Thermal Energy

Where does almost all chemical bond energy originally come from

Nuclear fusion in the Sun.

What does nuclear fusion in the Sun emit that provides energy for most processes on Earth Photons (light energy).

Which process is primarily driven by solar energy

Photosynthesis.

What is thermal energy

The total internal kinetic energy of all the particles in a system (measured in joules, J).

What is heat (q or Q)

The transfer of thermal energy from a hotter system to a cooler system.

In which direction does heat flow

Always from hot → cold.

What is temperature

The measure of the average kinetic energy of the particles in a system.

What formula is used to calculate heat transfer

Q = mcΔT

What does each variable represent

Q = thermal energy (J), m = mass (g), c = specific heat capacity (J/g°C), ΔT = change in temperature (°C or K)

How do you calculate ΔT

Final temperature – initial temperature (ΔT = Tf - Ti).

Do you use Celsius or Kelvin for ΔT

Either is fine — only the difference matters.

State the Law of Conservation of Energy.

Energy cannot be created or destroyed; it can only change form.

If a chemical system gains energy, what happens to the surroundings

The surroundings lose energy and feel cooler.

If a chemical system loses energy, what happens to the surroundings

The surroundings gain energy and feel warmer.

What does this mean for energy lost/gained

Energy lost by one system = energy gained by another.

What is an exothermic process

Heat is released by the system (Q is negative). The surroundings warm up.

Example of an exothermic process

Hot pack.

What is an endothermic process

Heat is absorbed by the system (Q is positive). The surroundings cool down.

Example of an endothermic process

Cold pack.

Tip for identifying: if the question says “How much energy is released,” what type of process is it

Exothermic.

If it says “What heat is required

Endothermic.

What is kinetic energy (Ek)

Energy related to the motion of particles — translational, rotational, or vibrational motion.

What is potential energy (Ep) in chemistry Energy stored in chemical bonds and intermolecular forces.

What happens when bonds are broken

Energy is absorbed (endothermic).

What happens when bonds are formed

Energy is released (exothermic).

Why does the graph plateau during a phase change

Because the energy is changing potential energy (breaking/forming bonds), not kinetic.

When the graph is sloped upward, what’s happening

Temperature and kinetic energy are increasing.

When energy is added during a plateau, what’s changing

The phase — bonds are being broken or formed.

Why is understanding thermochemistry important

It helps explain energy transformations in everything from metabolism to combustion engines and environmental systems.