blb 18 blb 8 supplemental hw: blb 5 this is true...

Dr. L. S. Van Der Sluys Ch. 5 Part 2

Thermochemistry Part 2

Read: BLB 5.6–7; 8.8

HW: BLB 5:63, 67a,b, 69, 73, 75, 83, 85;

BLB 8:65a, 67a,c, 92a

BLB 18:72ab, 74

Supplemental 5:1–7;

Supplemental 8:11–13

Know:

• Hess!s Law

• heats of formation

• enthalpy of reactions

• estimating reaction enthalpy from bond energies

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Dr. L. S. Van Der Sluys Page 2 Ch. 5 Part 2

HESS’S LAW

The sum of the !H values for each step

is the same as !H for the overall

process.

This is true because !H is a state

function. C !H2

!Hrxn

B

!H1

A A B !H1

B C !H2

A+B "# B + C !H1 + !H2= !Hrxn


Example: Given the following information A H2(g) + F2(g) # 2HF(g) !HA = –537kJ

B 2H2(g) + O2(g) # 2 H2O(g) !HB = –572kJ

Determine !H for the reaction:

C 2F2(g) + 2H2O(g)# 4HF(g) + O2(g) !HC =?

IDEA: find combinations of reactions

such that

n A + m B = C then

n !HA + m !HB = !HC

Here: 2xA 2H2(g) + 2F2(g) # 4HF(g) !H = 2(–537kJ) –1xB 2 H2O(g) # 2H2(g) + O2(g) !H = –(–572kJ) _

2F2(g) + 2H2O(g)# 4HF(g) + O2(g)

!HC = 2(–537kJ) –1(–572kJ) = –502kJ


Given the following information: !H

2SO2(g)+ O2(g) # 2SO3(g) $196kJ

2S(s) + 3 O2(g) # 2SO3(g) $790kJ

What is !Hrxn for the following reaction?

S(s) + O2(g) # SO2(g)

A. – 986 kJ

B. – 594 kJ

C. + 594 kJ

D. – 297 kJ


Heat of Formation

!Hf enthalpy of formation heat given off (or absorbed) when elements combine to give one mole or one molecule of a compound

combine

Elements Compounds !Hf

!H° f standard enthalpy of formation

enthalpy of formation when all substances are in their Standard State

DEFINITION OF STANDARD STATE 1. P = 1 atm 2. T = 25°C (298K)

3. element is in its most stable state (gas/liquid/solid)

For an element in its standard state:

!H° f = ___ (by definition)


Standard States of the Elements (Most stable phase under standard

conditions of 298K and 1 atm)

1. Metals: all are _______ at 298K

and 1 atm except one (Which one?)

2. Semi metals (metalloids): all are _________ at 298K and 1 atm

3. Nonmetals at 298K and 1 atm

i: Noble gases (Group 8):

atomic ___________

ii: Diatomics: H2, N2, O2, Group 7 (F2, Cl2, Br2, I2)

H2, N2, O2, F2, Cl2, are ______

Br2 is a __________

I2 is a ___________

iii: all other non-metals are _____

C (graphite), S8, P(s), Se


Are these Elements in their standard states?

YES NO

O2(g)

N3$

(s)

F2(g)

N2(g)

O3(g)

C(diamond)

C(graphite)

Fe(s)

Br2(g)

H(g)



For which of the following reactions (at 25° C and 1 atm) is !Hrxn = !H° f ?

Answer A: !Hrxn = !H° f

Answer B: !Hrxn ! !H° f

1) H2(g) + F2(g) "# 2HF(g)

2) NO(g)+ 1/2 O2(g) # NO2(g)

3) 2C(graphite)+3H2(g)+1/2O2(g)"#C2H5OH(l)

4) Pb(s) + Cl2(g) "#PbCl2(s)

5) S(s) + O3(g) "# SO3(g)

6) Br2(l) "# Br2(g)


We know that !H° rxn is the Standard

Enthalpy of reaction, or Heat of reaction; all elements must be in their

________________. We know that !H° f is the Heat of formation;

formation of _____________(how much?) from ______________ when all reactants

are in their __________________

How are they related?

Obtaining !H° rxn from !H° f:

!H° rxn = %n !H° f (prod) $ %m !H° f (react)

where n and m are stoichiometric

coefficients of products and reactants.

(This is an application of Hess’s Law.)


Using the following information, what is !H° rxn for the reaction:

C2H5OH(l) + 3O2(g)#2CO2(g) + 3H2O(l) !H° f (in kJ/mole):

-277.7 -393.5 -285.8

a) – 115.8 kJ b) + 115.8 kJ c) – 1366.7 kJ d) + 13366.7 kJ

NOTE: Appendix C in the text has a

more extensive table of !H° f


If a piece of fruit contains 16.0 g of

fructose, and !Hrxn = $2803 kJ, how many

food calories does it contribute to the

body? 1 cal = 4.184J

1kcal = 1 Cal (= 1 food calorie)

Combustion! C6H12O6(s) +6____# 6____+ 6_____


Changes in Enthalpy with Breaking and Formation of Bonds

! Bond Enthalpies (bond energy) can be used to estimate !Hrxn.

! The overall reaction has two steps; breaking the original bonds, and forming new ones.


Bond Properties Review COVALENT BOND LENGTHS and ENERGIES

Bond length: distance between nuclei

bond Bond energy

kJ/mol Bond length pm

C"C 348 154

C=C 614 134

C&C 839 121

more electrons shared, shorter bond length

bond Bond energy kJ/mol

Bond length pm

C"H 413 110

C"Cl 328 176

C"Br 276 196

Shorter bond length, stronger the bond

Dr. L. S. Van Der Sluys Page 15 Ch. 5 Part 2 Dr. L. S. Van Der Sluys Page 16 Ch. 5 Part 2

Estimating !Hrxn

• Bond energies provide estimates of reaction enthalpies:

!Hrxn ' %nDbroken $ %mDformed

n, m = # of bonds Energy is given off ($) when bonds form.

Helps understand origins of !Hrxn


Example: Predict the heat of reaction for the decomposition of hydrogen peroxide.

2 H2O2 # 2 H2O + O2 !Hrxn = ? Draw Lewis structures of reactants and products:

Reactants (broken) Products (formed)

bond # D bond # D

Compare to Actual value of !H°rxn = -196 kJ/mole


Peroxide Decomposition: Elephant Toothpaste Demonstration

2 H2O2(l) # 2 H2O(l) + O2(g) !Hrxn = ?

Add KI to catalyze the reaction:

I-(aq) + H2O2(l) # H2O(l) + IO-(aq)

IO-(aq) + H2O2(l) # H2O(l) + O2(g)+ I-(aq)

2 H2O2(l) # 2 H2O(l) + O2(g)

Can also catalyze the reaction with MnO2

blb 18 blb 8 supplemental hw: blb 5 this is true...

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