Plan for Fri, 17 Oct 08

• Lecture– Main points of 6.1

• System vs. surroundings• Kinetic vs. potential energy• Work vs. heat

– Enthalpy and Calorimetry (6.2)

• Quiz 3

Thermodynamic Talking Points• Energy of the universe is constant, but it is not

uniformly distributed.• We can monitor the changes in energy due to

physical or chemical processes by artificially partitioning the universe into a “system” and its “surroundings.”

• The energy content of a system is partitioned into:– Kinetic: the energy of an object in motion– Potential: the energy of an object derived from its

location in a field of force

Thermodynamic Talking Points• Energy can be exchanged between the system and

surroundings in two main ways:

– Heat (q): energy flow between objects due to a temperature difference.

• Heat is a transfer of energy manifested by changes in the microscopic thermal motions of particles

• Changes in vibrations, rotations and translations of molecules• Changes in bonding patterns

– Work (w): energy acting over a distance that results in the movement of an object.

• Work is an energy transfer manifested by changes in the macroscopic physical variables of a system.

• To accelerate a baseball from 0 mph to 60 mph, we must perform work on it.

q without w• If we run an experiment in

which the volume is fixed, then

w = -PV = 0E = q + 0

• Combustion produces gaseous products.

• If we don’t allow the gaseous products to change their volume, the energy lost by the system is in terms of heat only.

C(s) + O2(g) CO2(g) + heat

w without q

Some energy is expended by the system to push back the atmosphere as it expands.

• If we run an experiment in which the volume is allowed to change, but no heat transfer is allowed, then

q = 0 E = w

• Isothermal expansion of a gas only loses energy as work.

• Is any PV-work done if the gas expands into a vacuum?

q and w together

Zn (s) + 2HCl (aq) Zn2+(aq) + H2 (g) + 2H2O + Cl(aq)

System loses energy as heat.

Production of H2(g) causes piston to move... system loses energy as work done on surroundings.

The combustion of fuel in the space shuttle results in:

1. The emission of heat (q)

2. The expansion of the system against constant atmospheric pressure (PV-work)

3. The rapid movement of the shuttle over a distance (non-PV-work)

q, PV work, and non-PV work

If we could keep the shuttle from moving as its fuel combusted, what would happen to this non-PV-work energy?

It would be transferred as heat to the surroundings.

1. Intake. Mixture of air and gas enters combustion chamber.

2. Compression. The mixtures are placed under pressure.

3. Combustion/Expansion. Spark ignites the mixture, resulting in rapid production of gaseous products and heat. The hot mixture expands, pressing on and moving parts of the engine and performing useful work.

4. Exhaust. The cooled combustion products are exhausted.

Four-stroke cycle

1. Intake2. compression3. power4. exhaust

Internal Combustion Engine

Work• There really isn’t anything useful about PV-work…it is

simply an accounting of the energy required to change the volume of your system against constant external pressure.

• In this way, the combustion of gas in your car, even though it results in the production of gases, is not PV-work…

• The volume change in the system relative to the amount of gas produced is so small that the pressure does not remain constant.

• Useful work comes from a clever partitioning of the heat energy (the enthalpy), which you will learn about in 162 (HAND-WAVING ALERT).

Heat vs. Temperature

Copyright © Houghton Mifflin Company. All rights reserved.

Go to Slide Show View (press F5) to play the video or animation. (To exit, press Esc.)

This media requires PowerPoint® 2000 (or newer) and the Macromedia Flash Player (7 or higher).[To delete this message, click inside the box, click the border of the box, and then press delete.]

Temperature is an index of the average molecular KE.

Heat is an exchange of energy due to a temperature difference.

q

Heat vs. Temperature• Add 100 J of heat to a collection of 100 water molecules.• Add 100 J of heat to a collection of 1,000,000 water molecules.

• Which system will exhibit the greatest increase in T?– The collection of 100 water molecules.– Same amount of heat delivered, different average velocities, different

final temperatures.

