#46 and #47 Sodium carbonate can be made by beating sodium bicarbonate: 2NaHCO_3

Question

#46 and #47

Sodium carbonate can be made by beating sodium bicarbonate: 2NaHCO_3 (s) rightarrow Na_2 CO_2(g) + H_2O(g) Given that Delta degree - 128, 9 kj/mol and Delta G degree - 33.1 kj/mol at 25 degree C, above what minimum temperature will the reaction becomes spontaneous under standard state conditions? 0.4 K 3.9 K 321 K 401 K 525 K Aluminum forms a layer of aluminum oxide when exposed to air which protects the bulk metal from further corrosion. 4(Al(s) + 3O_2 (g) rightarrow 2Al_2O_2(3) Calculate delta G degree for this reaction, given that delta G degree, of aluminum oxide is - 1576.4 kj/mol. -3152, 8 kj/mol -1576, 4 kj/mol -788, 4 kj/mol 1576, 4kj/mol 3152, 8 kj/mol

Explanation / Answer

Following relation is used to find out the temperature.

Delta G = Delta H – T delta S

We have the value of delta H and delta G but we delta S value is required.

Calculation of delta S using given values at T = 25 deg C ( 298.15 K )

33.1 kJ /mol= 128.9 kJ /mol – 298.15 K x ( Delta S ) kJ / K mol)

Delta S = 0.3213 kJ /K mol

When reaction is spontaneous, Delta G should be negative and we have to calculate the temperature above which it is spontaneous.

Therefore at this temperature, Delta G should be 0

Lets plug in the value to get Temperature

0 = Delta H – T x Delta S

T Delta S = Delta H

T = Delta H / Delta S = ( 128.9/0.3213) K = 401 K

Therefore the Temperature above which the reaction is spontaneous is “401 K “

47 )

Delta Grxn = n Sum ( Delta G product ) – n Sum ( Delta G reactant)

We know, Delta G is the free energy change and that is zero for species in its natural form.

O2( g) and Al(s) are in their natural form so their free energy change is zero and then delta G rxn = 0 – ( 2 mol x ( -1576.4 kJ /mol) =3152.8 k/mol

So the Delta G of the reaction would be 3152.8 kJ /mol

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