6.1.4Thermochemistry
Thermal Energy and Molecular Collisions — AP CHEMISTRY Revision Notes & Topical Questions
5 min read 5 parts Lesson 15 of 15 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Forms of Energy
Chemistry uses several energy types, all of which are interchangeable under suitable conditions.
Kinetic energy
- Kinetic energy Energy due to motion, which for particles in a sample depends on how fast the particles move.
Potential energy
- Potential energy Energy stored because of position or arrangement, such as particles held apart by attractive forces.
Chemical energy
- Chemical energy Potential energy stored in the arrangement of atoms and bonds, released or absorbed when a reaction occurs.
Thermal energy
- Thermal energy The total random kinetic energy of all the particles in a sample, which depends on the amount of matter.
Section 2 of 5Temperature and Thermal Energy
These two ideas are related but are not the same quantity.
Temperature and Thermal Energy
Temperature
- Temperature A measure of the average kinetic energy of the particles, so it does not depend on how much material there is.
Kelvin scale
- Kelvin scale Average kinetic energy is proportional to absolute temperature, so doubling T in kelvin doubles the average energy.
Thermal energy
- Thermal energy A bathtub of warm water holds more thermal energy than a hot cup of tea because it contains far more particles.
Distribution of energies
- Distribution of energies Particles in a sample have a range of speeds, and raising temperature shifts and widens the distribution.
| Property | Temperature | Thermal energy |
|---|---|---|
| Depends on amount of matter | No | Yes |
| Represents | Average kinetic energy | Total kinetic energy |
| Units | K or °C | J |
Section 3 of 5Energy Transfer by Collisions
Heat is the transfer of thermal energy from a warmer object to a cooler one through particle collisions.
1
When a hot object touches a cooler one, particles at the boundary collide and the faster ones pass kinetic energy to the slower ones.
2
The particles of the hotter object lose average kinetic energy, so its temperature falls.
3
The particles of the cooler object gain average kinetic energy, so its temperature rises.
4
Net energy flow continues only while a temperature difference exists between the objects.
5
At thermal equilibrium both objects have the same temperature and there is no net transfer of energy, although collisions continue.
Section 4 of 5Worked Example
Question
- The average kinetic energy of particles is proportional to absolute temperature.
- By what factor does it increase when a gas is heated from 25 °C to 100 °C?
- Convert to kelvinT₁ = 25 + 273.15 = 298.15 K and T₂ = 100 + 273.15 = 373.15 K.
- The ratio of average kinetic energies equals the ratio of temperatures: 373.15 ÷ 298.15.
- 373.15 ÷ 298.15 = 1.25.
Answer The average kinetic energy increases by a factor of about 1.25 (not 4, which a comparison of the Celsius values would wrongly give)
Section 5 of 5Exam Checklist & Tips
6.1.4 – Key Points for the Exam
- Kinetic energy Energy a particle has because of its motion.
- Potential energy Stored energy that depends on position or arrangement.
- Thermal energy The total kinetic energy of the random motion of all particles in a sample.
- Heat Energy transferred between objects because of a temperature difference.
- Thermal equilibrium A state in which two objects in contact are at the same temperature and no net energy flows.
Common Mistakes
- Treating temperature and thermal energy as the same thing, ignoring the amount of substance.
- Using Celsius temperatures when comparing average kinetic energies.
- Saying cold flows from the cooler object to the warmer one, rather than heat flowing from hot to cold.
- Believing that particles stop colliding once thermal equilibrium is reached.
Last updated:
\n