Unit 2: Kinetic Model & States of Matter
Exploring the relationship between particle movement, energy, and the physical state of matter.
2.11 The Kinetic Theory of Matter
The Kinetic Theory of Matter states that all matter is composed of tiny particles (atoms, ions, or molecules) that are in constant, random motion. This motion gives the particles kinetic energy. The theory explains the physical properties and behaviours of different states of matter based on the motion and spacing of these particles.
The key postulates of the kinetic theory are:
- Matter consists of particles in continuous, random motion.
- There are forces of attraction between particles.
- The average kinetic energy of the particles is directly proportional to the absolute temperature (in Kelvin).
- The volume of the particles themselves is negligible compared to the volume of the container they occupy (for gases).
Solved Examples:
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Explain how diffusion provides evidence for the kinetic theory.
Solution: The scent of a perfume spreading throughout a room is due to the random motion of perfume molecules colliding with air molecules. This demonstrates that particles are in constant, random motion as stated by the kinetic theory. -
What causes gas pressure according to kinetic theory?
Solution: Gas pressure is a result of the constant, random collisions of gas particles with the walls of their container. The more frequent and forceful these collisions, the higher the pressure. -
Explain thermal expansion using kinetic theory.
Solution: When a metal bar is heated, its particles gain kinetic energy, vibrate more vigorously, and push each other further apart, causing the bar to expand. This demonstrates the relationship between temperature and particle motion. -
How does Brownian motion support the kinetic theory?
Solution: The random, jerky movement of pollen grains in water, caused by the constant bombardment of water molecules, provides visual evidence for the continuous random motion of particles described in kinetic theory. -
Why do liquids mix when combined?
Solution: When a drop of ink is placed in water, the ink molecules move randomly and mix with the water molecules due to their constant motion, leading to a uniform solution over time.
2.12 Temperature
Temperature is a measure of the average kinetic energy of the particles in a substance. The higher the temperature, the faster the particles are moving on average. It is a fundamental property that determines the direction of heat flow: heat always flows from a region of higher temperature to one of lower temperature.
It's important to distinguish temperature from heat. Heat is the transfer of thermal energy, while temperature is the measure of that energy. The Kelvin scale ($K$) is the absolute temperature scale, where $0 K$ (absolute zero) is the theoretical temperature at which all particle motion ceases. The relationship between Kelvin and Celsius is: $T(K) = t(°C) + 273.15$.
Solved Examples:
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Why does a hot cup of tea have a higher temperature than iced
tea?
Solution: A hot cup of tea has a higher temperature than a glass of iced tea because its water molecules have a greater average kinetic energy. The particles are moving faster on average. -
Convert $25°C$ to Kelvin.
Solution: Using the formula $T(K) = t(°C) + 273.15$: $T = 25 + 273.15 = 298.15 K$. -
What happens to mercury in a thermometer when it cools?
Solution: When you take a thermometer out of your mouth, the mercury contracts because the particles in the mercury lose kinetic energy to the cooler surroundings, moving more slowly and coming closer together. -
Why is the Sun's surface extremely hot?
Solution: The surface of the Sun has a very high temperature, meaning its particles are moving at extremely high speeds with very high average kinetic energy. -
What is significant about absolute zero?
Solution: The lowest temperature ever reached in a laboratory is just a fraction above absolute zero, where scientists observe quantum mechanical effects as classical particle motion has essentially stopped.
2.13 States of Matter
The state of matter (solid, liquid, or gas) depends on the balance between the kinetic energy of the particles and the strength of the intermolecular forces of attraction between them. As a substance gains energy, its particles gain kinetic energy, leading to changes in its state.
- Solids: Particles are closely packed in a fixed, regular lattice. They vibrate about fixed positions but cannot move past one another. They have a definite shape and volume. Intermolecular forces are very strong.
- Liquids: Particles are closely packed but in a random arrangement. They can slide past one another and are in constant, random motion. They have a definite volume but take the shape of their container. Intermolecular forces are strong but weaker than in solids.
