The Nature of Heat and Energy

Q: What happens to the wood when it is burning?

A: The matter that makes up wood is changed into other forms. Some of it gets emitted into a gas, while other parts become ashes. Energy is also released, mostly in the form of heat and light.

Thermochemistry

• This is the study of energy changes involved in chemical and physical processes.

• Anytime matter is altered (whether physically or chemically), the energy content is adjusted as well.

  • Energy (usually in the form of heat) is released or absorbed.

• As a result, this topic will highlight different forms of energy, transfer of energy, and relationships between energy and heat.

Q: What are the two types of energy categories?

A: Kinetic energy: energy that is in motion.
Potential energy: stored energy in its current state (e.g., found in bonds).

Q: What is the SI unit of energy?

A: Joule (J). Energy in chemical reactions is often expressed in kilojoules (1 kJ = 1000 J).

Q: What are System and Surroundings?

System: the object or collection of objects being examined (e.g., substance inside a beaker).
Surroundings: anything outside the system that can exchange energy and matter with it.

What are the three types of systems?

A: Open system: exchanges both energy and matter.
Closed system: exchanges energy but not matter.
Isolated system: exchanges neither energy nor matter.

Q: What are measurable and non-measurable variables of a system?

A: Cannot measure: Thermal energy (sum of kinetic energies of all particles).
Can measure: Temperature (average kinetic energy), Volume, Mass, Pressure (forces between system and surroundings).

Specific Heat Capacity (c)

A: The amount of energy needed to increase the temperature of one gram of a substance by one degree Celsius. Each substance has a different specific heat capacity.

How do you calculate the amount of heat entering or leaving a system?

Q = mcat

If ∆T is negative: Q is negative, meaning heat left the system (Exo)
If ∆T is positive: Q is positive, meaning heat entered the system (endo)

Q: What is the First Law of Thermodynamics?

First law: Energy cannot be created nor destroyed. It can be converted from one form to another.

Total energy of the universe remains constant:

Euniverse=Esystem+Esurroundings

A system and its surroundings will always exchange energy between each other (unless the system is closed).

Energy can be in the form of heat, chemical energy, mechanical energy, or kinetic energy:

Esystem=−Esurroundings

Enthalpy, H

A: Referred to as the heat content of the system: total energy of the system plus pressure times volume. H = E + PV. Enthalpy cannot be measured directly, but changes in heat transfer can be.

How can Enthalpy be expressed?

Endothermic: heat enters the system (absorbed), H is positive.
Exothermic: heat leaves the system (emitted), H is negative.

What is the Second Law of Thermodynamics?

When two objects are in thermal contact, heat is always transferred from the object at a higher temperature to an object at a lower temperature until the two are the same.

Example: When you drink a hot tea, eventually the system will reach the same temp.

Key Point: When they reach the same temperature, it has reached equilibrium.