Year 12s: what will be the first way you research your future options? >>

Confused

park1996

I really don't understand why the answer is A....isn't it D...

I just read in my revision guide that the enthalpy change is the heat energy transferred in a reaction at a constant pressure....

then what's the difference between writing this, and writing that it occurs under standard conditions of 100kPa and 298K???

Reply 1

KombatWombat

The answer is A because you want to work out the change in enthalpy by measuring the change in temperature. You work out the enthalpy change by doing:
∆H = ∆T * C, where C is the heat capacity; the amount of energy needed to raise a substances temperature.

Reply 2

Protoxylic

If temperature stays constant then there is no enthalpy change which is what you're trying to measaure, hence only pressure has to stay constant. The definition of enthalpy is the heat changed measured at constant pressure. You may be confusing it with the standard enthalpy change of combustion/formation in which that is slightly different in the fact that those are measured at constant pressure and temperature.