3.2.1 enthalpy changes

Endothermic reaction

• A reaction with a +∆H
• Enthalpy of products \> enthalpy of reactants
• Temperature of surroundings decrease

Exothermic reaction

• A reaction with a -∆H
• Enthalpy of reactants \> enthalpy of products
• Temperature of surroundings increase

Activation energy

the minimum energy required for a reaction to take place

Standard conditions ⦵

100kPa and 298K and 1 moldm⁻³

Standard state

the physical state of a substance at ⦵

Enthalpy change of formation

The enthalpy change when one mole of a compound is formed from it's elements under standard conditions and all elements in their standard states

Enthalpy change of combustion

The enthalpy change when one mole of a compound is completely burned in excess oxygen under standard conditions and all elements in their standard states

Enthalpy change of neutralisation

The enthalpy change when one mole of water is formed from a neutralisation reaction under standard conditions

Calculating energy change

q \=mc∆t

∆H

q (in KJ) / moles of reactant

Reasons why experimental methods for enthalpy determination not be accurate for enthalpy change of combustion (e.g. Methanol)

• Heat loss to the surroundings other than water (less exothermic)
• Incomplete combustion (less exothermic)
• Evaporation of methanol from the wick (less exothermic)
• Non-standards conditions (more exothermic)

Average bond enthalpy

The enthalpy required to break one mole of a certain type of covalent bond in gaseous molecules averaged over a range of compounds

Limitations of average bond enthalpies

• Actual bond enthalpy may differ from the average value
• All species need to be gaseous molecules

Bond breaking

Energy is required to break bonds ∴ it is endothermic

Bond making

Energy is released when bonds form ∴ it is exothermic

Using enthalpy changes of combustion

∆rH \= Σ∆cH(reactants) - Σ∆cH(products)

Using enthalpy changes of formation

∆rH \= Σ∆fH(products) - Σ∆fH(reactants)

Use of average bond enthalpies to calculate enthalpy changes

∆H \= Σbonds broken - Σbonds formed