CHE 112 (KOTZ). Chapter 5 Energy & 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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Energy & Chemical Reactions
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.
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
closed - isolated –
know: specific C H2O(l) = 4.18 J/g 0C or 1 cal /g 0C(can use Celsius or Kelvin; ΔT is same)
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
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 out
w transfer in
w transfer out
∆U = q + w
extensive a state function qp = ΔH, unique value ; “heat of rxn”
Example: Given H2 + Cl2 → 2 HCl ΔHrxn = - 184 KJfind ΔHrxn if a) 12.8 g H 2 react
b) 6.25 mol. HCl form
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