Thermochemistry

Chapters 6 and11

TWO Trends in Nature

• ___________ _________

• _____ energy ____ energy

_________ is any process that gives off heat – transfers thermal energy from the ______ to the _________.

__________ is any process in which heat has to be supplied to the _______ from the _________

2H2 (g) + O2 (g) 2H2O (l) + energy

H2O (g) H2O (l) + energy

energy + 2HgO (s) 2Hg (l) + O2 (g)

6.2

energy + H2O (s) H2O (l)

_________ is used to quantify the heat flow into or out of a system in a process that occurs at constant ________.

H = H (________) – H (________)

H = heat given off or absorbed during a reaction at constant pressure

H < 0 H > 0 6.4

Thermochemical Equations

H2O (s) H2O (l) H = 6.01 kJ

Is H negative or positive?

System ______ heat

_____thermic

H ____ 0

6.01 kJ are _______ for every 1 mole of ice that _____at 00C and 1 atm.

6.4

Thermochemical Equations

CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) H = -890.4 kJ

Is H negative or positive?

System gives off heat

______thermic

H____ 0

890.4 kJ are ________for every 1 mole of methane that is combusted at 250C and 1 atm.

6.4

H2O (s) H2O (l) H = 6.01 kJ/mol ΔH = 6.01 kJ

• The ______________________always refer to the number of moles of a substance

Thermochemical Equations

• If you reverse a reaction, the ______ of H changes

H2O (l) H2O (s) H = -6.01 kJ

• If you multiply both sides of the equation by a factor n, then H must change by the same factor n.

2H2O (s) 2H2O (l) H = 2 mol x 6.01 kJ/mol = 12.0 kJ

6.4

H2O (s) H2O (l) H = 6.01 kJ

• The ___________of all reactants and products must be specified in thermochemical equations.

Thermochemical Equations

6.4

H2O (l) H2O (g) H = 44.0 kJ

How much heat is evolved when 266 g of white phosphorus (P4) burn in air?

P4 (s) + 5O2 (g) P4O10 (s) Hreaction = -3013 kJ

_________________H0) is the heat change that results when ________ of a compound is formed from its _______ at a pressure of 1 atm.

f

The standard enthalpy of formation of any element in its most stable form is ______.

H0 (O2) = 0f

H0 (O3) = 142 kJ/molf

H0 (C, graphite) = 0f

H0 (C, diamond) = 1.90 kJ/molf

6.6

The ________________(H0 ) is the enthalpy of a reaction carried out at 1 atm.

rxn

aA + bB cC + dD

H0rxn dH0 (D)fcH0 (C)f= [ + ] - bH0 (B)faH0 (A)f[ + ]

H0rxn H0 (________)f= H0 (________)f-

6.6

Hess’s Law: ??????

(Enthalpy is a _________ function. It doesn’t matter how you get there, only where you start and end.)

Benzene (C6H6) burns in air to produce carbon dioxide and liquid water. How much heat is released per mole of benzene combusted? The standard enthalpy of formation of benzene is 49.04 kJ/mol.

2C6H6 (l) + 15O2 (g) 12CO2 (g) + 6H2O (l)

H0rxn H0 (products)f= H0 (reactants)f-

6.6

Calculate the standard enthalpy of formation of CS2 (l) given that:C(graphite) + O2 (g) CO2 (g) H0 = -393.5 kJrxn

S(rhombic) + O2 (g) SO2 (g) H0 = -296.1 kJrxn

CS2(l) + 3O2 (g) CO2 (g) + 2SO2 (g) H0 = -1072 kJrxn

1. Write the enthalpy of formation reaction for CS2

C(graphite) + 2S(rhombic) CS2 (l)

2. Add the given rxns so that the result is the desired rxn.

6.6

Chemistry in Action:

Fuel Values of Foods and Other Substances

C6H12O6 (s) + 6O2 (g) 6CO2 (g) + 6H2O (l) H = -2801 kJ/mol

1 cal = 4.184 J

1 Cal = 1000 cal = 4184 J

The __________________-is the heat generated or absorbed when a certain amount of solute dissolves in a certain amount of solvent.

6.7

Which substance(s) could be used for melting ice?

Which substance(s) could be used for a cold pack?

The Solution Process for NaCl

Hsoln = Step 1 + Step 2 = 788 – 784 = 4 kJ/mol6.7

Energy Diagrams

Exothermic Endothermic

(a) Activation energy (Ea) for the forward reaction

(b) Activation energy (Ea) for the reverse reaction

(c) Delta H

50 kJ/mol 300 kJ/mol

150 kJ/mol 100 kJ/mol

-100 kJ/mol +200 kJ/mol

________________is a measure of the ________________ _______________of a system.

order SdisorderS

If the change from initial to final results in an increase in randomness

S ____ 0

For any substance, the solid state is more ordered than the liquid state and the liquid state is more ordered than gas state

S_______ < Sl________ << S_____

H2O (s) H2O (l) S ____ 018.3

______ Law of Thermodynamics

Energy _________ converted from one form to another but energy ___________ be created or destroyed.

