F.6 Chemistry - Enthalpy

The enthalpy of a substance can refer to the heat content of that substance as it concerns the study of thermochemisty. As some of other physical quantity, absolute enthalpy of a substance cannot be determined but the enthalpy change of a process can be calculated.

For your second qusetion, to be honest, it is not always true because when considering stability of a substance, enthalpy is not the only thing being considered, entropy and kinetics also play important roles for stability of a substance or a system.

However, in AL chemistry, in order to simplify the curriculum, it is assumed that,at most of the cases, substances with lower enthalpies more stable than those with higher enthalpies in point of view of energetics only. In fact, kinetics should also be taken into considerations.

You will learn more in the AL chem syllabus.

2009-08-26 00:31:44 補充：
樓上備到咁多課,好勤力@@!!

1) Ionization enthalpy is the definite amount of energy required to ionize one mole of that substance in gaseous state. (note that the amount of energy for different substance is different)


2) 唔好意思....會唔會係lower enthalpies more unstable than higher enthalpies.....it's because those with lower enthalpies require less energy to remove electrons from them,ie easier to remove the outermost shell electrons. Let's take Li and Ne as examples, they both have same numbers of shell. And Li has lower enthalpy than Ne ( 500kJmol-1 and 2000kJmol-1 respectively).Obviously,by recalling the knowledge in F5,Li is more unstable than Ne,which's a GroupEight element.

希望你明白我講咩..=]我都係升F6備緊課