Enthalpy of CombustionCalculations

Example

Q. 4.6g of ethanol (C2H5OH) is burned. The energy released raised the temperature of 0.5kg of water by 20oC.

• What is the enthalpy of combustion of ethanol?

Answer

EH = c x m x ΔT

= 4.18 x 0.5 x 20= 41.8kJ

Therefore 1 mole C2H5OH = 46g (worked out from FM)

so, number of moles for 4.6g= 4.6/46= 0.1

mole C2H5OH

so, 0.1 mole C2H5OH releases 41.8kJ

1 mole C2H5OH releases 418kJ

i.e. enthalpy of combustion of C2H5OH is -418kJ mol-1 (negative is due to exothermic reaction).

Calculating ΔTUsing EH = c x m x ΔT

Q. 1g of butanol is burned and the energy used to heat 250cm3 of water.

Assuming all of the heat energy is transferred to the water, what would the temperature rise be?

Answer

Butanol: C4H9OH

FM =(4x12) +(9x1) + 16 + 1=48+9+16+1=74

1 mole of butanol=74g

n= m/FM= 1/74= 0.0135 moles

• Enthalpy of combustion of butanol = -2800kJ mol-1

So, 1 mole butanol 2800kJ0.0135 moles butanol 2800 x

0.0135=37.8kJ

i.e. EH= 37.8kJ

Mass of water= 0.25kg

ΔT= ?

EH = c x m x ΔT

Change the subject of the equation

ΔT = EH/(c x m)

=37.8 /(4.18 x 0.25)=37.8 / 1.047

ΔT =36.2oC

Calculating the Mass of Water Used

Q. 2g of methane was used to heat water. The temp of the water rose by 30oC, from 20oC to 50oC. What mass of water was heated?

EH = c x m x ΔT

1. Calculate EH

1 mole CH4=16g

N=m/FM= 2/16=0.125 moles

Data book: ΔH= -891kJ mol-1

So, 1 mole methane 891kJ0.125 moles methane 891x0.125= 111.375kJ

2. Calculate the Mass of Water

EH = c x m x ΔT

Change the subject of the equation

m= EH / (c x ΔT)

=111.375 /( 4.18 x 30)=111.375 / 125.4=0.89kg