______Law of Thermodynamics

The entropy of the ______increases in a ________process and remains unchanged in an _________ process.

Suniv = Ssys + Ssurr ____ 0___________ process:

Suniv = Ssys + ssurr_____ 0___________ process:

18.4

Entropy Changes in the System (Ssys)

aA + bB cC + dD

S0rxn dS0(D)cS0(C)= [ + ] - bS0(B)aS0(A)[ + ]

S0rxn S0()________)= S0(________)-

The ________________ (S0 ) is the entropy change for a reaction carried out at 1 atm and 250C.

rxn

18.4

What is the standard entropy change for the following reaction at 250C? 2CO (g) + O2 (g) 2CO2 (g)

S0(CO) = 197.9 J/K•molS0(O2) = 205.0 J/K•mol

S0(CO2) = 213.6 J/K•mol

S0rxn

S0rxn l

Entropy Changes in the System (Ssys)

18.4

When gases are produced (or consumed)

• If a reaction produces more gas molecules than it consumes, S0 ____ 0.

• If the total number of gas molecules diminishes, S0 ____ 0.

• If there is no net change in the total number of gas molecules, then S0 may be _______________BUT S0 will be a _____ number.

What is the sign of the entropy change for the following reaction? 2Zn (s) + O2 (g) 2ZnO (s)

_______________ Physical and Chemical Processes

• A waterfall runs ________

• A lump of sugar ________ in a cup of coffee

• Heat flows _____ a hotter object ____ a colder object

• A gas _______ in an evacuated bulb

• Iron exposed to oxygen and water forms ___

spontaneous

nonspontaneous

18.2

Suniv = Ssys + Ssurr ___ 0Spontaneous process:

Suniv = Ssys + Ssurr ____ 0Equilibrium process:

Gibbs Free Energy

For a constant-temperature process:

G = Hsys -TSsysGibbs free energy (G)

G __ 0 The reaction is ________ in the forward direction.

G __ 0 The reaction is ________ as written. The reaction is _______ in the _______direction.

G ___ 0 The reaction is at ______.

18.5

G = H - TS

18.5

18.5

aA + bB cC + dD

G0rxn dG0 (D)fcG0 (C)f= [ + ] - bG0 (B)faG0 (A)f[ + ]

G0rxn G0 (_______)f= G0 (_______________)f-

The __________________ (G0 ) is the free-energy change for a reaction when it occurs under ________ conditions.

_______________(G0) is the free-energy change that occurs when ______ of the compound is formed from its elements in their standard states.

f

G0 of any element in its stable form is __________.

f

2C6H6 (l) + 15O2 (g) 12CO2 (g) + 6H2O (l)

G0rxn G0 (products)f= G0 (reactants)f-

What is the standard free-energy change for the following reaction at 25 0C?

G0rxn =

Is the reaction spontaneous at 25 0C?

18.5

Recap: Signs of Thermodynamic ValuesRecap: Signs of Thermodynamic Values

Negative Positive

Enthalpy (ΔH)

Entropy (ΔS)

Gibbs Free Energy (ΔG)

Gibbs Free Energy and Chemical Equilibrium

G = G0 + RT lnQ

R is the gas constant (8.314 J/K•mol)

T is the absolute temperature (K)

Q is the reaction quotient

At Equilibrium

18.6

G0 = RT lnK

18.6

The __________[most books use lower case c] of a substance is the amount of heat (q) required to raise the temperature of ________of the substance by ______ Celsius.

The ________of a substance is the amount of heat (q) required to raise the temperature of ___________ (m) of the substance by ______ Celsius.

Heat (q) absorbed or released:

6.5

How much heat is given off when an 869 g iron bar cools from 940C to 50C?

6.5

Constant-Pressure Calorimetry

No heat enters or leaves!6.5

6.5

The _____________is the temperature at which the (equilibrium) ______________of a liquid is equal to the __________pressure.

The _______________ is the temperature at which a liquid boils when the _______________ pressure is ____atm.

11.8

Phase Changes

The _______________ is the temperature above which the gas cannot be made to _______, no matter how great the applied pressure.

The __________ (Pc) is the minimum pressure that must be applied to bring about _______ at the critical temperature.

11.8

Where’s Waldo?Can you find…

The Triple Point?

Critical pressure?

Critical temperature?

Where fusion occurs?

Where vaporization occurs?

Melting point (at 1 atm)?

Boiling point(at 6 atm)?

Carbon Dioxide

11.8

H2O (s) H2O (l)

The melting point of a solid or the freezing point of a liquid is the temperature at which the _____ and _____ phases coexist in ______

11.8

H2O (s) H2O (g)

Molar heat ________(Hsub) is the energy required to sublime 1 mole of a solid.

( Hess’s Law)

Molar heat of fusion (Hfus) is the energy required to melt 1 mole of a solid substance.

11.8

11.8

Sample Problem• How much heat is required to change 36 g of

H2O from -8 deg C to 120 deg C?

Sample Problem• How much heat is required to change 36 g of

H2O from -8 deg C to 120 deg C?