Revision:Enthalpy

What are the thermodynamic standard conditions?

298K, 1 atm pressure, concentration of 1mol per dm3


What is enthalpy change?

The heat change measured at a constant pressure and temperature.


Define standard enthalpy of formation

The enthalpy change when a single mole of compound is formed from its
elements under standard conditions of 298K and 1atm pressure with all
the reactants in their standard states.


Why is enthalpy change for exothermic reactions negative?

Because energy has been released to the surroundings. The products
therefore have a lower enthalpy than the reactants and the final
enthalpy is less than the initial enthalpy.


Classify as endothermic or exothermic

* NaOH dissolves in water & temp. of solution rises - Exothermic
* Ammonium Chloride dissolves in water and temp. of surroundings drop - Endothermic
* H and O combine explosively into water - Exothermic
* Liquid water condenses into ice at 0 C - Exothermic
* Liquid Nitrogen boils spontaneously at room temp.- Endothermic


Why is standard enthalpy changes of combustion always negative?

Because heat is always evolved, the reaction is exothermic and
therefore heat is lost to the surroundings. The energy required to
break bonds is always less than the energy released when bonds are
formed.