Enthalpy Flashcards
Define enthalpy
A measure of the energy stored in a chemical system
When is the enthalpy negative
When the reaction is exothermic, releasing energy, causing temperature to increase
When is the enthalpy positive
When the reaction is endothermic, it gains energy, decreasing the surrounding temperature
What is the peak of an energy profile diagram
the activation energy
If Cl-Cl has a mean bond enthalpy of 242kj.mol, what does this mean
Cl-Cl requires an input of +242kj to break 1 mole of these bonds, and would release -242kj when 1 mole of these bonds is formed
Is breaking bonds endo or exo thermic
Endothermic
Is making bonds endo or exo thermic
Exothermic
why is official data more accurate than mean bond enthalpies
the same bond has different enthalpies depending on which compound it is present in, eg, C=O in carboxylic acids has a different enthalpy than in an aldehyde. Mean bond enthalpies are just an average
What is Hr
Enthalpy of reaction in standard conditions
What is Hf
Enthalpy of formation of 1 mole of a substance from its constituent elements in their standard states in standard conditions
What is Hc
Enthalpy of combustion in standard conditions where 1 mole of a substance is burned in excess oxygen
What is Hn
Enthalpy of neutralisation in standard conditions when 1 mole of water is formed through a neutralisation reaction
State Hess’ law
The enthalpy change for a reaction is independent of the route taken
Explain Hess’ law
The enthalpy change of a direct reaction is equal to the enthalpy change of a reaction using intermediates. This is useful is cases where enthalpy cannot be measured, eg, in the thermite reaction
