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A level Chemistry Question

I have an A Level Chemistry question about enthalpies.

It states:
A Data Book value for the enthalpy change of combustion of 2-methylpropan-2-ol is -2643.8 kJ mol−1.
.
Give the main reason for the difference between this value and your answer to part (c)(i) (which was -2512 kJ mol-1).

In my answer, I talked about how mean bond enthalpies are averaged over a range of compounds and so would not be specific to 2-methylpropan-2-ol. However, the mark scheme specifically states to not award this type of answer. Instead, it states:

"The enthalpy change of combustion figures are at 298 K / data book bond energies refer to gaseous state AND water and/or 2-methylpropan-2-ol are both liquids (at 298 K)"

I understand the second part of the answer but not the first part. I thought the data book value references to the standard enthalpy of combustion, where the calculations would have involved water and methylpropan-2-ol in their standard states under standard conditions (ie as liquids). However, the mark scheme states that the data book value references to all substances in their gaseous states, so now I am confused.

Any help would be appreciated!
Original post by TwisterBlade596
I have an A Level Chemistry question about enthalpies.
It states:
A Data Book value for the enthalpy change of combustion of 2-methylpropan-2-ol is -2643.8 kJ mol−1.
.
Give the main reason for the difference between this value and your answer to part (c)(i) (which was -2512 kJ mol-1).
In my answer, I talked about how mean bond enthalpies are averaged over a range of compounds and so would not be specific to 2-methylpropan-2-ol. However, the mark scheme specifically states to not award this type of answer. Instead, it states:
"The enthalpy change of combustion figures are at 298 K / data book bond energies refer to gaseous state AND water and/or 2-methylpropan-2-ol are both liquids (at 298 K)"
I understand the second part of the answer but not the first part. I thought the data book value references to the standard enthalpy of combustion, where the calculations would have involved water and methylpropan-2-ol in their standard states under standard conditions (ie as liquids). However, the mark scheme states that the data book value references to all substances in their gaseous states, so now I am confused.
Any help would be appreciated!

The previous part of the question, (c)(i), must have asked you to use bond enthalpy values to determine the combustion enthalpy. The definition of bond enthalpy terms refers to gaseous molecules only.
But wouldn't the enthalpy change of combustion which the Data Book value states involve all substances in their standard states under standard conditions (so not all of them may be gaseous)?
Original post by TwisterBlade596
But wouldn't the enthalpy change of combustion which the Data Book value states involve all substances in their standard states under standard conditions (so not all of them may be gaseous)?

That is precisely the point of the question.
Combustion enthalpy values come from substance is their standard states, which in the case of the alcohols is liquid.
Bond enthalpy values refer to only gaseous substances.
So, the values are different.
So when the mark scheme states: "data book bond energies", this does not reference to the Data Book Value but instead the table (which was given in the question) showing the mean bond enthalpies for different bonds?

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