CHEMISTRY 161

Chapter 6

www.chem.hawaii.edu/Bil301/welcome.html

REVISION

Measurement of Heat Changes

HHmm = = ΔΔQQmm = = ccmpmp ΔΔTT HHmm = = ΔΔQQmm = = ccmpmp ΔΔTT

ccmpmp(H(H22O) = 75.3 J molO) = 75.3 J mol-1-1 K K-1-1ccmpmp(H(H22O) = 75.3 J molO) = 75.3 J mol-1-1 K K-1-1

Calculation of Heat Changes

HHmm = H = Hm,productsm,products – H – Hm,reactantsm,reactantsHHmm = H = Hm,productsm,products – H – Hm,reactantsm,reactants

REFERENCE SYSTEM

molar volume at standard temperature and pressureVm = 22.4 l

oxidation numbers of elements are zero

Standard Enthalpy of Formation

HHffOOHHff

OO

heat change when 1 mole of a compound is formed from its elements at a pressure of 1 atm

(T = 298 K)

HHffO O (element) = 0 kJ/mol(element) = 0 kJ/molHHff

O O (element) = 0 kJ/mol(element) = 0 kJ/mol

HHffO O (graphite) = 0 kJ/mol(graphite) = 0 kJ/molHHff

O O (graphite) = 0 kJ/mol(graphite) = 0 kJ/mol HHffO O (diamond) = 1.9 kJ/mol(diamond) = 1.9 kJ/molHHff

O O (diamond) = 1.9 kJ/mol(diamond) = 1.9 kJ/mol

EN

TH

AL

PY

, H

HHreactantsreactantsHHreactantsreactants

HHproductsproductsHHproductsproducts

C(s, graphite) + O2(g)

CO2(g)

Hf0 = - 393.51 kJ mol-1

0

-393.51

C(s, graphite) + O2(g) CO2(g) Hf0 = - 393.51 kJ mol-1

Standard Enthalpy of Formation

C(s, graphite) + 2H2(g) CH4(g) Hf0 = - 74.81 kJ mol-1

½ N2(g) + 3/2 H2(g) NH3(g) Hf0 = - 46.11 kJ mol-1

(1/2) N2(g) + (1/2) O2(g) NO(g) Hf0 = + 33.18 kJ mol-1

EN

TH

AL

PY

, H

HHreactantsreactantsHHreactantsreactants

HHproductsproductsHHproductsproducts

a A + b B → c C + d D

Standard Enthalpy of Reaction

a A + b B

c C + d D

HHOOrxn rxn = = ΣΔΣΔHHff

00(prod) – (prod) – ΣΔΣΔHHff00(react)(react) HHOO

rxn rxn = = ΣΔΣΔHHff00(prod) – (prod) – ΣΔΣΔHHff

00(react)(react)

a a ××HHffO O (A) + b (A) + b × × ΔΔHHff

OO(B)(B)a a ××HHffO O (A) + b (A) + b × × ΔΔHHff

OO(B)(B)

c c ××HHffOO(C) + d (C) + d × × ΔΔHHff

OO(D)(D)c c ××HHffOO(C) + d (C) + d × × ΔΔHHff

OO(D)(D)

HHOOrxn rxn = = ΣΣnnΔΔHHff

00(prod) – (prod) – ΣΣmmΔΔHHff00(react)(react) HHOO

rxn rxn = = ΣΣnnΔΔHHff00(prod) – (prod) – ΣΣmmΔΔHHff

00(react)(react)

Standard Enthalpy of Reaction

CaO(s) + CO2(g) → CaCO3(s)

-635.6 -393.5 -1206.9 [kJ/mol]

HHOOrxn rxn = -177.8 kJ/mol= -177.8 kJ/molHHOO

rxn rxn = -177.8 kJ/mol= -177.8 kJ/mol

Standard Enthalpy of Reaction

CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l)

HHOOrxn rxn = = ΣΣnnΔΔHHff

00(prod) – (prod) – ΣΣmmΔΔHHff00(react)(react) HHOO

rxn rxn = = ΣΣnnΔΔHHff00(prod) – (prod) – ΣΣmmΔΔHHff

00(react)(react)

CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g)

2 H2O(g) → 2 H2O(l)

CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l)

CH4(g) + 2O2(g)

CO2(g) + 2H2O(l)

EN

TH

AL

PY

, H

Reactants

Products

CO2(g) + 2H2O(g)

- 802 kJ

- 88 kJ

- 890 kJ

Hess’s Law

The overall reaction enthalpy is the

sum of the reaction enthalpies

of the steps in which the reaction

can be divided

HHrxnrxn for S(s) + 3/2 O for S(s) + 3/2 O22(g) (g) SOSO33(g)(g)

S(s) + OS(s) + O22(g) (g) SOSO22(g) (g) HH11 = -320.5 kJ = -320.5 kJ

SOSO22(g) + 1/2 O(g) + 1/2 O22(g) (g) SOSO33(g) (g) HH22 = -75.2 kJ = -75.2 kJ

S solid

SO3 gas

direct path

+ 3/2 O2

H3 = -395.7 kJ

SO2 gas

+O2H1 = -320.5 kJ

+ 1/2 O2H2 = -75.2 kJ

Indirect Path

Enthalpy of Solution

NaCl(s) → Na+(aq) + Cl-(aq) HHOOsolution solution = ?= ?HHOO

solution solution = ?= ?

NaCl(s) → Na+(g) + Cl-(g)

Na+(g) + Cl-(g) → Na+(aq) + Cl-(aq)

HH11OO=+788 kJ/mol=+788 kJ/molHH11

OO=+788 kJ/mol=+788 kJ/mol

HH22OO=-784 kJ/mol=-784 kJ/molHH22

OO=-784 kJ/mol=-784 kJ/mol

HHOOsolution solution = 788 kJ/mol – 784 kJ/mol = + 4 kJ/mol = 788 kJ/mol – 784 kJ/mol = + 4 kJ/mol HHOO

solution solution = 788 kJ/mol – 784 kJ/mol = + 4 kJ/mol = 788 kJ/mol – 784 kJ/mol = + 4 kJ/mol

( solutebility tables)

EN

TH

AL

PY

, H

NaCl(s)

Na+(g) + Cl-(g)

Na+(aq) + Cl-(aq)

HH11OO=+788 kJ/mol=+788 kJ/molHH11

OO=+788 kJ/mol=+788 kJ/mol HH22OO=-784 kJ/mol=-784 kJ/molHH22

OO=-784 kJ/mol=-784 kJ/mol

SUMMARY

Standard Enthalpy of Formation

HHffOOHHff

OO HHffO O (element) = 0 kJ/mol(element) = 0 kJ/molHHff

O O (element) = 0 kJ/mol(element) = 0 kJ/mol

Standard Enthalpy of Reaction

Hess’s Law

Homework

Chapter 6, p. 217-222problems