Thermal Energy & Molecular Collisions (College Board AP Chemistry)

• 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