The standard entahlpies of formation of methanol, menthanal and water are -240,

Question

The standard entahlpies of formation of methanol, menthanal and water are -240, -118, and -286 kJ/mol, respectively, and the standard enthalpies of combustion of methanol (CH3OH) and methanal (HCOH) are -725 and -561 kJ/mol respectively. (You don't need to use all the data given.)

(A) Define standard enthalpy of formation.

(B) Define standard enthalpy of combustion.

(C)Calculate the enthalpy change for the oxidation of methanol to methanal. Using: CH3OH + (1/2)O2-->CH20+H20

(D)Distinguish between bond enthalpy term and bond dissociation enthalpy.

Explanation / Answer

A)
The standard enthalpy of formation is defined as the change in enthalpy when one mole of a substance in the standard state is formed from its pure elements in standard state.By standard state we mean state of the substance at standard temperature and pressure.

For example:
H2(g) +O2 (g) -->H2O (l)
The enthalphy change in above reaction will be standard enthalpy of formation of water.

B)
The standard enthalpy change of combustion of a compound is the enthalphy change when 1 mol of compound in its standard state is burnt completely in oxygen gas under standard teperature and pressure.

C)
CH3OH + (1/2)O2-->CH20+H20
enthalphy change = delta Hf (H2O) + delta Hf (CH2O) - delta Hf (CH3OH) -0.5delta Hf (O2)
            = (-286) + (-118) - (-240)
             = -164 KJ/mol
D)
Bond enthalpy is the average energy required to break a bond of a certain

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