Thermochemistry+1c+-+Enthalpy+and+Molar+Enthalpy (3)

Thermochemistry Overview

  • Thermochemistry: The study of energy changes, particularly heat, in chemical reactions.

Section 1c: Enthalpy and Molar Enthalpy

  • Definition of Enthalpy (H): Refers to the total heat content of a system at constant pressure. Enthalpy change (ΔH) is the heat absorbed or released during a reaction.

  • Molar Enthalpy (ΔrHm): The amount of heat absorbed or released per mole of a substance during a reaction.

The Law of Conservation of Energy

  • Energy can neither be created nor destroyed; it can only change forms.

  • In any physical or chemical change:

    • If a system gains energy, the surroundings lose energy.

    • If a system loses energy, the surroundings gain energy.

Energy in Thermodynamic Systems

  • Internal Energy (E): The total energy contained within a system, including kinetic and potential energy.

  • Kinetic Energy (Ek): Energy of motion, including energy associated with molecules in a gas, liquid, or solid.

  • Potential Energy (Ep): Stored energy based on the position of particles in a field (gravity, electromagnetic, etc.).

Heat Transfer and Molar Enthalpy Calculations

  • Molar Enthalpy is calculated in kJ/mol and denoted as ΔrHm.

  • To calculate ΔH for reactions, the formula is:

    • ΔrH = n × ΔrHm

    • Where 'n' is the number of moles of the substance reacting.

Examples of Enthalpy Change Calculations

  • Combustion of Sucrose:

    • For 150.0 g of sucrose (C12H22O11):

      • Molar mass of sucrose = 342.30 g/mol.

      • Moles of sucrose burned = 150.0 g / 342.30 g/mol = 0.4382 mol.

      • Molar enthalpy of combustion of sucrose = -5 155.7 kJ/mol.

      • Total ΔH = 0.4382 mol × -5 155.7 kJ/mol = -2 259.29 kJ.

  • Burning Methane:

    • For releasing 2.0 MJ of energy with molar enthalpy of methane = -802.5 kJ/mol:

      • Required moles = ΔcH / ΔcHm = -2000 kJ / -802.5 kJ/mol = 2.4922 mol

      • Total mass of methane = 2.4922 mol × 16.05 g/mol = 40 g.

Problems Involving Molar Enthalpy

  1. Enthalpy of Formation: If 750 kJ is absorbed forming benzene.

    • Molar mass of benzene (C6H6) = 78.12 g/mol.

    • Moles formed = 1200 g / 78.12 g/mol = 15.36 mol.

    • Molar enthalpy = ΔfHm = 750 kJ / 15.36 mol = 48.83 kJ/mol.

  2. Combustion of Ethane: What is the enthalpy change for 105 g of ethane?

    • Molar mass of ethane = 30.08 g/mol.

    • Moles = 105 g / 30.08 g/mol = 3.49 mol.

    • ΔrHm = -1251 kJ/mol

    • Calculate ΔrH: ΔrH = 3.49 mol × -1251 kJ/mol = -4,366.86 kJ.

Reactions Producing Energy in Organisms

  • Combustion of sucrose involves oxygen and produces CO2, water, and energy. Molar enthalpy = -5640.3 kJ/mol.

  • To produce 500 kJ, the mass of sucrose required:

    • m = n × M = (500 kJ / -5640.3 kJ/mol) × 342.30 g/mol = 30.3 g of sucrose.

Calculation of Molar Enthalpy of Reaction for Ammonia

  • When 1.7 g of ammonia (NH3) causes 3500 J of heat release, calculate the molar enthalpy of reaction:

    • Molar mass of ammonia = 17.04 g/mol.

    • Moles = 1.7 g / 17.04 g/mol = 0.09976 mol.

    • ΔsolH = -3500 J = -3.500 kJ.

    • ΔfHm = ΔsolH / n = -3.500 kJ / 0.09976 mol = -35.08 kJ/mol.

Quotes for Inspiration

  • Albert Einstein: "Annotation is more important than knowledge."

  • Albert Einstein: "It's not that I'm SO smart, it's just that I stay with problems longer."