Notes on

Energy

• Is the capacity to do work or to produce heat

• Forms:

• Kinetic energy = energy of motion

• Potential energy = energy of position

• Thermal energy = a form of KE caused by random internal motion of particles of matter

Heat

• A form of energy

• Is measured in calories (cal) or Joules (J)

• 1 cal = 4.184 J

• 1 Cal = 1000 cal

• Is not the same as temperature !

Temperature

• A measure of the Kinetic Energy in a system

• Units are: Fahrenheit or Celsius (Centigrade) or Kelvin

Absolute Zero

• Corresponds to about -273 oC

• Is the (theoretical) point at which the motion of particles of matter – their kinetic energy - ceases

More about Temperature

• For most applications in Chemistry, we will use the Celsius scale.

• There is at least one topic for which we must change our temperature units to Kelvins – that is for Gas Laws.

• On the Kelvin scale there is no “zero” and no negative values. That becomes important in calculations involving gases, since a negative temp might make it look like, in calculations, that there is a negative volume.

Law of Conservation of Energy

Energy is neither created nor destroyed…it just Changes form.

We can record a reading as, for example, 102.3 degrees Celsius - but - if we were to record a temperature using the Kelvin scale, we would simply state, for example, “275 Kelvins” (not “degrees kelvin”)

By the way…

Conversions

• C = K – 273 so therefore K = C + 273

• C = 5/9 (F – 32) and F = (9/5*C) + 32

22.5oC = ___ K

55oF = ___ oC

290 K = ___oC

97 oF = ___ oC

Temperature “Sense” – Memorize these:

Room Temp in C: 22

Body Temp in C: 37

Boiling Pt/Freezing Pt: 100/0

Phase Changes Depend on Energy!

Adding Heat

Energy!

Removing Heat

Energy!

Random motion; VERY weakForces of attraction

Freely moving; definite volume; forces of attraction btw particles

Particles vibrating about fixed positions; very strong forces of attractions

Heating Curve

Temperature is NOT changing!

KE not changing!

Temp DOES NOT CHANGE

during a phase

change!

Cooling Curve

During a phase change

• There are 2 phases present.• Temperature DOES NOT change!• KE does NOT change.• PE does change.• Forces of attraction is changing.

Heat energy is being added/removed; energy is being used to weaken or strengthen forces of attraction NOT to increase or decrease KE of particles.

Melting – Heat energy is added to break up the forces of attraction between the solid particles. Particles move more freely.

Melting Point and Freezing point of a substance are the same temperature! (Depends on whether heat energy is being added or removed.)

Evaporation and Boiling/Vaporization are NOT the same thing! They are both a phase change from liquid to solid, but evaporation can happen below the boiling point and only surface particles are escaping.Boiling can only happen at a particular temperature and requires a much higher input of heat energy. Particles throughout the liquid are escaping.

SUBLIMATION

Dry Ice!

Condensation

As water vapor molecules cool off and slow down, they get close enough to clumptogether and change phase to liquid.

Specific Heat

• A property of matter that is unique to each element or compound –

• It does not depend on the amount of matter present, so is an intensive property

• Also called “heat capacity”• “the amount of energy needed to raise

the temperature of 1 g of a substance by 1 degree Celsius”

Formula for Specific Heat

• Q = mcT

• Where Q = “heat” and is measured in J

• Where m = mass and is measured in g

• Where c = specific heat and is measured in …(see next slide!)

• Where T = (“delta T”) is “change in temperature” and is measured in C

Unit for Specific Heat

• So what is the unit for “c”?

• Let’s rearrange the formula Q = mcT in terms of c (we call this “deriving”)

• c = Q / mT

• All units in Chemistry and Physics come from the formulas that they’re in

• Therefore, c = Q / mTJ g/ oC

Specific Heat of Water

• Water, H2O, has an extremely high “heat capacity”, which in plain language, means that it takes a lot of energy to heat it up and it has a tremendous ability to retain heat (aka: takes a lot to cool it down)

• The specific heat of water is 4.18 J/goC (which you may recognize as 1 calorie)

• Memorize this!!!!!!!!!!!

Why That’s Important• It affects your life every day. • The reason it tends to be cooler at the ocean

than in the valley is due to the cH2O

• The reason that hurricanes get stronger over warm water is that they’re deriving their energy from the heat contained in the water (maybe that doesn’t affect you every day); it gives them energy

• The reason a steam burn is worse than a regular burn is due to the energy in the steam

• Can you think of other examples?

Reminder: Law of Conservation of Energy

• We know that energy cannot be created or destroyed

• We know that heat is a form of energy

• So…it must be true that the heat lost by one substance must be “accountable” by another substance or substances, or…

• Q lost = Q gained

If Qlost = Qgained

• …and if Q = mcT, then it must be true that…

• (mcT) lost = (mcT) gained

• And that’s the foundation for our next lab: “Specific Heat of a Metal”

Pre-Lab: Specific Heat of a Metal

• Discussion: You will be given a sample of a metal. You will have to determine its specific heat.

• We already know the specific heat of water (4.18 J/goC)

• We will use the concept of …

• (mcT)lost =(mcT)gained to solve for the unknown c for the metal

Lab: Specific Heat of a Metal

• (mcT)gained = (mcT)lost

• (mcT)gained by water = (mcT)lost by metal

• (mcT)water = (mcT)metalMeasure

the mass

of the

water

You know

the “c”

for

water

(4.18 J/gC)

Measure the

initial temp of the

water before the metal was added; the final temp after

the metal heats it up

Measure the mass of the metal

You will be calculatingthe “c” for the metal

If the metal starts at the boiling pointof water, that’syour T1. The temp after thewater has stoppedrising has to bethe temp of the metal as well, so that is T2.