Calculations involving Heat

Themes: Conservation of Energy

Thermal Equilibrium

Temperature vs. Heat

Temperature: Measure of the average kinetic energy of molecules.

Heat: Measurement of the transfer of energy from one object to another

$200 dollar analogy…is 10 degrees always considered a lot of heat?

Temperature in Kelvin

Zero Kelvin is absolute zero…the temperature at which the molecules within an object stop moving!

Celsius is a more practical way to measure everyday temperature.

C= K + 273

Measurements

Heat measurements: Calories:

Amount of heat needed to raise 1 g of water 1 degree Celsius

Joules Unit that measures heat directly.

4.184 J = 1 calorie

Both Joules and calories are often measured in KJ or KCal

Conversions

How many joules are contained in 800 calories?

How many calories are contained in 100 KJ?

Measuring Heat Transfer

Q system = - Q surroundings

How is q defined?

Q = smΔT

S=specific heat

M= mass

ΔT = change in temperature (Tf-Ti)

What is specific heat?

The amount of energy required to raise a certain mass of a specific substance by a certain amount. THINK:

What has more specific heat? Air or water? Water or alcohol? Metal or water? Copper or aluminum?

Solving for variables looking at just the

“system” Q = smΔT

If you take a cube of ice weighing 12 g out of the freezer and let it melt on a bench top at 20 C. What is the energy change that would occur?

Using specific heat to identify a metal:

Q = smΔT

The temperature of a piece of Metal X with a mass of 95.4g increases from 25.0°C to 48.0°C as the metal absorbs 849 J of heat. What is the specific heat of Metal X? 

What is the identity of metal X?

Using the surroundings to

learn about the heat transfer of the

system A 245.7g sample of metal at 75.2 degrees Celsius was placed in 115.43g water at 22.6 degrees Celsius. The final temperature of the water and metal was 34.6 Celsius. If no heat was lost to the surroundings what is the specific heat of the metal?

Strategy

What do you know about the system?

What do you know about the surroundings?

Which is going to gain heat…this one will be +

Set system = to surroundings and solve for unknown.