CHE 112 (KOTZ)

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CHE 112 (KOTZ). Chapter 5 Energy &amp; Chemical Reactions. Che 112 Course Introduction. Prerequisites Course Policy and Grading Homework Shopping List Lab Start up. Chapter 5 Overview. Energy, work and heat 1 st Law of Thermodynamics Enthalpy, H. Energy and Heat.

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### CHE 112 (KOTZ)

Chapter 5

Energy & Chemical Reactions

Che 112 Course Introduction
• Prerequisites
• Homework
• Shopping List
• Lab Start up
Chapter 5 Overview
• Energy, work and heat
• 1st Law of Thermodynamics
• Enthalpy, H
Energy and Heat

Definitions:Energy 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
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

Systems
• System vs surroundings
• Three types of systems open -

closed - isolated –

Specific Heat Capacity, C
• C = q / m ΔT mass dependent
• Algebra q = ?

know: specific C H2O(l) = 4.18 J/g 0C or 1 cal /g 0C(can use Celsius or Kelvin; ΔT is same)

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

Heating Curves, review
• Heat of fusion, ∆Hffor water 80 cal/g
• Heat of vaporization, ∆Hvapfor water 540 cal/g
• See other values Appendix D (table 12) See Heating Curve for water p. 219
Internal Energy, U
• U defined: total energy withinsystem KE and PE molecular motion, KE

bonding, PE

• Heat, “q” transferred energy (joules)
• Work, w = - P Δ V
• State functions
1st Law Δ U = q + w

Sign Conventions [see summary table p. 224]

+ energy absorbed by system; U increases when:q > 0 Endo

w > 0 compression

- energy lost by system; U decreases when:q < 0 Exo w< 0 expansion

NOTES: work = f x d ( where f = ma ) work and energy have SAME units, joules!

energy transfer in

(endothermic), +q

energy transfer out

(exothermic), -q

w transfer in

(+w)

w transfer out

(-w)

SYSTEM

∆U = q + w

Terms
• Adiabatic [ thermal insulation], q =0
• Exothermic , q < 0
• Endothermic, q > 0
Che Reactions
• @ V = constant w = 0 So, ΔU = qv
• @ P = constant ΔU = qp + w = qp - P ΔV
• Or qp = ΔU + P ΔV
Enthalpy, Hdefined as H = U + PV
• So, ΔH = ΔU + PΔV
• Thus ΔH = qp

extensive a state function qp = ΔH, unique value ; “heat of rxn”

• Recall, exo and endo
• General Enthalpy Diagrams (Energy Profiles) (skip energy level diagrams-textbook)
ΔH and Stoichiometry

Example: Given H2 + Cl2 → 2 HCl ΔHrxn = - 184 KJfind ΔHrxn if a) 12.8 g H 2 react

b) 6.25 mol. HCl form

Hess’ Law of Heat Summation

A + B → C + D ΔH1

C + B → E + F ΔH2

___________________________________

A + 2B → D + E + F ΔHrxnΔHrxn = ΔH1 +ΔH2

You see ex.5.8 page 235

Standard Enthaply of FormationΔfH0
• Define: standard state
• “standard heat of formation”
• ΔfH0 = 0 for elements
• See table Appx.L ΔHrxn0 = ∑ ΔfH0prods - ∑ ΔfH0reactnts [using molar coeffs ]see ex. 5.9 page 238
End Ch 5
• You may skip sec 5.8
• Homework questions?

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