Question

The gas ethane, C2H6(g), can be used in welding. When ethane is burned in oxygen, the reaction is: 2 C2H6(g) + 7 O2(g)------>4 CO2(g) + 6 H2O(g)

(a) Using the following data, calculate ΔH° for this reaction. ΔH°_f kJ mol^-1:   C₂H₆(g) = -84.0 ; CO₂(g) = -393.5 ; H₂O(g) = -241.8

ΔH° = ___________ kJ

(b) Calculate the total heat capacity of 4 mol of CO₂(g) and 6 mol of H₂O(g), using C_CO2(g) = 37.1 J K^-1 mol^-1 and C_H2O(g) = 33.6 J K^-1 mol^-1.

C = _____________ J K^-1

^{(c) When this reaction is carried out in an open flame, almost all the heat produced in part (a) goes to raise the temperature of the products. Assuming that the reactants are at 25°C, calculate the maximum flame temperature that is attainable in an open flame burning ethane in oxygen. The actual flame temperature would be lower than this because heat is lost to the surroundings.}

^{Maximum temperature = _____________°C}

Answer 1

(a) Consider the following reaction-

2 C₂H₆(g) + 7 O₂(g)------> 4 CO₂(g) + 6 H₂O(g)

ΔH°_f (kJ mol-1) :   C₂H₆(g)) = -84.0 ;CO₂(g) = -393.5 ; H₂O(g) = -241.8

ΔH°_rxn = (4 x  ΔH°_f CO₂(g) + 6 x ΔH°_f H₂O(g) ) -(2 x ΔH°_f  C₂H₆(g) + 7 x  ΔH°_f O₂(g))

ΔH°_rxn =(4 x -393.5 + 6 x -241.8 )-(2 x -84.0 + 7 x 0 ) kJ mol-1

= (-3024.5 + 168.0 +0 )

ΔH°_rxn = -2856.8 kJ

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(b)

Heat Capacity = ∑ (no of moles x molar specific heat )

=( 4 x C_CO2(g) + 6 x C_H2O(g) )

=( 4 mol x 37.1 J K-1 mol^-1 + 6 mol x 33.6 J K-1 mol^-1 )

Heat Capacity = 350 J K^-1

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(c)

ΔH°_rxn =heat capacity x ( T_max - T_min )

( T_max - T_min ) = ΔH°_rxn / heat capacity

= 2856.8 kJ / 350 J K^-1

=2856800 /350

  T_max =8162.3 K +( 25 +273.15)

  T_max = 8461.02 K