Enthalpy Change

Some reactions involve ONLY bond breaking while others involve ONLY bond making Most reactions → include both phases - bond breaking then bond making

Enthalpy change helps us determine whether reaction is overall exothermic or endothermic.

Definition

Enthalpy refers to the total energy content in a substance. It is represented by symbol $H$ #ChemistryDefinitions

Definition

Enthalpy Change $\Delta H$ measures the difference in energy content of the reactants and products

$\Delta H$ = Total energy of products - Total energy of reactants $\Delta H$ = Total energy taken in to break bonds - Total energy released to form new bonds

ChemistryDefinitions

Units for Enthalpy change:

• Kilojoules
• Kilojoules per mole of reaction kJ/mol

Note

It is not possible to measure energy of a substance directly. But it is possible to measure enthalpy change, when substance (reactant) undergoes chemical reaction to form products

Important

Endothermic Reactions have a positive enthalpy change

Important

Exothermic Reactions have a negative enthalpy change

Note

bond breaking phase always has positive enthalpy value and bond making phase always has negative enthalpy value

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Calculating Enthalpy Changes

Overall Enthalpy Change

Formulae

Overall enthalpy change ($\Delta H$) = total energy absorbed during bond breaking - total energy released during bond making

whether overall reaction has +ve or -ve enthalpy change depends on which of these phases has a larger magnitude of change

Formulae

Endothermic reaction: total energy absorbed during bond breaking > total energy released during bond making

Exothermic reaction: total energy absorbed during bond breaking < total energy released during bond making

Formulae

$\Delta H$ = $\Delta H$ of bond breaking + $\Delta H$ of bond making OR $\Delta H$ = total amt of energy absorbed for bond breaking + (- total amt of energy released during bond making)

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Related to: Energy Level Diagrams and Energy Profile Diagrams

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