Introduction to Thermodynamics

Unit 03 - Thermodynamics

The Concept of Energy

• Although you may know what energy is, it may be difficult to define it.

• Energy: • the ability to do work or produce heat.

The Concept of Energy

• What forms can energy come in?• Light energy, sound energy,

mechanical energy, nuclear energy, and electrical energy

• Biomass, petroleum, natural gas, and coal are examples of stored chemical energy• Chemical energy is converted to

thermal energy (heat) when we break those bonds

• In this unit, we will focus on heat energy.

What is Heat?

• Previous science courses may have taught us that molecules in a substance, no matter its state, are in constant motion.

• Molecules move slowest in solids, faster in liquids, and fastest in gasses.

• The energy a particle has because of its motion is called kinetic energy.

What is Heat

• The energy associated with the movement of particles is called heat and thermal energy.

• What happens to the motion of particles as the temperature of a substance is increased?

• http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/thermochem/eqilibrium-v1.html

• Since the particles begin to move faster, they have more kinetic energy. Therefore, the heat an object gives off is directly related to the motion of its particles.

Heat – A TRANSFER of Energy

• Heat is the transfer of energy. Note that heat always flows from hot to cold, never from cold to hot.• For example, imagine you had an ice cube

in your hand. The heat from your hand gets transferred to the ice cube.

• That is, fast moving particles hit the slow moving particles in the ice cube.

• This interaction slows the particles down in your hand (making it cold) and speeds up the particles in the ice (making it warm).

• Thus, thermal energy is transferred from your hand into the ice cube (coldness does not travel from the ice cube to your hand).

Heat – A TRANSFER of Energy

• If you were holding a cold piece of metal (instead of ice) your hand will eventually warm the metal to the same temperature as your body.• At this point your hand and the metal are in

thermal equilibrium.• http://www.chem.iastate.edu/group/Gree

nbowe/sections/projectfolder/flashfiles/thermochem/thermoEquiv.html

Potential & Kinetic Energy

• Kinetic Energy• Energy of motion• i.e.:

• a rock rolling down a hill has kinetic energy

• Atoms and molecules have kinetic energy as they are always in motion

• Potential Energy• Energy that is stored in something,

and has the potential to do work, is called potential energy.

• i.e.: • a rock sitting at the top of a hill has

potential energy

1st Law of Thermodynamics

Potential Energy – Chemical Bond Energy

• This energy exists because of the attractive forces between molecules and atoms.

• It plays a large part in chemical reactions.

• Therefore, the bonds that hold molecules together are stored energy and may do work (or give off heat) if they are broken.

The Law of Conservation of Energy

• The Law of Conservation of Energy states that energy cannot be created or destroyed, but only change forms.

• This law is also known as the First Law of Thermodynamics.

• i.e.:

• When octane (C3H8, the main component of gasoline) is burned in your car engine, chemical bond energy (potential energy) is converted into mechanical energy (pistons moving in the car engine; kinetic energy) and heat.

• When we eat, our bodies convert the chemical energy of the food into movement of our muscles; again heat is also a product of this conversion.

• When we turn on a light switch, electrical energy is converted into light energy and, you guessed it, heat energy.

The Law of Conservation of Energy

• When talking about chemical reactions, all the energy in the reaction must come from somewhere. That is, it is not spontaneously created.

• The only place that energy is stored is in the bonds between atoms as potential energy.

• This potential bond energy will usually be converted to kinetic energy (heat).

Heat vs. Temperature

• Heat:• Heat is a form of energy. It is the total amount of kinetic

energy in a sample of matter.• Heat also flows from a warmer object to a cooler object.• Therefore, the feeling of getting colder is not “coldness”

entering your body, but heat (energy) leaving your body.• The S.I. unit for heat is the Joule (J).

Heat vs. Temperature

• Temperature:• is a measure of the average kinetic energy of a sample of

matter. It is the intensity of heat energy.

Measurements of Heat

• There are two ways we will measure temperature:• 1. Celsius scale based on the freezing and boiling points

of water.• 2. Kelvin scale based on energy.

• The Kelvin scale has no negative numbers. In fact, the lowest temperature that can ever be reached is absolute zero (0 K).

• At this temperature, a particle has no energy, and thus, all movement is ceased.

• To convert between degrees Celsius and degrees Kelvin, we will use:• K = ˚C + 273 where:

• K = degrees in Kelvin• ˚C = degrees in Celsius

Heat vs. Temperature

• In terms of heat, or energy, an iceberg would have more than a boiling cup of coffee.• Although the temperature of the boiling water is

much higher than the iceberg, the iceberg has more particles. Thus, it will have a higher total energy.

• A good way to think of temperature is the intensity of heat.

• The coffee will have a more intense heat than the iceberg.

Calories: Another way to measure energy

• A thermochemical “calorie” is defined as:• the amount of heat required to raise the temperature of

one gram of water by one degree Celsius (1°C).

• Units for calories are written as “cal”

• To convert between joules and calories, use the following conversion factors:• 1 J = 0.2390 cal and/or• 1 cal = 4.184 J

• Examples:• Convert 8181 joules to calories• Convert 2019 calories to joules• Convert 1125 kJ to calories• Convert 4307 kcal to joules

Food for Thought…

• Be careful – dietary calories can be deceiving:• A dietary calorie (as written on

food labels), written with a big C ( not “cal”) is actually a kilocalorie, or 1000 calories.

• What this means is when you look at a label and it says “100 calories per serving!” it really means:• 100 C x 1000 cal = 100 000 cal

per serving 1 C