Thermochemistry: Enthalpy Change (ΔH)

Thermochemistry: Enthalpy Change (ΔH)

• Thermochemistry: The study of heat released or absorbed during chemical reactions, phase changes, or other chemical activities.
• Enthalpy (ΔH): The amount of energy released or absorbed during a chemical reaction. A negative ΔH indicates an exothermic reaction (heat released), and a positive ΔH indicates an endothermic reaction (heat absorbed).
• Hess's Law: The total enthalpy change of a reaction is the same regardless of the pathway taken, as long as the initial and final conditions are the same. This allows calculating enthalpy changes for complex reactions using known enthalpy for simpler steps.
• Enthalpy Change (ΔH): The difference in enthalpy (total heat content) between products and reactants in a chemical or physical process at constant pressure. ΔH = H_products - H_reactants.
• Types of Enthalpy Change:
  • Exothermic: Releases heat to the surroundings (ΔH < 0).
  • Endothermic: Absorbs heat from the surroundings (ΔH > 0).
• Units of ΔH: Joules (J) or kilojoules (kJ), commonly expressed in kJ/mol in chemistry.
• Rules for Manipulating Thermochemical Equations:
  • Reversing an equation: Reverses the sign of ΔH.
  • Changing Coefficients: Multiplying or dividing all coefficients in an equation by a factor also multiplies or divides the ΔH value by the same factor.
  • Identical Physical States: Reactants and products must be in the same physical state (e.g., solid, liquid, gas) for cancellation.

Examples of Thermochemical Equations and Calculations

• Example 1 (Exothermic): Combustion of Methane (CH₄):

  • CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) ΔH = −890 kJ/mol
  • Meaning: 890 kJ of heat is released when 1 mole of methane combusts.
  • Example Calculation: Calculate the heat released during the combustion of 2 moles of methane: (2 moles × −890 kJ/mol) = −1780 kJ.

• Example 2 (Endothermic): Decomposition of Calcium Carbonate (CaCO₃):

  • CaCO₃(s) → CaO(s) + CO₂(g) ΔH = +178 kJ/mol
  • Meaning: 178 kJ of heat is absorbed when 1 mole of CaCO₃ decomposes.
  • Example Calculation: Calculate the heat absorbed during the decomposition of 50 g of CaCO₃. Determining the moles (0.5 moles), leads to heat absorbed (89 kJ).

• Example 3 (Neutralization): Reaction of HCl and NaOH:

  • HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) ΔH = -57 kJ/mol
  • Meaning: 57 kJ of heat is released when 1 mole of HCl reacts.
  • Example Calculation: Calculate the heat released from 0.1 moles of HCl reacting. This results in -5.7 kJ of heat.

Hess's Law Concept

• Hess's Law States: The total enthalpy change of a reaction is the same, regardless of the pathway taken, provided the initial and final conditions are the same.

Example: Formation of CO₂

• Reaction: C(s) + O₂(g) → CO₂(g)
• Given Data:
  • C(s) + ½O₂(g) → CO(g) ΔH₁ = −110.5 kJ
  • CO(g) + ½O₂(g) → CO₂(g) ΔH₂ = −283.0 kJ
• Calculation: Adding the equations for ΔH₁, and ΔH₂, results in ΔH = −393.5 kJ.

Example: Decomposition of Hydrogen Peroxide (H₂O₂)

• Reaction: H₂O₂(l) → H₂O(l) + ½O₂(g)
• Find ΔH by reversing the first step and summing it with the second step as per Hess's law. Given Data:
  • H₂(g) + O₂(g) → H₂O₂(l) ΔH₁ = -188 kJ
  • H₂(g) + ½O₂(g) → H₂O(l) ΔH₂ = -286 kJ

Explore the concepts of enthalpy change in thermochemistry, including the significance of ΔH in chemical reactions. Understand the difference between exothermic and endothermic reactions, and learn how Hess's Law applies to calculate enthalpy changes. This quiz will deepen your knowledge of energy transformations during chemical processes.