ai) In an experiment to determine the enthalpy change of combustion of ethanol, a calorimeter containing 200.0g of water was used. Burning 0.185g of ethanol caused the temperature of the water in the calorimeter to rise by 6.0 degree celsius.ii) Assuming that the heat capacity of the calorimeter is negligible, calculate the enthalpy change of combustion of ethanol, in kJ mol^-1, under the conditions of the experiment.( Specific heat capacity of water = 4.2Jg^-1K^-1)bi) Do you agree with the following statement? Explain your answer.' The standard enthalpy of formation of ethanol can be determined directly by experiment.'ii) The table below lists the standard enthalpy changes of combustion of three substances.C(graphite)]= -394KJ/mol
H2(g)= -286kJ/mol
C2H5OH(l)= -1368kJ/molCalculate the standard enthalpy change of formation of ethanol.
F5 Chem enthalpy calculation
2011-05-09 3:20 am
回答 (2)
2011-05-09 4:04 am
✔ 最佳答案
a ii) Energy change in combustion of 0.185 g of C2H5OH(l)
= -Energy absorbed by water
= -m*c*ΔT
= -200.0 * 4.2 * 6.0 J
= -5040 J
= -5.04 kJ
No. of moles of C2H5OH used
= 0.185/(12*2 + 1*6 + 16) mol
= 0.185/46 mol
Enthalpy change of combustion
= -5.04 / (0.185/46) kJ/mol
= -1253 kJ/mol
b i)
No, I do not agree.
This is because the reaction that ethanol is produced from its constituentelements does not occur.
i.e. 2C(graphite) + 3H2(g) + (1/2)O2(g) → C2H5OH(l)
The above reaction does not occur.
b ii)
2C(graphite) + 2O2(g) → 2CO2(g) .. ΔH = 2(-394) kJ/mol
3H2(g) + (3/2)O2(g) → 3H2O(l).. ΔH = 3(-286)kJ/mol
2CO2(g) + 3H2O(l) → C2H5OH(l)+ 3O2(g) ..ΔH = -(-1368) kJ/mol
Add the above 3 thermochemical equation together, and eliminate 2CO2,3H2O and 3O2 on the both sides:
2C(s) + 3H2(g) + (1/2)O2(g) → C2H5OH(l)
ΔH = 2(-394) + 3(-286) - (-1368) = -278kJ/mol
參考: miraco
2011-05-09 8:56 am
太明顯是功課吧. 直接拋出問題的伸手黨, 那些懶惰鬼就別管了吧.
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