Mr. Shields Regents Chemisty U03 L04

HeatLet’s now discuss something calledHEAT and HEAT CAPACITY

Heat is the energy transferred between particles and the surroundings as a result of a temp. difference

It ALWAYS FLOWS from Hot to Cold. NEVER from Cold to hot!

Ex. You put a cold pack on your leg. Heat flows from your leg to the cold pack. Your leg Loses heat so it “feels” cold.

“COLD” DOES NOT FLOW FROM THE ICE PACK

70 deg C 30 deg C

Heat Energy (Q)• Heat energy results from the movement

of atoms & molecules in matter.

• This Energy depends upon how much of the substance is present (an extensive property)

• Remember the SI Unit for energy is the JOULE (calories are used in Eng. System)

1 calorie = 4.184 Joules and1 calorie (or 4.184J) can raise the

temp of 1g water 1°C

Heat Flow

• Remember …heat always flows from hotter objects to cooler objects.

• And the QuantityQuantity of heat transferred depends on:

• Temperature difference• Mass of both substances

• And the SPECIFIC HEAT of both substances

H C

Hey!What’s this?

Specific HeatSome materials absorb or transfer heat better than other materials.

Consider what happens to the temperature of a metal spoon handle vs. a plastic spoon handleimmersed in a cup of hot coffee.

We use cloth pot holders to pick up hot metal pots.We also use cast iron radiators (not plastic or wood)to heat our homes by running hot water through it.

The SPECIFIC HEAT of a substance is the amount of heat required to raise the temperature of 1 gof the substance by 1 degree centigrade.

Each substance has it’s own unique specific heat (Cp)

Also 1 cal

1 deg(Cp)

Specific heats of various Materials

Material Cp Joules/gram deg C

Aluminum 0.900

Silver 0.230

Gold 0.129

Graphite 0.720

Diamond 0.502

Copper 0.385

Brass 0.377

Iron 0.444

Mercury 0.139

Water 4.184

Ice 2.01

Ethanol 2.46

Materials withLOW Cp areGood heatConductors

Which materialIs the best HeatInsulator?

What materialwould make The best Radiator?

Cu, Brass orIron?

Specific Heat Equations

HEAT is typically represented by the term “q”

Specific Heat is represented by the term “Cp”

Recall that SPECIFIC HEAT of a substance is theamount of heat required to raise the temperatureof 1 g of the substance by 1 degree C.

Therefore …. q = m x Cp x (Tf – Ti)

m = mass Tf = Final TemperatureTi = Initial Temperature

Change in Temp

Specific Heat CalculationsLet’s try a problem.

How much heat energy in Joules is needed to raise the temperature of 30 grams of water from 20 deg. C to 30 deg. C?

Recall that q = m x Cp x (Tf – Ti)

q = 30 g x 4.18 J/g deg C x (30 deg C – 20 deg C)

q = 125.4 x 10 deg C = 1254 Joules

Specific Heat CalculationsLet’s try another problem.

If 1500 joules are added 100 grams of water at 20 deg C what is the final temperature of the water?

Heat = mass x specific heat x (final temp – initial temp)

q = m x Cp x (Tf – Ti)

1500 = 4.18 J/g deg C x 100 g x (final temp – 20 deg C)

1500= 418 x (final temp – 20) final temp -20 = 1500/418

Final temp = 3.59 + 20 = 23.59 deg C