Chapter 6: Thermochemistry

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Chapter 6 Thermochemistry

• CHE 123 General Chemistry I
• Dr. Jerome Williams, Ph.D.
• Saint Leo University

2
Overview

• Enthalpy
• Calorimetry

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Enthalpy

• Relationship between Enthalpy Heat
• Enthalpy Equation ? H ? E ?PV
• First Law ?E qp w
• Note qp heat _at_ constant P
• Work Equation w - P?V

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Enthalpy

• Relationship between Enthalpy Heat
• Since P constant ?PV P?V
• Rewrite First Law ?E qp - P?V
• Rearrange terms qp ?E P?V

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Enthalpy

• Relationship between Enthalpy Heat
• Substitution and simplification gives
• qp ?H - P?V P?V
• ?H qp

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Enthalpy

• At constant pressure, ?H flow of energy as
  heat.
• ?H endothermic process
• ?H - exothermic process
• ?Hrxn ? Hproducts - ? Hreactants

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Enthalpy

• Standard Enthalpies of Formation ?Hf0
• ?H for formation of 1 mole compound from its
  elements (all substances in their standard
  states).
• ?Hreaction0 ? np ?Hf0 (products) - ? nr ?Hf0
  (reactants)

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Enthalpy

• Standard states (Conventional Definitions)
• ?Hf0 for elements in their standard states is
  defined as zero.
• ?Hf0 for substances (T 25 C)
• Gases pressure is 1.0 atm
• Solutions concentration is 1.0 M
• Pure liquids solids std. state pure liquid
  or solid.

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Enthalpy

• Enthalpies of Physical Changes
• Enthalpy of Fusion (Melting) solid ? liquid
• Enthalpy of Vaporization (Boiling) liquid ? gas
• Enthalpy of Sublimation solid ? gas
• Expressing Physical States in Reaction becomes
  very important.

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Enthalpy
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Calorimetry

• Science of measuring heat absorbed or discharged
  by a body.
• Calorimeter device used to measure heat
  associated with a chemical reaction.

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Calorimetry

• Specific heat capacity Energy required to raise
  temperature of 1 g of any substance by 1o C (or
  K).
• Units are J / g C or J / g K
• Molar heat capacity Energy required to raise
  temperature of 1 mole of any substance by 1o C
  (or K).
• Units are J / mol C or J / mol K

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Calorimetry

• Coffee Cup Calorimeter
• Q m c ?T
• - Q metal Q water