1. Visualizing Heat and The Laws of Thermodynamics Unit 3 - Thermodynamics

2. Thermodynamics The study of thermodynamics is the study of energy changes Because this is chemistry, we are studying the energy changes in chemical reactions and matter Thermo = energy Dynamic = changing In this course, we examine the energy of HEAT. However, the theory we are using could be applied to any type of energy in a chemical reaction

3. Understanding Heat In order to understand heat, you will need to be able to: 1.Visualize how heat works (Understand the theory of heat) 2.Understand that Laws that govern heat. Keep in mind a scientific law is a statement that describes repeated scientific observations. A law NEVER tries to explain why something happens. Scientific THEORIES explain WHY we observe repeated observations

4. Theory of Heat The theory of heat is based in how particles move This theory is called Kinetic Molecular Theory Essentially, the faster a particle is moving, the more heat the particle contains The greater the kinetic energy of a particle, the greater the heat of the particle Not all particles in a system are at the same heat so they do not all move at the same speed

5. Theory of Heat Temperature is an AVERAGE of every particles kinetic energy (heat) in a system When you take the temperature of a liquid, you are finding the average speed the particles are moving in the liquid

6. Theory of Heat Heat is transmitted to other objects by particles striking each other and transferring kinetic energy A system wants to reach Thermal Equilibrium All of the particles in a system will collide and transfer energy until they all have the same kinetic energy

7. Theory of Heat Thermal Equilibrium Example: Imagine filling a cup with hot coffee. Immediately the cup heats up and the air above the coffee heats up. This is because those are the particles directly touching the hot coffee. The hot particles in the coffee strike the cup and the air above and transfer their kinetic energy. Eventually, the surrounding air continues to strike the cup and energy is transferred throughout the entire surrounding particles. This happens until the coffee, the air, and the cup all reach the same heat levels.

8. Theory of Heat Because heat is kinetic energy an is transferred by particles striking and increasing the speed of others, we use an important convention in thermodynamics: Heat ALWAYS flows from HOT to COLD You can never let the cold out, you can only let the heat in!

9. 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).

10. Important Definitions System – A volume of space containing matter that is being studied in a thermodynamic sense. Surroundings – The remainder of space that lies outside the boundary of the system The system and surroundings are always in contact. In our example, the coffee was the system. The cup and the air were the surroundings.

11. Systems There are 3 different types of systems: 1.Isolated System – does not exchange energy or matter with its surroundings Example – sealed thermos **Real Isolated Systems do not actually exist! We can make systems that are NEARLY isolated but we can never stop energy from transferring. 2.Closed System – does not exchange matter with its surroundings but exchanges energy Example – sealed paper cup 3.Open System – Exchanges matter and energy with its surroundings Example – hot cup of coffee

12. 1 st Law of Thermodynamics The 1 st Law of Thermodynamics is one of the most important concepts in all of physics and chemistry: “Energy cannot be created or destroyed. Only converted from one form into another.” When you heat your water for a cup of tea, the heat energy came from electrical energy, that came from kinetic energy (movement in a generator), that came from heat energy, that came from chemical energy (bonds that hold coal together)…

13. 1 st Law of Thermodynamics

14. A Quick Note About Signs Thermodynamics is notoriously complicated when it comes to positives and negatives If heat is GAINED it is positive If heat is LOST it is negative In the 1 st Law, the negative implicates that what is gained by one is lost by the other.

15. Impact of the 1 st Law The 1 st Law sheds light on a lot of things but I thought I would share an important impact that it has…the end of the world. Consider Earth as a system. Its surroundings would then be space Earth is continually losing heat to space, trying to reach thermal equilibrium with its surroundings. Earth cannot make more energy. It does gain heat through various nuclear reactions, gravity, etc. but there is a finite amount of energy for Earth Eventually, the Earth will radiate this heat outward and after a LONG time (estimated at about 5 billion years) the core of Earth will become solid When this happens, the Magnetosphere, the magnetic field that envelopes, that protects us from incredibly harmful cosmic rays will disappear. Earth will eventually become uninhabitable. This process already happened on Mars making it the way it is today. However, Mars is smaller than Earth so it had less heat to work with from the start and lost it faster.

16. Heat in Chemical Reactions Heat in a chemical reaction comes from chemical bonds Chemical bonds store potential energy This potential energy is converted into kinetic energy in the molecules We see this as thermal energy (heat energy)

17. Measuring Thermal Energy All energy is measured the SI unit Joules (J) A joule is a very small unit of energy You release roughly 100 joules of body heat every second when you are sitting still You may often see kJ because of this

18. Calories: Another way to measure energy Calories (cal) are also a common measurement of 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). 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