Challenge Question: The heat of combustion (delta H_C) for alpha-D-glucpse is.-6

Question

Challenge Question: The heat of combustion (delta H_C) for alpha-D-glucpse is.-670 kcal/mol, and that of beta-D-fructose is -6.72/mol. The heat of combustion of the disaccharide sucrose is -1348 kcal/mol. What is the delta H for the hydrolysis of sucrose into glucose and fructose based on these numbers? Is this hydrolysis exothermic or endothermic? As part of your answer, write all four reactions in balanced form with the delta H shown. You can use molecular formulas, but label which sugar they apply to.

Explanation / Answer

A) Balanced combustion reactions of D-Glucose, D-Fructose, Sucrose are,

1)            C6H12O6 (s)         +         6O2 (g) -----------> 6CO2 (g) + 6H2O (l)

            - D-Glucose

            or -D-Fructose

2)         C12H22O11 (s)        +        12O2 (g) ---------------> 12CO2 (g) + 11 H2O (l)

             Sucrose

B) Balanced reaction for hydrolysis of Sucrose into D-Glucose and D-Fructose

C12H22O11 (s) + H2O (l) ---------------->    C6H12O6     + C6H12O6.

Sucrose                                                 D-Glucose      D-Fructose

Hc for D-Glucose = -670kcal/mol

Hc for D-Glucose = -672 kcal/mol

Hc for sucrose = -1348 kcal/mol

Enthalpy of hydrolysis (H)

H = Hc for sucrose – (Hc for D-Glucose + Hc for D-fructose)

H = (-1348) – [(-670 )+ (-672)]

H = - 8 kcal/mol

-ve sign indicate that its an endothermic reaction.

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