Thermochemistry and Enthalpy Changes

Thermochemistry

• Definition: Thermochemistry is the study of energy changes during chemical reactions, specifically focusing on heat (enthalpy) transfer.

Enthalpy Changes

• Enthalpy Change (ΔH): Represents the heat absorbed or released under constant pressure.
• Units for ΔH: Typically expressed in kilojoules (kJ).

Reaction Example

• Consider the reaction:
  2 ext{Mg}(s) + ext{O}_2(g)
  ightarrow 2 ext{MgO}(s)
• Given Data:
  • Moles of Mg: 2.30 mol
  • Enthalpy change (ΔH) for the reaction: $ext{ΔH} = -1204 ext{ kJ}$
• Interpretation: The negative sign in ΔH indicates that the reaction is exothermic, meaning it releases energy in the form of heat.

Calculation of Enthalpy Change for Given Moles

• To find the change in enthalpy when 2.30 mol of Mg is reacted:
  1. Molar Ratio: The reaction shows that 2 mol of Mg produces an enthalpy change of -1204 kJ.
  2. Calculation: Use dimensional analysis to scale the enthalpy change to 2.30 mol of Mg:
     • $ext{ΔH}_{ ext{for 2.30 mol}} = \frac{-1204 ext{ kJ}}{2 ext{ mol}} imes 2.30 ext{ mol}$
     • $= -1204 imes 1.15 ext{ kJ}$
     • $= -1384.6 ext{ kJ}$
• Final Result: The change in enthalpy when 2.30 mol of Mg is completely reacted is $ext{ΔH} = -1384.6 ext{ kJ}$.