Energy – the ability to do work or produce Potential energy


Heating and cooling curves



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Heating and cooling curves

Starting with a solid below its melting point the follow effects can be observed.

1. The temperature of the solid __________________________ until it begins to melt.

2. When melting begins the temperature ____________________ the solid has all turned into a liquid.

3. The temperature of the liquid ____________________________ until it begins to boil.

4. When boiling begins the temperature _____________ until the liquid has all turned into a gas.

5. The temperature of the gas_____________________________.

Heating Curve

The heating curve can provide a lot of information.

In the regions where the temperature of the solid, liquid or gas is being increased the____________________________ yield a value for the heat capacity. The specific heat capacity is the heat capacity divided by the _____________________________.

A plateau represents a stage __________________________________ with one another.

The heat (or enthalpy) of fusion can be calculated by dividing the length of the melting plateau by the ______________________________

The heat (or enthalpy) of vaporization can be calculated by dividing the length of the boiling plateau by the ____________________________.

Entropy


1st Law of Thermodynamics

Energy is neither created nor destroyed

The energy of the universe is constant

Energy just changes from one form to another

This law lets us know the energy of the system, but does not give us any information about the direction of the energy flow

Spontaneity

Spontaneous process – _____________________________________________

Spontaneous processes can be ____________

Spontaneity tells us the _________________ energy flow

It tells us NOTHING about the ______ of the reaction

For example…

A ball spontaneously rolls down a hill

It does not spontaneously roll up

If iron is exposed to air, it spontaneously rusts

The rust does not spontaneously turn back into air & iron

Entropy


Entropy (s) ______________________________________________

Think of entropy as the amount of _____

The driving force for a spontaneous process is ________________________

Entropy


The natural progression of things is from ________________

Or from _______________________________

Think of a deck of cards…when you drop one it goes from order to disorder

Think of your room… it goes from order to disorder

Think of entropy as a ___________…not a certainty

The more ways a state can exist, _______________________________

Nature spontaneously proceeds toward those states that have the _______________________________

Entropy


Predict which has the highest entropy

CO2 (s) or CO2 (g)

1 mol of N2 at 1 atm or 1 mol of N2 at 0.001 atm

1 mol of N2 at 0.001 atm

Entropy

Predict the sign of the entropy change for the following…

Sugar is added to water to form a solution

Iodine vapor condenses on a cold surface to produce a liquid

-

2nd Law of Thermodynamics



2nd Law of Thermodynamics – ___________________________________________________________________________________________

Energy is conserved…entropy is _______________

3rd Law of Thermodynamics

The entropy of a perfect crystal at 0K is _______

Free Energy

_______________ – a thermodynamic function equal to the enthalpy minus the product of the entropy and the Kelvin temperature

DG = D H – TD S

A process is only spontaneous in the direction where___________________ive

Example

At what temperatures is the following process spontaneous at 1 atm?

Br2(l) à Br2(g)

DH = 31.0 KJ/mol à 31000 J/mol

DS = 93.0 J/ K mol

Dependence of H & S on Spontaneity

What is the sign for D S?

N2(g) + 3H2(g) à 2NH3(g)

First look at the states…if you go from a solid or a liquid to a gas, you will have a + entropy

In this case, all of the states are the same, so we look at the number of moles

N2(g) + 3H2(g) à 2NH3(g)

1 + 3 vs. 2

The entropy decreases because you go from 4 moles to 2

DS is negative

What is the sign for D S?

4NH3 + 5O2 à 4NO + 6H2 O

9 moles vs. 10 moles

DS increases (+)

Calculating D S

Calculating D S is just like calculating D H

Simply use the Appendix…just look at the column for S instead of H

DS° of any element or diatomic molecule is __________

You must look these up!

Example


Calculate D S for the following reaction:

2NiS(s) + 3O2 (g) à 2SO2 (g) + 2NiO (s)


Example


Calculate D S for the following reaction:

Al2 O3 (s) + 3H2 (g) à 2Al(s) + 3H2 O(g)


Gibbs Free Energy & Chemical Reactions

You can calculate D G in 3 ways…

________________

_________

___________________________________

Example

Calculate DH, DS, & DG at 25°C using the following data…

2SO2 + O2 à 2SO3

Example


2SO2 + O2 à 2SO3

DH = 2(+297)+(0) + 2(-396) = -198 KJ

DS = 2(-248)+ (-205) + 2(257) = -187J/K

DG = D H – T D S

=(-198) – (298 x -0.187)

=-142 KJ


Calculate D G

Using the following data at 25°C

Cdiamond + O2(g) à CO2 (g) DG = -397KJ

Cgraphite + O2(g) à CO2 (g) DG = -394KJ

Calculate DG for the reaction:

C diamond à C graphite


Calculating DG

Methanol is a high octane fuel used in high performance racing engines. Calculate DG for the following reaction

2 CH3OH(g) + 3O2(g) à 2CO2(g) + 4H2O(g)

Given the following free energies of formation:

Calculating DG

2 CH3OH(g) + 3O2(g) à 2CO2(g) + 4H2O(g)

2(163) + 3(0) + 2(-394) + 4(-299)


AP Like Questions

C(s) + CO2 (g) « 2CO(g)

All 3 of the gases above form an equilibrium mixture by the equation above

a. Predict the sign of D S of the reaction & justify your prediction


b. In the table below are data that show the % CO in the equilibrium mixture at 2 different temperatures. Predict the sign of DH & justify



Temp %CO

700K 60%


900K 94%

AP Like Questions

c. Appropriately complete the energy diagram for the reaction by finishing the graph below. Also clearly label D H for the reaction…

AP Like Questions

Will the following be endo or exo?

Boil H2O


Dissolve NH4Cl (gets colder)
Dissolving CaCl2 (gets hot)
Melting ice

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