Question

If a reaction for the combustion of ethanol has a -29.7KJ/g, what is the molar heat of combustion?

if a reaction for the combustion of ethanol has a -29.7KJ/g, what is the molar heat of combustion?

Answer 1

`-"1370 kJ mol"^(-1)`

Explanation:

The molar heat of combustion, or molar enthalpy change of combustion, `DeltaH_"comb/mol"`, expresses the enthalpy change associated with the combustion of one mole* of a given compound.

In your case, the molar heat of combustion of ethanol will represent the enthalpy change that accompanies the combustion of one mole of ethanol.

Notice that the problem provides you with the enthalpy change that accompanies the combustion of one gram of ethanol.

`DeltaH_"comb/g" = - "29.7 kJ g"^(-1)`

The minus sign indicates that the this much heat is being given off by the combustion of one gram of ethanol.

To convert this to the molar enthalpy of combustion, use ethanol's molar mass

`-29.7"kJ"/color(red)(cancel(color(black)("g"))) * overbrace((46.07color(red)(cancel(color(black)("g"))))/"1 mole ethanol")^(color(purple)("molar mass of ethanol")) = -"1368.3 kJ mol"^(-1)`

Rounded to three sig figs, the answer will be

`DeltaH_"comb/mol" = color(green)(|bar(ul(color(white)(a/a)-"1370 kJ mol"^(-1)color(white)(a/a)|)))`