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IB DP Chemistry – S1.1.3 Temperature and Kinetic Energy- Study Notes

IB DP Chemistry -S1.1.3 Temperature and Kinetic Energy - Study Notes - New Syllabus - 2026, 2027 & 2028

IB DP Chemistry – S1.1.3 Temperature and Kinetic Energy – Study Notes – New Syllabus

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Structure 1.1.3 - Temperature and Changes of State

Structure 1.1.3 – Temperature and Changes of State

Temperature and Kinetic Energy:

Temperature is a physical quantity that reflects the average kinetic energy \( (E_k) \) of the particles in a substance. In kinetic molecular theory, the greater the temperature, the faster the particles move, and hence the higher their kinetic energy.

 

  • The SI unit of temperature is the Kelvin (K).
  • Temperature in Kelvin is directly proportional to the average kinetic energy of particles.
  • Absolute zero (0 K) is the theoretical point at which all particle motion stops.

Relationship between Temperature and Kinetic Energy:

\( E_k \propto T \)

This means that if temperature increases, the average kinetic energy of particles increases as well.

Temperature Scales and Conversion:

The Kelvin and Celsius scales have the same incremental value (1 K = 1°C in magnitude).

 

However, they have different starting points:

    • 0°C = melting point of water
    • 0 K = absolute zero

Conversion formulas:

    • \( T(K) = T(°C) + 273.15 \)
    • \( T(°C) = T(K) – 273.15 \ )

Physical Changes and Temperature During State Changes:

During a change of state (e.g., melting, boiling), energy is either absorbed or released, but the temperature remains constant. This energy is used to overcome or strengthen intermolecular forces, not to increase kinetic energy.

  • Heating a solid: Temperature rises as kinetic energy increases until melting point is reached.
  • Melting point: Temperature remains constant during the melting process. Energy goes into breaking bonds between particles.
  • Heating the liquid: Temperature rises again until boiling point.
  • Boiling point: Temperature remains constant during vaporization. Energy is used to overcome intermolecular forces.
  • Cooling processes (condensation, freezing): Opposite of heating – energy is released as particles slow down and come closer together.

Graph of Heating Curve:

Observable Physical Changes During State Changes:

  • Solid → Liquid: Particles become less ordered, start to flow (melting).
  • Liquid → Gas: Particles escape the liquid, move rapidly and randomly (vaporization).
  • Gas → Liquid: Particles lose energy, move closer together (condensation).
  • Liquid → Solid: Particles slow down, become fixed in place (freezing).
  • Solid → Gas: Particles rapidly separate and move freely (sublimation).

State Changes Are Physical: No new substances are formed. Reversible processes, no change in chemical identity.

Example

Which of the following statements is true during the boiling of water at 100°C?

  1. The temperature of the water increases as it boils.
  2. The kinetic energy of water molecules increases during boiling.
  3. Heat energy is used to overcome intermolecular forces between water molecules.
  4. The water changes chemically into a new substance.
▶️Answer/Explanation

Correct answer: C
During boiling, temperature remains constant. The heat energy supplied is used to overcome intermolecular forces, not to increase kinetic energy or temperature.

Example

(a) Water freezes at 0°C. What is this temperature in Kelvin?
(b) Explain, in terms of particle energy, what happens to water molecules as water freezes.

▶️Answer/Explanation

(a) Using the conversion formula:
\( T(K) = T(°C) + 273.15 = 0 + 273.15 = 273.15 \, \text{K} \)

(b) As water freezes, particles lose kinetic energy. They move more slowly and get locked into fixed positions in a regular lattice. Intermolecular forces become stronger, holding the particles in place to form a solid.

Example

A sample of oxygen gas is cooled from 300 K to 27°C. Calculate the temperature change in Kelvin and explain whether the gas’s average kinetic energy increases or decreases.

▶️Answer/Explanation

Step 1: Convert 27°C to Kelvin:
\( T = 27 + 273 = 300 \text{ K} \)

Step 2: Initial and final temperatures are both 300 K – so there is no change.

Conclusion: There is no change in temperature in Kelvin, so the average kinetic energy of the gas particles remains the same.

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