review of chapter 6

Review of Chapter 6Review of Chapter 6Thermodynamics = study of energy

and its interconversionsChemists focus on HEAT energy and

the CHANGE IN HEAT of a reactionWays to measure Enthalpy of a

reaction– Calorimetry (q = mcT)– Hess’s Law– Standard enthalpy of formation– Bond Energy

Spontaneous ReactionsSpontaneous Reactions

A Spontaneous reaction occurs without outside assistance.

Using enthalpy to predict spontaneity– If the energy of the products is lower than the

reactants, reaction is likely to occur– When this is true, the reaction is exothermic– Most exothermic reactions are spontaneous

Spontaneous ReactionsSpontaneous Reactions

Not all spontaneous reactions are exothermic

Nor are all exothermic reactions spontaneous

Enthalpy of a reaction is not the only factor that determines if a reaction is spontaneous

EntropyEntropy

Entropy (S) = measurement of randomness or disorder of a system

Left alone, systems tend to become more disordered

Chemists study the change in entropy (S) that accompanies a reaction

Entropy and SpontaneityEntropy and Spontaneity

If the products are in a greater state of disorder (increase in entropy) than the reactants, reaction is likely to be spontaneous.

Entropy’s effect on spontaneity depends on temperature

Unit of entropy = J / K mol

Spontaneous ReactionsSpontaneous Reactions

Most spontaneous reactions are

– Exothermic– Increase in Entropy

11stst Law Thermodynamics Law Thermodynamics

Total energy of the universe remains constant

This is synonymous with the Law of Conservation of Energy

22ndnd Law Thermodynamics Law Thermodynamics

Entropy of the universe INCREASES for any spontaneous process.– For any spontaneous process, the

sum of the change in entropy of the system and surroundings must be positive

Suniverse (+) = Ssystem + Ssurroundings

22ndnd Law (con’t) Law (con’t)

Reactions are only spontaneous in ONE direction.

If a reaction is spontaneous, it will NOT be spontaneous in the reverse direction.

If a reaction is NOT spontaneous as written, it WILL be spontaneous in the reverse direction.

33rdrd Law Thermodynamics Law Thermodynamics

The entropy of a perfect crystal at 0 Kelvin is zero.

All other entropy values are positive.

Change in entropy values however, can be positive or negative.

EntropyEntropy

Change in entropy of a reaction will be positive if– Solids are melted to form pure liquids

or dissolved to form solutions.– Liquids or solids are converted to gas.– The number of moles of gas increases.

(More moles of gas on product side)– The temperature increases.

EntropyEntropy

Predict if the following will have a positive change in entropy or negative– 2 C (s) + O2 (g) 2CO (g)

– 2 K (s) + Br2 (l) 2KBr (s)

– 2 MnO2 (s) 2 MnO (s) + O2 (g)

– O (g) + O2 (g) O3 (g)

EntropyEntropy

Entropy is a state functionChange in entropy can be calculated

just like enthalpySo

reaction = npSo products – nrSo

reactants

Calculating EntropyCalculating Entropy

Predict and find So for the reaction

2 NH3 (g) + CO2 (g) NH2CONH2 (aq) + H2O(l)

Entropy at a Phase Entropy at a Phase ChangeChange

To calculate entropy at a phase change, use the equation

S = H / T**change H to joules

You must know H and the temperature to do this!

Try the example in the notes.

Free EnergyFree Energy

What can tell us if a reaction is spontaneous?

A property called Free energy (G)Free energy = measurement of

amount of energy available to flow out of a system

Free EnergyFree Energy

Free energy depends on three things– Change in enthalpy– Change in entropy– Temperature

Change in free energy can be calculated using the equation

Go = Ho – TSo **This equation is

important

GGoo = = HHoo – T – TSSoo

Ho tells us the amount of work that could potentially be done by the system

So tells us the amount of work that needs to be done on the system

Go is the maximum amount of work that can be done by the system

Free Energy SignFree Energy SignThe sign of free energy tells us

about the reaction

G value

Type of Reaction

Negative

Spontaneous as written

Positive NOT spontaneous as written(Spontaneous in reverse

direction)

Zero System is at equilibrium

Free EnergyFree Energy

Free energy is a state functionStandard free energy of formation can

be calculated and used to determine change in free energy of a reaction

Change in free energy can be foundGo

reaction = Gof (products) - Go

f (reactants)

Calculating Free EnergyCalculating Free Energy

Use the Gof values in the book to

determine Go for the reactionC2H5OH (l) + 3 O2 (g) 2 CO2 (g) + 3 H2O (g)

Non-Standard ConditionsNon-Standard Conditions

To find G under non-standard conditions, you must know– Change in free energy under standard

conditions (Go)– Temperature (T)– Ratio of products to reactants (Q)

(Reaction Quotient)

Reaction QuotientReaction Quotient

Most reactions do not go all the way to completion

Reaction quotient tells us how far a reaction has gone

Molar concentrations or pressures for gases are used to calculate the reaction quotient

Reaction QuotientReaction Quotient

Equation to calculate reaction quotientQ = molar concentrations products

molar concentrations reactantFor the reaction aA + bB qQ + rR

Q = [Q]q [R]r

[A]a [B]b

** Amounts of pure solids and liquids not used

Free Energy Non-StandardFree Energy Non-Standard

Once you know the– Standard change in free energy– Temperature– Reaction quotientYou can use the equationG = Go + RT lnQR = universal gas constant (8.314 J/K

mol)

Free Energy at EquilibriumFree Energy at Equilibrium

In fact, these reactions can occur in the forward direction and the reverse direction simultaneously

For reactions that do not go all the way to completion, there comes at point at which the rate of reaction in the forward direction equals the rate of reaction in the reverse direction

Free Energy at EquilibriumFree Energy at Equilibrium

Ta-Da!!! This point is known as equilibrium All reactions that come to equilibrium do

so at a very specific set of conditions The ratio of products to reactants at

equilibrium is expressed by the equilibrium constant (K)

At equilibrium the reaction quotient is equal to the equilibrium constant (Q = K)

Free Energy at EquilibriumFree Energy at Equilibrium

Additionally, a system at equilibrium has no free energy available to flow out of the system. Therefore, at equilibrium, G = 0 and Q = K.

We can find the standard change in free energy for a system at equilibrium by using the equation:

Go = - RT ln K

Free Energy at EquilibriumFree Energy at Equilibrium The sign of Go and the value of K can

give us information about how far a reaction comes toward completion before equilibrium is reached.

Sign of Go K value Favored direction

Negative Greater than 1

Products favored (greater concentration of products than reactants)

Zero Equal to 1 Equal products and reactants

 Positive Less than 1 Reactants favored (greater concentration of reactants than products)