Branch of chemistry that studies the relationship between the chemical action and the amount of heeeet absorbed or generated.

Thermochemistry Thermochemistry Terminology Terminology

Heeet (not to be confused with temp.) Heeet (not to be confused with temp.) Transfer of energy to an object of Transfer of energy to an object of

higher energy to an object of lower higher energy to an object of lower energy.energy.

Symbolized by ( q )Symbolized by ( q ) Heeet is stoichiometric which means Heeet is stoichiometric which means

more stuff in reaction more heeet more stuff in reaction more heeet involved. involved.

( Continued )( Continued )

System is the concentrated object System is the concentrated object Surroundings are everything around the Surroundings are everything around the

systemsystem EnthalpyEnthalpy

Total heeet of a systemTotal heeet of a system Symbolized by ( H ) Symbolized by ( H ) State function: Only initial and final State function: Only initial and final

conditions matter not how you get thurr conditions matter not how you get thurr

Endothermic or Endothermic or Exothermic?Exothermic?

Endothermic: Endothermic: Requires input of Requires input of heeet from heeet from surrounding for surrounding for reaction to take placereaction to take place The system feels to The system feels to

cool to touch cool to touch ∆∆H > 0H > 0

Exothermic: Exothermic: Releases heeet into Releases heeet into the surroundings as the surroundings as the process occursthe process occurs The system feels to The system feels to

hot to touchhot to touch ∆∆H < 0H < 0

3 Ways to determine 3 Ways to determine Enthalpy change (Enthalpy change (∆H) of a ∆H) of a reactionreaction

1.1. CalorimetryCalorimetry

2.2. Hess’s LawHess’s Law

3.3. Standard Enthalpies of Standard Enthalpies of FormationFormation

Calorimetry:Calorimetry:

Measurement of Measurement of heeet flowheeet flow Coffee cup Coffee cup

Calorimeter Calorimeter

C x M x C x M x ∆T=q∆T=qSpecific heeet capacity ( j/g°C)

Mass

( g )

Change in Temp. ( C° )

Heeet

“ BOMB “ Calorimeter

q= C x ∆T

HeeetSpecific heeet capacity ( j/g°C)

Change in Temp. ( C° )

Calorimetry ExampleCalorimetry Example

H+(aq) + OH-(aq) → H2O(l)H+(aq) + OH-(aq) → H2O(l) The temperature of 110 g of water rises from The temperature of 110 g of water rises from

25.0°C to 26.2°C when 0.10 mol of H+ is 25.0°C to 26.2°C when 0.10 mol of H+ is reacted with 0.10 mol of OH-. reacted with 0.10 mol of OH-.

Calculate q of the waterCalculate q of the water Calculate Calculate ∆H∆H

Hess’s lawHess’s law Hess’s Law states that the heat of a whole Hess’s Law states that the heat of a whole

reaction is equivalent to the sum of it’s steps.reaction is equivalent to the sum of it’s steps. For example: C + OFor example: C + O22 CO CO22

This occurs as 2 stepsThis occurs as 2 stepsC C + + ½O½O22 CO CO HH = – 110.5 kJ = – 110.5 kJ

CO CO + + ½O½O22 CO CO22 HH = – 283.0 kJ = – 283.0 kJ

C + CO + OC + CO + O22 CO + CO CO + CO22 HH = – 393.5 = – 393.5

kJ kJ I.e. C + OI.e. C + O22 CO CO22 HH = – 393.5 = – 393.5

kJ kJ Hess’s law allows us to add equations.Hess’s law allows us to add equations. We add all reactants, products, & We add all reactants, products, & HH values. values.

Hess’s law: ExampleHess’s law: ExampleWe may need to manipulate equations further: We may need to manipulate equations further:

2Fe + 1.5O2Fe + 1.5O22 Fe Fe22OO33 HH=?=?, given, given

FeFe22OO33 + + 3CO 3CO 2Fe + 3CO 2Fe + 3CO22 HH= = –– 26.74 kJ 26.74 kJ

CO CO + + ½½ OO22 CO CO22 HH= = –– 282.96 kJ282.96 kJ1: Align equations based on reactants/products.1: Align equations based on reactants/products.2: Multiply based on final reaction.2: Multiply based on final reaction.3: Add equations.3: Add equations.

2Fe 2Fe + 1.5O+ 1.5O22 Fe Fe22OO33

3CO + 1.53CO + 1.5 OO22 3CO 3CO22 HH= = –– 848.88 848.88 kJkJ

2Fe 2Fe + 3CO+ 3CO22 Fe Fe22OO33 + 3CO + 3CO HH= += + 26.74 26.74 kJkJ CO + CO + ½½ OO22 CO CO22 HH= = –– 282.96 282.96 kJkJ HH= = –– 822.14 kJ822.14 kJ

Standard Enthalpies of Standard Enthalpies of FormationFormation

Standard conditions: Most stable form of Standard conditions: Most stable form of the substance the substance 1atm and 251atm and 25°C ( 298K )°C ( 298K ) Standard Enthalpy, ∆H°, is enthalpy Standard Enthalpy, ∆H°, is enthalpy

measured when everything is measured in measured when everything is measured in standard statestandard state

Multiple ChoiceMultiple Choice

1.) Which of the following is NOT a 1.) Which of the following is NOT a characteristic of an exothermic characteristic of an exothermic reaction?reaction?

A.A. Reaction feels warmReaction feels warm

B.B. System gains energySystem gains energy

C.C. Enthalpy change of reaction is negativeEnthalpy change of reaction is negative

BB22HH66 + 6H + 6H220 0 6H 6H2 2 + + 2H2H33BOBO33

2.) ∆H=? KJ/mol2.) ∆H=? KJ/molA.A. -3604 KJ/mol-3604 KJ/mol

B.B. -772 KJ/mol-772 KJ/mol

C.C. 3604 KJ/mol3604 KJ/mol

D.D. 772 KJ/mol772 KJ/mol

BB22HH66: ∆H°= +36 KJ/mol: ∆H°= +36 KJ/mol

HH220: ∆H°= -242 KJ/mol0: ∆H°= -242 KJ/mol

HH33OBOOBO33: ∆H°= -1094 KJ/mol: ∆H°= -1094 KJ/mol

3.) Which of the 3.) Which of the ∆H’s is exothermic?∆H’s is exothermic?

A.A. 563563

B.B. 00

C.C. -375-375

D.D. 989 989