garrels argued that halite precipitation would not be observed in today’s ocean. Does your answer support or refute that argument? Explain.

Mackenzie & Garrels argued that halite precipitation could be observed in marginal seas which may have been more prevalent on the Earth in the past. The Dead Sea is the world’s saltiest water body with a salinity of about 337 ppt. Determine whether NaCl is precipitating in the Dead Sea.   

Answer 1

(a): Gibbs free energy for the given reaction is

G^o = Sum[G^o(products) - G^o(reactants)]

=> G^o = G^o(NaCl, s) - [G^o(Na+, aq) + G^o(Cl-, aq)]

=> G^o = - 384 kJ/mol - (-261.9 kJ/mol) - (-131.2 kJ/mol)

=> G^o = 9.1 kJ/mol

(b) Since the Gibbs free energy value is positive, the formation of halite is not feasible. Also the solubility of NaCl(s) is 359 g/L which is much more than the salinity (35 ppt of 35 g/L) of sea water. Hence the answer supports the argument.

(c): Given the salinity of sea water is about 337 ppt or 337 g/L. Since the salinity of dead sea is near to the solubility of NaCl, precipitation of NaCl will occur in dead sea at the bottom.