• An extensive physical property depends directly on the amount of substance present.– Examples: enthalpy, mass, volume, length

• An intensive physical property is not related to the amount of substance.– Examples: temperature, density, concentration, color

We would like to know about the internal energy change accompanying chemical processes:

Consider a process carried out at constant P:

The heat transferred at const P contains a “pure heat” component and a “non-PV-work” component (HAND-WAVING ALERT)…you will learn about this in 162.

The work term at const P represents the energy required to change the volume of the system…this is “PV-work.”

Measuring the heat flow at const P gives you the internal energy minus PV-work.

Measuring E

E q w

E q w PE q P V

Pq E P V

pq H

Recall our definition of enthalpy:

Recall the heat transferred at constant P:

Enthalpy vs. Internal Energy

H E PV

Pq E P V

Pq E P V H We can track the heat flow in a process occurring at constant P and that will give us direct info about E.

Enthalpy examples• Heat transfer often accompanies physical and chemical processes.

• “Heat of Fusion” - enthalpy change associated with meltingH2O(s) H2O(l) Hfus = +0.334 kJ/mol

• “Heat of Vaporization” - enthalpy change associated with boilingH2O(l) H2O(g) Hvap = +2.26 kJ/mol

• “Heat of Reaction” - enthalpy change associated with a chemical reactionCH4(g) + 2O2(g) CO2(g) + 2H2O(g) Hrxn = -50.1 kJ/mol

• “Heat of Solution” – enthalpy change associated with the dissolution of ionic solids in water

NH4NO3(s) NH4+(aq) + NO3

-(aq) Hsoln = +82.93 kJ/molCaCl2(s) Ca2+(aq) + 2Cl-(aq) Hsoln = -26.2 kJ/mol

EXO

ENDO

ENDO

ENDO

EXO

Example• 5.00 g of ammonium nitrate is dissolved in

500. mL of water at 25.00oC. What is the final temperature of the water?

NH4NO3(s) NH4+(aq) + NO3

-(aq)

Hsoln = +82.93 kJ/mol

Heat Flow

System:

1 L of water

T = 50oC

Surroundings:

T = 25oC

1 L of water

T = 25oC

How much heat did the water lose?

Well, first we need to know how much heat it had.

The liter of water is allowed to cool,

until it reaches room temperature.

Heat CapacityHeat Capacity (C): • the energy required to raise the

temp. of a sample of a substance by 1oC;

• the ability of a substance to absorb heat.

• C is an extensive property

Recall: thermal energy is associated with the random motions of atoms and molecules

Heat capacity gives you a measure of how much more random those motions can get.

q C T

f iT T T Units of C: J/oC

Specific HeatSpecific heat (s): the energy

required to raise the temperature of 1 gram of a substance by 1oC.

You can also have molar heat capacities.

Specific heat is an intensive version of heat capacity (like density is an intensive version of mass)

If we know the specific heat of a substance, the mass, and the temperature change, we can determine the heat flow.

q s m T C

Units of s: J/oC.g

Specific Heat Specifics

• Typically, specific heats depend on:– Degrees of freedom: the number of different motions

available to a molecule or atom– Molar mass: bigger molecules have more bonds that

can vibrate– Strength of intermolecular attraction: energy can

be stored in these attractions…they can vibrate too

• Compounds have higher heat capacities than elements…how come?– Greater complexity more ways to store energy

higher heat capacity

Specific vs. Molar heat capacities

Specific heat (J/g.oC)

Molar heat capacity (J/mol.oC)

Al(s)

Cu(s)

C2H5OH(l)

H2O(l)

H2(g)

0.901

0.384

2.43

4.18

14.30

24.3

24.4

112.2

75.3

28.82

System:

1 L of water

T = 50oC

How much heat did the water lose?

The liter of water is allowed to cool, until it

reaches room temperature, 25oC.

q s m T J

4.184 g C

s

1 L = 1000 mL = 1000 gm

f iT T T 50 C+25 C = 25 C

J4.184 1000 25 CCq gg 104,600 J104.6 kJ