Unit 3: PVTn…what about Energy

Warm Up:

What will raise the temperature of 500mL water more? A 50g block of iron at 25°C or a 10g block of iron at 25°C?

So… about Thermal Energy

What’s up with Temperature vs Heat?

Temperature is related to the average kinetic energy of the

particles in a substance.

Thermal energy relationships

As the temperature of an individual substance increases, so does thermal energy (because the kinetic energy of the particles increased).

Given 2 substances of the same material, one with a mass of 75g and the other with a massive of 25g and equal temperatures, the more massive substance has more thermal (because it is a total measure of energy – speed of particles AND number of particles).

Heat

The flow of thermal energy from one object to another.

Heat always flows from warmer to cooler objects.

Ice gets warmer while

hand gets cooler

Cup gets cooler while hand gets

warmer

Things heat up or cool down at different rates.

Specific Heat: c

Land heats up and cools down faster than water.

Specific heat is the amount of heat required to raise the temperature of 1 g of a material by 1 degree (C or K, they’re the same size).

C water = 4184 J / kg °C (“holds” its heat)

C sand = 664 J / kg °C (less E to change)

This is why land heats up quickly during the day and

cools quickly at night- and why water takes longer.

Why does water have such a high specific heat?

Water molecules have strong intermolecular forces with other water molecules.

It takes more heat energy to overcome these interactions.

water metal

q = m c T q = change in thermal energy (J for joules)

m = mass of substance

T = change in temperature (Tf – Ti)

C = specific heat capacity of substance

Calculating heat, yep the math has arrived

Heat can be Transferred even if there is No Change in State

q = (c)(mass)(∆T)

Heat Transfer can cause a Change of State

Changes of state involve energy (at constant T)Ice + 333 J/g (heat of fusion) -----> Liquid water

Is there an equation? Of course!q = (heat of fusion)(mass)

WHY DO I NEED THIS:

q = (heat of fusion)(mass)

WHEN I HAVE:

q transferred = (c)(mass)(∆T)

When a phase changes THERE IS NO change in temperature… but there is definitely a change

in energy!

Heat Transfer and Changes of State

Liquid (l) Vapor (g) requires energy (heat)

Why do you cool down after swimming ?

-the water vapor carries the heat energy away from you as it evaporates

So, let’s look at this equation again…

q = (heat of fusion)(mass) for melting/freezing

There’s also q = (heat of vaporization)

(mass)for

vaporization/condensation

So… if I want the total heat to take ice and turn it to steam I need 3 steps…

1) To melt the ice I need to multiply the heat of fusion with the mass…q = (heat of fusion)

(mass)

2) Then, there is moving the temperature from 0°C to 100°C … for this there is a change in temperature so we can use…

q transferred = (c)(mass)(∆T)

3) But wait, that just takes us to 100°C , what about vaporizing the molecules? Well, then we need

q = (heat of vaporization)(mass)…

Add ‘em all up and there it is!

Now, lucky for us, just like there are tables for specific heats, there are also tables for heats of fusion and heats of vaporization.

At least we don’t have to worry memorizing those!

Specific Heat CapacityIf 25.0 g of aluminium cools from 310 °K to 37 °K, how many joules of heat energy are lost by the aluminium?

heat gain/loss = q = (c)(mass)(∆T)where ∆T = Tfinal-Tinitial

q = (0.897 J/g• °K)(25.0 g)(37 - 310) °K

q = - 6120 J

Notice that the negative sign on q signals heat “lost by” or transferred OUT of Al.