The enthalpy of combustion of benzoic acid (C6H5COOH) is commonly used as the st

Question

The enthalpy of combustion of benzoic acid (C6H5COOH) is commonly used as the standard for calibrating constant-volume bomb calorimeters; its value has been accurately determined to be – 3226.7 kJ/mol. (a) When 0.4654 g of a-D-glucose (C6H12O6) was oxidized the temperature rose from 21.22 °C to 22.28 °C. Calculate the enthalpy of combustion of glucose and the value of DUrxn for the combustion. (Hint: Be sure to write a balanced equation for each reaction first and assume the product H2O is a liquid).

Explanation / Answer

equation of the reaction is given by C6H12O6 (s) + 6O2(g) à 6CO2(g) + 6H2O(l)

q = C D T = nglu D cU o gives D cU o = (6.81 kJ/K) 1.06K (180 g/mol)/0.4654 g or D cU o = - 2792 kJ/mol

Note that D cU o + D (pV) = D cU o + D (nRT)o = D cH o (assuming a perfect gas). Since D n = net change in #moles of gas = 0, D cU o = D cH o.

Note D cH o = - 2808 kJ/mol in A1.1

Find D fH o for 6C(s) + 6H2(g) + 3O2(g) à C6H12O6 (s)

6CO2(g) + 6H2O(l) à C6H12O6 (s) + 6O2(g) D cH o = +2792 kJ/mol

6C(s) + 6O2(g) à 6CO2(g) 6 x D fH o = - 2361

6H2(g) + 3O2(g) à 6H2O(l) 6 x D fH o = -1715

Add these 3 eqns together to get the enthalpy of formation for glucose

D fH o = - 1283 kJ/mol Note D fH o = - 1274 kJ/mol in A1.1

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