Thermal Energy & Molecular Collisions (College Board AP® Chemistry)
Study Guide
Written by: Oluwapelumi Kolawole
Reviewed by: Stewart Hird
Thermal Energy & Molecular Collisions
Energy is a much-used abstract concept that is defined as the capacity to do work
Work is done when a force applied to an object causes the object to move in the direction of the applied force
Some forms of energy which are relevant to chemistry include:
Kinetic
Potential
Thermal
Chemical
Kinetic and Potential Energy
Kinetic energy is energy due to the motion of the object
This energy is directly related to the temperature of the object as explained by the kinetic molecular theory
On the other hand, potential energy is energy available by virtue of an object’s position
Consider a rock at the top of a cliff, because of its height, it has more potential energy and will make a bigger splash if it falls into the water below than a similar rock located down the cliff
Representing potential energy
Boulder 1 is higher than boulder 2, which means it has more potential energy. This results in boulder 1 causing a bigger splash when it hits the water
Chemical Energy
This is a form of energy stored within the structural units of chemical substances
Its quantity is determined by the type and arrangement of atoms within the chemical
It is considered a form of potential energy because it is associated with the relative positions and arrangements of atoms within a given substance
When substances undergo chemical reactions, the chemical energy present in such substances is released, stored or converted to other energy forms
Thermal Energy
Thermal energy is the energy associated with the random motion of atoms and molecules
In this sense, it can be considered a form of kinetic energy and so related to temperature
In a general sense, thermal energy may be obtained from temperature measurement
The greater the motion of the particles that make up a substance, the higher its temperature (hotter the substance becomes) and the higher its thermal energy
However, we must differentiate between thermal energy and temperature
Thermal energy is a measure of the total kinetic energy of the particles that make up a substance
Temperature is a measure of average kinetic energy
Thermal energy is dependent on the mass of the substance while temperature is independent of the mass
For example, a bathtub filled with warm water at 45℃ has much more thermal energy stored than in a coffee cup with water at 70℃
This is because the bathtub has a much larger volume and greater mass than the coffee
Therefore, the bathtub contains more water molecules and has more molecular motion
When two bodies at different temperatures are in contact, there is a flow of thermal energy in the form of heat until the two bodies are at thermal equilibrium
Thermal equilibrium refers to a state where there is no flow of energy between two bodies at the same temperature
For example, consider an iron metal at 200℃ and a container of water at 25℃
On placing the hot iron in water, heat energy would flow from the iron to the water
This increases the temperature of the water and causes a decrease in the temperature of the iron until both are the same temperature and there’s no flow of energy
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