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+1 vote
3.5k views
in Chemical thermodynamics by (64.7k points)

The thermochemical equation for solid and liquid rocket fuel are given below : 

2Al (s) +1 1/2 O2 (g) →Al2 O3 s ;

ΔH= -1667.8 kJ mol-1

H2(g) +1/2 O2(g) →H2O(l);

ΔH= -285.9 kJ mol-1

(a) If equal masses of aluminium and hydrogen are used, which is a better rocket fuel ? 

(b) Determine ΔH for the reaction :

Al2O3(s) 2Al (s) +1 1/2 O2 (g).

1 Answer

+2 votes
by (58.3k points)
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Best answer

(a) From the first given equation,

2 moles of Al (i.e., 2 × 27 g = 54 g) on combustion give heat = 1667.8 kJ

∴1 g of Al on combustion gives heat = (1667.8/54) = 30.9 kJ

From the second given equation, 1 mole of H2 (= 2g) on combsution gives heat = 285.9 kJ

∴1 g of H2 on combustion gives heat = (285.9/2) = 142.95 kJ

Thus, H2 is a better rocket fuel.

(b) Writing the reverse of the first reaction, we have

Al2O3 (s)→ 2Al (s)+ 1 1/2 O2  ΔH= +1667.8 kJ mol-1 

Thus, for the reaction given in part (b) of the problem, ΔH = + 1667.8 kJ mol–1

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