ChemLiveLesson 6.01

Thermochemistry

Thermochemistry

• Thermochemistry: study of the changes in energy that accompany chemical reactions and physical changes.

• Thermochemistry is really all about energy.– Law of Conservation of Energy:

• Energy cannot be created or destroyed, it can only be transformed from one form into another.

-Fuel is burnt to produce energy - combustion (e.g. when fossil fuels are burnt)

-CH4(g) + 2O2(g) ----- > CO2(g) + 2H2O(l) + energy

Thermal Energy

• What is it?

– Thermal Energy is the kind of energy that is related to and/or caused by heat. When thermal energy is applied to a substance, the average velocity of the particles or molecules which make up the substance increases -- and it gets warmer!

– Example: Pot of boiling water on a fire.

Thermochemistry-Explained

There are two kinds of heat in chemistry. 1. The first is caused by physical activity. This extra activity

makes more molecular collisions occur. The collisions create the heat (such as when you increase the pressure in a system).

2. Chemical processes cause the second type of heat. Instead of exciting a system and feeling the heat, chemical bonds are made and broken, and the energy is then released. A release of energy charges up the system and the molecules bounce around faster, resulting in that physical activity we just explained. The opposite can also happen. Sometimes bonds are made and broken and energy is absorbed. The area then gets colder.

Heat- Explained• Heat: The transfer of thermal energy from one

substance to another due to the temperature difference between the two substances. It can be described as the transfer of thermal energy from one substance to another due to the temperature difference between the two substances.

• A sample of matter can have a certain amount of thermal energy, but the matter does not have heat. Heat is the movement, or flow, of thermal energy. Heat always flows spontaneously from matter at a higher temperature to matter at a lower temperature.

Heat- units of measurement

• Heat and thermal energy are both measured in units of energy. The SI unit for energy that is most often used by scientists around the world is the joule (J).

• Another unit of energy that is commonly used is the calorie, which is the amount of energy it takes to increase the temperature of one gram of water by one degree Celsius.

Temperature

• Temperature is a measure of the average kinetic energy of the particles in a sample of matter. The greater the average kinetic energy of the particles in a sample of matter, the higher the temperature of that matter. To assign a numerical value to temperature, scientists use defined temperature scales (Fahrenheit, Celsius, and Kelvin).

Systems & Surroundings

• In thermodynamics, the world is divided into a system and its surroundings

• A system is the part of the world we want to study (e.g. a reaction mixture in a flask)

• The surroundings consist of everything else outside the system

SYSTEM

CLOSED

OPEN ISOLATED

Types of Systems

OPEN SYSTEM: can exchange both matter and energy with the surroundings (e.g. open reaction flask, rocket engine)

CLOSED SYSTEM: can exchange only energy with the surroundings (matter remains fixed) e.g. a sealed reaction flask

ISOLATED SYSTEM: can exchange neither energy nor matter with its surroundings (e.g. a thermos flask)

• This lesson does not have an assignment for you to turn in. However, before you go on to the next lesson, make sure you are able to compare and contrast temperature, thermal energy, and heat, and that you can differentiate between open, closed, and isolated systems.