Question

240 cm3 of ethane (C2H6) was burnt in a controlled way and found to raise the temperature of 100 cm3 of water by 33.5 °C. (specific heat capacity of water = 4.18Jg–1K–1; 1mol of gas molecules occupies 24.0 dm3 at r.t.p.)

a. How many moles of ethane were burnt?

b. Calculate the heat change for the experiment.

c. Calculate the molar enthalpy change of combustion for ethane, as measured by this experiment.

d. Use the values below to calculate the standard molar enthalpy change for the complete combustion of ethane.

ΔHQf[CO2] = –394 kJ mol–1

ΔHQf[H2O] = –286 kJ mol–1

ΔHQf[C2H6] = –85 kJ mol–1

e. Give possible reasons for the discrepancy between the two results.

Answer 1

a. moles of ethane were burnt

 

At r.t.p 1 mole of ethane  = 24000cm3

 

240cm3  of ethane = 240 X 1

                                    24000

 

                               = 0.01 moles

b. heat change for the experiment

 

ΔH = m C Δ T

 

m = 100/1000 kg, c= 4.18,  Δ T= 33.5

 

    = 0.1 X 4.18 X 33. 5

 

   = 14.003kJ

c. molar enthalpy change of combustion for ethane

 

0.01 moles of ethane produce = 14.003 kJ

1 mole of ethane = 1 X 14.003

                                   0.01

 

                           = - 1400.3 kJ/mol

d. standard molar enthalpy change for the complete combustion of ethane

 

C2H6(g) + 7/2 O2(g) -------------> 2CO2(g) + 3H2O(g)

 

By Hess's law =  2(ΔHQf[CO2] = –394 kJ mol–1) + 3( ΔHQf[H2O] = –286 kJ mol–1) - (ΔHQf[C2H6] = –85 kJ mol–1)

 

 

                       = 2(-394) + 3(-286) -(-85)

 

 

                     = -1561kJ/mol

e.  possible reasons for the discrepancy between the two results

i. Heat energy absorbed by the apparatus or lost to the environment was not accounted for.

ii. Faulty apparatus.

iii. Wrong reading of temperature value.

 

 

 

 

 

 

 

 

 

 

 

 

 

possible reasons for the discrepancy between the two results

i. Heat energy absorbed by the apparatus or lost to the environment was not accounted for.

ii. Faulty apparatus.

iii. Wrong reading of temperature value.