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1. Aluminium reacts with oxygen to form aluminium oxide.
2Al ( s ) + 1 1/2 O2 (s) > Al2O3 EH = -1670 kJ mol-1

what is the enthalpy of combustion of aluminium in kJ mol-1?

A -835
B -1113
C -1670
D +1670

what answer is A but why???
2. (Original post by xmaryamxbhattix)
Aluminium reacts with oxygen to form aluminium oxide.
2Al ( s ) + 1 1/2 O2 (s) > Al2O3 EH = -1670 kJ mol-1

what is the enthalpy of combustion of aluminium in kJ mol-1?

A -835
B -1113
C -1670
D +1670

what answer is A but why???
The answer is correct because of enthalpy change definitions.

-1670kJ mol-1 is the enthalpy change of formation of aluminium hydroxide. The definition of enthalpy change of formation refers to 1 mole of compound from its' elements; in your example it is the energy to form 1 mole of aluminium oxide.

The definition of enthalpy change of combustion refers to 1 mole of the substance being combusted in excess oxygen; in your example this refers to 1 mole of aluminium.
The value -1670 is the energy when 2 moles of aluminium is burnt (as you can see from the balanced equation). Hence you need to halve this energy to calculate the enthalpy change of combustion for aluminium.
3. Thank you!
4. (Original post by TutorsChemistry)
The answer is correct because of enthalpy change definitions.

-1670kJ mol-1 is the enthalpy change of formation of aluminium hydroxide. The definition of enthalpy change of formation refers to 1 mole of compound from its' elements; in your example it is the energy to form 1 mole of aluminium oxide.

The definition of enthalpy change of combustion refers to 1 mole of the substance being combusted in excess oxygen; in your example this refers to 1 mole of aluminium.
The value -1670 is the energy when 2 moles of aluminium is burnt (as you can see from the balanced equation). Hence you need to halve this energy to calculate the enthalpy change of combustion for aluminium.
Tonight's top tip:

Exam boards love asking questions about enthalpy change definitions (and other definitions too), they come up time after time. Sometimes the questions are direct, sometimes they indirectly test your understanding of definitions.
My tip is learn and understand definitions. If it is something you struggle with, practice as many enthalpy related past paper questions as you can. It is almost certain you will get a question in your exam.

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