a) Propane is typically used for gas grills. Write a balanced combustion reactio

Question

a) Propane is typically used for gas grills. Write a balanced combustion reaction for propane (CH_3 CH_2 CH_3) and calculate the enthalpy of reaction at standard conditions. Note that propane is purchased as liquid (LPG) but combusted as a gas (expands upon exiting hose), other species are gas. b) Use CRC Handbook of Chemistry and Physics to look up the enthalpy of vaporization of propane. b) A typical propane tank is 20-lb, how many BTU's are present in a tank (use net of enthalpy from combustion less the enthalpy of vaporization)? If a grill has a 30,000 BTU/hr burner how many hours would the tank last?

Explanation / Answer

(a)

Combustion reaction is:

C3H8 + 5O2 ---> 3CO2 + 4H2O

Enthalpy of combustion of propane = -2202 kJ/mol

(b)

Enthalpy of vaporization of propane = 15.7 kJ/mol

(c)

Mass of propane taken = 20 lb = 9072 g

Moles of propane taken = Mass/MW = 9072/44 = 206.18 moles

Net enthalpy of reaction ( taking into account the utilised in vaporizing the liquid propane ) = -2202+15.7 = -2186.3 kJ/mol

So, heat released by combustion of 20 lb of liquid propane = 206.18*2186.3 = 450771.334 kJ = 427249 Btu

The number of hours the tank lasts is: 427249/30000 = 14.24 hrs

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