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Week 12 CHEM 1310 - Sections L and M 1

Energy, Enthalpy, & Thermochemistry

9.1 The Nature of Energy 9.2 Enthalpy 9.3 Thermodynamics of Ideal Gases 9.4 Calorimetry 9.5 Hess’s Law 9.6 Standard Enthalpies of Formation 9.7 Present Sources of Energy 9.8 New Energy Sources

Week 12 CHEM 1310 - Sections L and M 2

Example Problems

What is ∆E in kJ? ∆E = q + w Recall: q = -10 kJ (because heat is released) Recall: w = 125 L x atm Must convert units in order to add…

A gas is compressed from 40L to 15L at a constantpressure of 5 atm. In the course of this compression

10 kJ of energy is released.

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Week 12 CHEM 1310 - Sections L and M 3

Yesterday’s Chemical Explosion

Due to Unforeseen Reactivity in Calorimetry Experiment

WSBtv.com

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Week 12 CHEM 1310 - Sections L and M 4

What is Calorimetry?

Measurement of amounts of heat flow andthe accompanying temp changes.

In “bomb calorimetry”, a combustion reactionoccurs inside of the bomb.

Reaction chamber has a fixed volume, so P∆V is 0.

Week 12 CHEM 1310 - Sections L and M 5

What is Calorimetry?

Measurement of amounts of heat flow andthe accompanying temp changes.

A simple design uses astyrofoam cup sealed.

Heat lost to the styrofoamitself, the thermometer,

and to the surrounding airis negligible.

Week 12 CHEM 1310 - Sections L and M 6

Calorimetry Problem A student placed 50.0 mL of 1.00 M HCl at 25.5°C

into a styrofoam cup calorimetry. To this, sheadded 50.0 mL of 1.00 M NaOH at 25.5°C. Themixture was stirred and the temp increased to32.2°C.

What is the energy evolved in J/mol of HCl? Assume specific heat for reaction is that of

water (4.18 Jg-1°C-1) Density of HCl = 1.02 g/mL Density of NaOH = 1.04 g/mL

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Week 12 CHEM 1310 - Sections L and M 7

Hess’s Law If two or more chemical equations are added to

give a new equation, then adding the enthalpies ofthe reactions that they represent gives theenthalpy of the new reaction.

Significance: Experimental determination of Δ H for some rxns!

Recall: Enthalpy is a state function! Path is irrelevant.

Week 12 CHEM 1310 - Sections L and M 8

Hess’s Law

Enthalpy changeis the same

whether 1 step or 2.

Week 12 CHEM 1310 - Sections L and M 9

Standard Enthalpy of Formation

a A + b B → c C + d DΔ H°f = cΔ H°f (C) + dΔ H°f (D) - aΔ H°f (A) - bΔ H°f (B)

Products Reactants

Notice:(1) Coefficients in balanced chemical eqn are a part of the

molar enthalpy term.

(2) Magnitude of contribution from reactants is subtracted from that of the products to denote “unformation”.

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Week 12 CHEM 1310 - Sections L and M 10

Example Problem

N2H4(l) + 3 O2(g) → 2 NO2(g) + 2 H2O(l)

ΔH°f = cΔH°f (C) + dΔH°f (D) - aΔH°f (A) - bΔH°f (B)

Calculate ΔH°f Find standard enthalpy of formation for individualmolecules in Appendix

Week 12 CHEM 1310 - Sections L and M 11

Example Problem

N2H4(l) + 3 O2(g) → 2 NO2(g) + 2 H2O(l)

ΔH°f = cΔH°f (C) + dΔH°f (D) - aΔH°f (A) - bΔH°f (B)

ΔH°f

N2H4(l) = 50.63 kJ/molO2(g) = 0 kJ/molNO2(g) = 33.18 kJ/molH2O(l) = -285.83 kJ/mol

From Reference

source

Week 12 CHEM 1310 - Sections L and M 12

Example Problem

N2H4(l) + 3 O2(g) → 2 NO2(g) + 2 H2O(l)

ΔH°f = 2ΔH°f (NO2) + 2ΔH°f (H2O)- 1ΔH°f (N2H4) - 3ΔH°f (O2)

ΔH°f = 2 (33.18) + 2 (-285.83)- 1 (50.63) - 3 (0)

ΔH°f = 66.36 - 571.66 - 50.63 = -555.93 kJ/mol