Delta H a state function. Calculate heat of combustion of methane by the pathway

Question

Delta H a state function. Calculate heat of combustion of methane by the pathways below: CH_4 + 2 O_2(g) rightarrow CO_2 (g) + 2 H_2 O(g) Delta H degree = ? Path 1: Direct measurement by isobaric calorimetry: Delta H_rxn = q_rxn = -(q_H20 + q_cal) 3-785 g of methane, CH_4, is burned in excess oxygen in a calorimeter containing 2.50 kg of water resulting in a temperature change from 24.7 degree C to 42.8 degree C. What is the heat of combustion of methane (kJ/mol.) (assume no heat loss to the calorimeter i.e q_col = 0; specific heat capacity of water is 4.18J/g degree C) Using Delta H's of the following related reactions: 2 O rightarrow O_2(g) Delta H degree = -249 kJ/mol 2 H(g) + O(g) rightarrow H_2 O(g) Delta H degree = -800 kJ/mol C(graphite) + 2 O(g) rightarrow CO_2 (g) Delta H degree = -643 kJ/mol C(graphite + 2 H_2 (g) rightarrow CH_4 (g) Delta H degree = -75 kJ/mol 2 H(g) rightarrow H_2 (g) Delta H degree = -434 kJ/mol Using standard heats of formation: Delta H degree = summation(n Delta H_f degree)_products - summation(n Delta H_f degree) _reactants

Explanation / Answer

CH2 + O2 ----> CO2+ 2H2O

1)

nO OF MOL OF ch4 = 3.785/16 = 0.2365 mol

q released = m*s*DT

   = 2.5*10^3*4.18*(42.8-24.7)

   = 189.145 kj

Dhrxn = 189.145/0.2365 = 800 kj/mol

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