Enthalpy Change Terms & Definitions Flashcards
What is enthalpy change?
Enthalpy change is the change in heat energy of a substance at constant pressure.
What is a standard enthalpy change?
If the enthalpy change is measured under standard conditions of temperature and pressure, it is called a standard enthalpy change.
What is the enthalpy of formation?
The enthalpy of formation, AfH, is the enthalpy change when one mole of compound is formed from its constituent elements in their standard states under standard conditions.
What are the standard conditions for enthalpy of formation?
The standard conditions for enthalpy of formation are 298K and 100 kPa.
What is the enthalpy of combustion (ΔcH)?
The enthalpy change when one mole of substance in its standard state is burnt in excess oxygen under standard conditions.
Example: S (s) + O2 (g) → SO2 (s) ΔcH = -296.8 kJ mol-1
What is the enthalpy of atomisation (ΔatH)?
The enthalpy change when one mole of gaseous atoms is formed from the element in its standard state, under standard conditions.
What is mean bond enthalpy?
Mean bond enthalpy is the enthalpy change when a covalent bond in one mole of gaseous molecules breaks to give two gaseous atoms (or free radicals).
What is another name for mean bond enthalpy?
Mean bond enthalpy is also known as bond dissociation enthalpy.
What are the atoms formed when a covalent bond breaks?
The atoms formed are known as free radicals.
What is First Ionisation Energy (AIEH)?
AIEH is the enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form one mole of gaseous ions with a single positive charge.
What is first electron affinity (AEAH)?
The enthalpy change when one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous negative ions.
Example: Cl (g) + e- → Cl- (g)
What is the value of the first electron affinity for chlorine?
The first electron affinity for chlorine is -349 kJ mol-1.
Is the first electron affinity exothermic or endothermic?
The first electron affinity is exothermic for atoms that normally form negative ions.
Why is the first electron affinity exothermic?
Because the ion is more stable than the atom.
Is the second electron affinity exothermic or endothermic?
The second electron affinity is endothermic.
