Thermal Energy & Molecular Collisions (College Board AP® Chemistry)

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Oluwapelumi Kolawole

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

  • 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

7S1yXSn8_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

bathtub-and-coffee-cup
  • 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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Oluwapelumi Kolawole

Author: Oluwapelumi Kolawole

Expertise: Chemistry Content Creator

Oluwapelumi is a Pharmacist with over 15000+ hours of AP , IB, IGCSE, GCSE and A-Level chemistry tutoring experience. His love for chemistry education has seen him work with various Edtech platforms and schools across the world. He’s able to bring his communication skills as a healthcare professional in breaking down seemingly complex chemistry concepts into easily understood concepts for students.

Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.