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Chemical Thermodynamics

Spontaneous Processes

First Law of Thermodynamics Energy is Conserved – ΔE = q + w

Need value other than ΔE to determine if a process if favored (spontaneous)

Spontaneous processes have a direction Spontaneity can depend on temperature

Reversible Processes

Reversible Process When a change in a

system is made in such a way that the system can be restored to its original state by exactly reversing the change.

Irreversible Processes

Irreversible Process A process that cannot

simply be reversed to restore the system and surroundings.

Must take alternative pathway

Only system is returned to its original state

Entropy

Processes where the disorder of the system increases tend to occur spontaneously Ice melting Salts dissolving

Change in disorder and change in energy determine spontaneity

Entropy (S) is a state function (ΔS) that measures of disorder Units – J/K

Determining Entropy Change

Predict whether ΔS is positive or negative for each of the following processes. H

2O(l) → H

2O(g)

Ag+ (aq) + Cl-(aq) → AgCl(s) 4Fe(s) + 3O

2(g) → 2Fe

3(s)

CO2(s) → CO

2(g)

CaO(s) + CO2(g) → CaCO

3(s)

Calculating Entropy

In a reversible process there is only one heat for both processes (q

r e v)

When a process occurs at constant temperature entropy is related to both heat and absolute temperature – ΔS = q

r e v / T

When 1mol of water is converted to 1mol of steam at 1 atm, ΔH

v a p = 40.67kJ/mol, what is

the change in entropy?

Calculating Entropy

The element mercury, Hg, is a silvery liquid at room temperature. The normal freezing point of mercury is -38.9°C, and its molar enthalpy of fusion is ΔHf u s = 2.29 kJ/mol. What is the entropy change of the system when 50.0 g of Hg(l) freezes at the normal freezing point?

Answer: -2.44 J/K

Calculating Entropy

The normal boiling point of ethanol is 78.3°C and its molar enthalpy of vaporization is 38.56 kJ/mol. What is the change in entropy when 68.3g of ethanol at 1 atm condenses to liquid at the normal boiling point?

Answer: -163 J/K

Second Law of Thermodynamics

Second law = Entropy of the universe increases in any spontaneous process. ΔS

univ = ΔS

sys+ ΔS

surr = 0 → reversible process

ΔSuniv

= ΔSsys

+ ΔSsurr

> 0 → irreversible process

Unlike energy, entropy is not conserved Examples

Straightening up your room 4Fe(s) + 3O

2(g) → 2Fe

3(s)

Exceptions – Isolated Systems ΔS

sys = 0 → reversible process

ΔSsys

> 0 → irreversible process

Calculating Entropy

Consider the reversible melting of 1 mol of ice in a large isothermal water bath at 0°C and 1 atm pressure. The enthalpy of fusion of ice is 6.01 kJ/mol. Calculate the entropy change in the system and in the surroundings, and the overall change in entropy of the universe for this process.

Answer = 22.0 J/K, 0 J/K

Molecular Interpretation of Entropy

Why does the entropy increase when a gas expands?

Why does entropy decrease in the following reaction - 2NO(g) + O

2(g) → 2NO

2(g)?

Degrees of Freedom Translational Motion Vibrational Motion Rotational Motion

Molecular Interpretation of Entropy

Lowering temperature decreases energy which lowers the degrees of freedom.

Third law of thermodynamics = entropy of a pure crystalline substance at absolute zero is zero.

Entropy generally increases with temperature.

Entropy Changes in Reactions

Entropy increases for processes in which: Liquids or solutions are formed from solids. Gases are formed from either solids of liquids. The number of molecules of gas increases during a

chemical reaction. Choose the sample of matter that has greater

entropy and explain your choice. 1 mol of NaCl(s) or 1 mol HCl(g) at 25°C 1 mol of HCl(g) or 1 mol Ar (g) at 25°C 1 mol of H

2O(s) at 0°C or 1 mol of H

2O(l) at 25°C

Predicting Entropy Changes

Predict whether the entropy change of the system in each of the following isothermal reactions if positive or negative. CaCO

3(s) → CaO(s) + CO

2(g)

N2(g) + 3H

2(g) → 2NH

3(g)

N2(g) + O

2(g) → 2NO(g)

HCl(g) + NH3(g) → NH

4Cl(s)

2SO2(g) + O

2(g) → 2SO

3(g)

Entropy Changes in Chemical Reactions

Using variation of heat capacity with temperature absolute entropy are measured.

Molar entropy at standard states (S°) Molar entropies of elements are not zero Molar entropies of gases are greater

than liquids and solids Molar entropies generally increase with

increasing molar mass. Molar entropies generally increase with

increasing number of atoms

Entropy Changes in Chemical Reactions

Entropy Change in a reaction can be calculated using a table of values ΔS°= ΣnS°(prod)-ΣmS°(react)

Calculate ΔS° for the synthesis of ammonia from nitrogen and hydrogen at 298K. N2(g) + 3H

2(g) → 2NH

3(g)

Answer: -198.3 J/K

Entropy Changes in the Surroundings

Surroundings are essentially a large constant temperature heat source.

The change in entropy will then depend on how much heat is absorbed or given off by the system. ΔS

s u r r = -q

s y s /T

If the reaction happens at constant P, what is q?

Calculate the ΔSu n i v

given the heat of formation of ammonia (-46.19kJ/mol)

Gibbs Free Energy

Spontaneity depends on enthalpy and entropy. Gibbs Free Energy – G = H – TS

In a chemical reaction at constant T ΔG = ΔH – TΔS

Algebra fun When both T and P are constant

If ΔG is negative, the reaction is spontaneous in the forward direction

If ΔG is zero, the reaction is at equilibrium If ΔG is positive, the forward reaction is nonspontaneous, work

must be done to make it occur. The reverse reaction is spontaneous.

Standard Free Energy Changes

Gibbs free energy is a state function. For Standard free energies

Free energies for standard states are set to zero. Gases should be at 1 atm Solutions should be 1M in concetration Solids and Liquids should be in their pure forms

ΔG°= ΣnG°(prod)-ΣmG°(react)