5. Kinetic Molecular Theory, Specific Heat, Latent Heat Flashcards
(22 cards)
Kinetic Molecular Theory (KMT)
- Matter is made of constantly moving particles (atoms or molecules).
- Particle collisions are perfectly elastic (no energy lost).
- Bonds between particles store potential energy like springs.
- Greater distance between particles = higher potential energy.
- In solids and liquids the force of attraction between the particles are significant
- In gases, this is insignificant, and the volume of the particles is negligible compared to the volume of gas (large spacing between individual particles)
Evidence for Kinetic Molecular Theory (KMT)
- KMT provides strong support for the existence of atoms and molecules.
- In 1905, Einstein explained Brownian motion—the random movement of pollen grains in liquid—as a result of collisions with invisible molecules.
- He applied KMT to accurately predict the behavior of particles undergoing Brownian motion, offering key evidence for molecular motion
What is Heat? (Definition, Principles, Direction of Flow, Units)
What Is Heat?
Heat is a form of energy in transfer.
It flows from a hotter object to a cooler one due to a temperature difference.
Key Principle:
- Heat Always Moves - It always flows, and it is not stored
- It cannot be possessed by an object.
✅ Correct: “20 J of heat transferred to the object.”
❌ Incorrect: “The object contains 20 J of heat.”
Direction of Heat Flow:
- Always moves from high to low temperature.
- Continues flowing until thermal equilibrium is reached.
Measurement and Units:
- Heat is measured in Joules (J).
What Is Kinetic Energy (KE)?
- Kinetic Energy is the energy of motion.
- In the context of particles, it refers to the energy they have due to their movement.
Formular for Kinetic Energy?
eK = 1/2(m)(v)^2
Where:
m = mass of the particle (kg)
v = speed of the particle (m/s)
- This formula assumes no rotational motion—only translational motion is considered (straight-line movement).
What Is Potential Energy in Particles?
Particles (atoms/molecules) are held together by electrical forces, called bonds.
These bonds behave like springs—they can stretch and compress.
Each bond has an ideal (rest) length; stretching or compressing it stores or releases potential energy.
How Energy Affects Bonds?
Adding energy causes particles to move further apart, increasing potential energy.
Removing energy brings particles closer together, decreasing potential energy.
Potential Energy During Phase Changes?
Melting and Vaporisation:
- Particles spread out
➜ Distance between particles increases
➜ Potential energy increases
Freezing and Condensation:
- Particles come closer together
➜ Distance between particles decreases
➜ Potential energy decreases
What Is Total Internal Energy?
Total Internal Energy (𝐸ₜ) is the sum of:
- The kinetic energy (𝐸ₖ) of particles (due to their motion), and
- The potential energy (𝐸ₚ) of particles (due to their positions/bonds).
Expressed as: eT = eK + eP
How Internal Energy Changes?
- When energy is transferred to a cold object, its internal energy increases.
- At absolute zero, all internal energy is removed—particles stop moving.
Important Distinction: Microscopic vs Macroscopic
Internal energy only includes the energy of particles within the object.
- It does not include: Macroscopic kinetic energy (e.g. the whole object moving or spinning).
- Even if an object (e.g. a glass of water) appears still, its particles still have microscopic kinetic and potential energy.
How Potential and Total Energy change when a substance is melted?
During melting, energy is absorbed by the substance.
This energy does not increase temperature (kinetic energy stays constant).
Instead, it increases the potential energy by weakening the bonds between particles.
Particles move further apart as bonds are stretched.
Total internal energy increases because potential energy increases, even though kinetic energy stays the same.
What is Temperature
Temperature is a measure of the average kinetic energy of the particles in a substance. Particles have a range of kinetic energies, but most are near the average.
It is proportional to the average of the expression: 1/2(m)(v)^2
where:
- m = particle mass
- v = particle speed
The faster the particles move, the higher the temperature.
Units:
- Degrees Celsius (°C)
- Kelvin (K)
Difference between heat and temperature
Heat = energy in transfer (flows from hot to cold).
Temperature = average kinetic energy of particles.
An object cannot contain heat—it can only gain or lose energy through heat transfer.
Molecular Speed Distribution
As temperature increases, the average speed of the molecules increases.
On a graph of speed vs number of molecules:
- The curve flattens and shifts right at higher temperatures.
- At higher temperatures, more particles move at faster speeds.
This distribution shows that not all molecules move at the same speed, even at the same temperature.
What is Diffusion
Diffusion is the process where particles intermingle and spread out due to their random motion from kinetic energy, as described by the Kinetic Molecular Theory (KMT).
🔄 How It Happens
- If two gases are separated by a barrier and the barrier is removed, the particles will move randomly and mix evenly over time.
- This occurs without external force, simply due to the natural motion of particles.
📌 Key Point
- Diffusion is evidence for particle motion and supports KMT—particles are constantly moving and can spread into available space.
What is Kelvin?
Kelvin is an absolute temperature scale where 0 K represents absolute zero, the point of minimum particle energy. It starts from zero and increases with the average kinetic energy of particles. Temperature in Kelvin is found by adding 273 to the Celsius value.
Absolute Zero
0 K = -273°C, called absolute zero.
At absolute zero, particles stop moving — they have zero kinetic energy.
This is the lowest possible temperature; nothing can be colder.
Why Kelvin is Useful
- Kelvin scale is directly proportional to particle kinetic energy.
- No negative values on the Kelvin scale, unlike Celsius.
- Makes calculations in physics and chemistry more logical and accurate.
SI Unit for Temperature
Conversions Between Celsius and Kelvin
K = C + 273
C = K - 273
- A 1 K change is the same as a 1 C change
Volumes and Temperature Relationship (Ideal Gases)
- As temperature decreases, the volume of an ideal gas also decreases.
- This relationship is linear.
- If extrapolated, the volume becomes zero at -273°C (0 K), where molecular motion stops.
What are Inelastic Collisions
Inelastic collisions are collisions where kinetic energy is not conserved. Some of the kinetic energy is transformed into other forms of energy such as heat, sound, or deformation.
