CHE 112 (KOTZ)

1
CHE 112 (KOTZ)

• Chapter 5
• Energy Chemical Reactions

2
Che 112 Course Introduction

• Prerequisites
• Course Policy and Grading
• Homework
• Shopping List
• Lab Start up

3
Chapter 5 Overview

• Energy, work and heat
• 1st Law of Thermodynamics
• Enthalpy, H

4
Energy and Heat

• DefinitionsEnergy is the capacity to do work or
  transfer heat.
• Heat is the form of energy that flows between
  2 objects because of their difference in
  temperature.
• KE ½ m v 2
• examples
• PE (composition or location) examples
• KE ?PE interconvertedLAW OF CONSERVATION OF
  ENERGY

5
Energy Units

• 1 calorie DEFINED
• 1000 cal 1 kilocalorie 1 kcal
• 1 kcal 1 Calorie (a food calorie)
• But we use the unit called the JOULE
• 1 cal exactly 4.184 joules

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Systems

• System vs surroundings
• Three types of systems open -
• closed - isolated

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Specific Heat Capacity, C

• C q / m ?T mass dependent
• Algebra q ?
• know specific C H2O(l) 4.18 J/g 0C or 1 cal /g
  0C(can use Celsius or Kelvin ?T is same)

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Lab Determination of Specific C of a Metal

• Assume isolated system
• Thermal equilibrium
• or q metal - q waterNB textbook
  lab manual do not agree!! specific heat capacity
  specific heat C textbook SH
  manual textbook uses K manual uses 0C
• See problem 17 page 243

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Heating Curves, review

• Heat of fusion, ?Hf for water 80 cal/g
• Heat of vaporization, ?Hvap for water
  540 cal/g
• See other values Appendix D (table 12) See
  Heating Curve for water p. 219

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Internal Energy, U

• U defined total energy within system KE and
  PE molecular motion, KE
• bonding, PE
• Heat, q transferred energy (joules)
• Work, w - P ? V
• State functions

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1st Law ? U q w

• Sign Conventions see summary table p. 224
• energy absorbed by system U increases
  when q gt 0 Endo
• w gt 0 compression
• - energy lost by system U decreases when q lt
  0 Exo wlt 0 expansion
• NOTES work f x d ( where f ma )
  work and energy have SAME units, joules!

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SYSTEM
?U q w
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Terms

• Adiabatic thermal insulation, q 0
• Exothermic , q lt 0
• Endothermic, q gt 0

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Che Reactions

• _at_ V constant w 0 So, ?U qv
• _at_ P constant ?U qp w qp - P ?V
• Or qp ?U P ?V

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Enthalpy, Hdefined as H U PV

• So, ?H ?U P?V
• Thus ?H qp
• extensive a state function qp ?H, unique
  value heat of rxn
• Recall, exo and endo
• General Enthalpy Diagrams (Energy Profiles)
  (skip energy level diagrams-textbook)

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?H and Stoichiometry

• Example Given H2 Cl2 ? 2 HCl ?Hrxn
  - 184 KJfind ?Hrxn if a) 12.8 g H 2
  react
• b) 6.25 mol. HCl form

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Hess Law of Heat Summation

• A B ? C D ?H1
• C B ? E F ?H2
• ___________________________________
• A 2B ? D E F ?Hrxn
  ?Hrxn ?H1 ?H2
• You see ex.5.8 page 235

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Standard Enthaply of Formation?fH 0

• Define standard state
• standard heat of formation
• ?fH 0 0 for elements
• See table Appx.L ?Hrxn0 ? ?fH0prods - ?
  ?fH0reactnts using molar coeffs
  see ex. 5.9 page 238

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End Ch 5

• You may skip sec 5.8
• Homework questions?
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