AS Chemistry Chapter 1.3 - Energetics and Enthalpy Changes

Chapter 1.2 ­ Energetics and Enthalpy Changes

Breaking bonds requires energy (endothermic)

Forming bonds releases energy (exothermic)

A + B C + D
Endothermic Exothermic
Enthalpy change of reaction is the energy releases to the surroundings in an exothermic reaction.

Enthalpy is energy. It's represented by a `H'

H =…

Standard enthalpy change of atomisation H at of an element is the enthalpy change when one
mole of its atoms in the gaseous state is formed from the element under standard conditions.

E.g. C (graphite) C (g)

It's always endothermic since it involves increasing the separation between atoms, which requires…

Example: -

HO (l) HO (l) + ¹/ O (g)

H (g) + O (g) H HO (l) + ¹/ O (g)
-285.8
To find H you do H - H, so
-285.8 - - 187.8 = -98 kJ mol¯¹
H H
-187.8
HO (l)

Bond dissociation enthalpy (bond energy)…