Enthalpy of Combustion Calculations. Example Q. 4.6g of ethanol (C 2 H 5 OH) is burned. The energy released raised the temperature of 0.5kg of water by.

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Enthalpy of Combustion Calculations

Example Q. 4.6g of ethanol (C 2 H 5 OH) is burned. The energy released raised the temperature of 0.5kg of water by 20 o C. What is the enthalpy of combustion of ethanol?

Answer E H = c x m x ΔT = 4.18 x 0.5 x 20= 41.8kJ Therefore 1 mole C 2 H 5 OH = 46g (worked out from FM) so, number of moles for 4.6g= 4.6/46= 0.1 mole C 2 H 5 OH

so, 0.1 mole C 2 H 5 OH releases 41.8kJ 1 mole C 2 H 5 OH releases 418kJ i.e. enthalpy of combustion of C 2 H 5 OH is -418kJ mol -1 (negative is due to exothermic reaction).

Calculating ΔT Using E H = c x m x ΔT Q. 1g of butanol is burned and the energy used to heat 250cm 3 of water. Assuming all of the heat energy is transferred to the water, what would the temperature rise be?

Answer Butanol: C 4 H 9 OH FM=(4x12) +(9x1) = =74 1 mole of butanol=74g n= m/FM= 1/74= moles

Enthalpy of combustion of butanol = -2800kJ mol -1 So, 1 mole butanol  2800kJ moles butanol  2800 x =37.8kJ i.e. E H = 37.8kJ Mass of water= 0.25kg ΔT= ?

E H = c x m x ΔT Change the subject of the equation ΔT = E H /(c x m) =37.8 /(4.18 x 0.25) =37.8 / ΔT =36.2 o C

Calculating the Mass of Water Used Q. 2g of methane was used to heat water. The temp of the water rose by 30 o C, from 20 o C to 50 o C. What mass of water was heated? E H = c x m x ΔT

1. Calculate E H 1 mole CH 4 =16g N=m/FM= 2/16=0.125 moles Data book: ΔH= -891kJ mol -1 So, 1 mole methane  891kJ moles methane  891x0.125= kJ

2. Calculate the Mass of Water E H = c x m x ΔT Change the subject of the equation m= E H / (c x ΔT) = /( 4.18 x 30) = / =0.89kg