- Gases: Particles are far apart and move randomly and rapidly in all directions. They have no definite shape or volume and will expand to fill their container. Intermolecular forces are negligible.
Solved Examples:
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Describe the particle arrangement in ice.
Solution: A block of ice is a solid where the water molecules are locked in a crystalline lattice with strong intermolecular forces, giving it a fixed shape and volume. -
Why is molten iron considered a liquid?
Solution: When heated to its melting point, the iron atoms gain enough kinetic energy to overcome some intermolecular forces and move past each other, making the substance fluid while maintaining a definite volume. -
Explain the behavior of oxygen gas in the atmosphere.
Solution: The oxygen molecules in the air are far apart and moving rapidly and randomly, with negligible intermolecular forces, filling any space they occupy completely. -
What happens when butter is heated on a pan?
Solution: When butter is heated, the kinetic energy of its molecules increases, overcoming the intermolecular forces and causing it to transition from solid to liquid state (melting). -
Why does water vapor condense on a cold window?
Solution: The condensation of water vapor on a cold window pane demonstrates the transition from gaseous state to liquid state as the gas particles lose kinetic energy and intermolecular forces pull them closer together.
2.14 Change of State
Changes of state are physical processes where a substance transitions from one state to another. These changes are reversible and involve the absorption or release of energy, but the chemical identity of the substance remains the same. The temperature remains constant during a change of state as the absorbed energy is used to break intermolecular bonds rather than increase kinetic energy.
- Melting ($s → l$): Absorption of heat.
- Freezing ($l → s$): Release of heat.
- Boiling/Vaporization ($l → g$): Absorption of heat.
- Condensation ($g → l$): Release of heat.
- Sublimation ($s → g$): Absorption of heat.
- Deposition ($g → s$): Release of heat.
Solved Examples:
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Describe what happens when an ice cube melts.
Solution: An ice cube turning into liquid water on a warm day is melting. Energy is absorbed to break the rigid crystal structure, allowing water molecules to move more freely while maintaining the same chemical composition. -
Why does water temperature remain constant during boiling?
Solution: Water boiling at $100°C$ maintains this temperature until all water has boiled because all absorbed energy goes into breaking intermolecular forces to convert liquid to gas, not increasing kinetic energy. -
Explain how dew forms on grass.
Solution: The formation of dew on grass in the morning occurs as water vapor in the cool night air loses energy and condenses into liquid droplets through condensation. -
What is special about dry ice?
Solution: Solid carbon dioxide (dry ice) undergoes sublimation, turning directly into gaseous $CO_2$ without forming a liquid, absorbing energy in the process. -
How does frost form on car windows?
Solution: The formation of frost on a cold car window on a clear winter night occurs when water vapor from the air directly solidifies through deposition, releasing energy in the process.
2.15 Effects of Impurities on Melting and Boiling Points
The presence of impurities in a substance affects its melting and boiling points. Impurities interfere with the regular arrangement of particles, disrupting the intermolecular forces and the phase transition process. This phenomenon is categorized under colligative properties, which depend on the concentration of solute particles, not their identity.
- Melting Point Depression: Impurities lower the melting point of a solid. They disrupt the crystal lattice, making it easier for particles to move apart, thus requiring less energy to melt. The melting point becomes a range rather than a sharp value.
- Boiling Point Elevation: Impurities increase the boiling point of a liquid. The impurity particles occupy space at the liquid's surface, lowering the substance's vapor pressure. More energy (and a higher temperature) is required to overcome this and reach the boiling point.
Solved Examples:
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Why is salt spread on icy roads in winter?
Solution: Spreading salt on icy roads lowers the melting point of the ice through melting point depression, causing it to melt at temperatures below $0°C$, making roads safer for travel. -
What is the purpose of antifreeze in car radiators?
Solution: Antifreeze (a mixture of water and ethylene glycol) is used in car radiators to lower the freezing point of the liquid and prevent the engine from freezing in winter temperatures. -
Why do some people add salt when cooking pasta?
Solution: Adding salt to boiling water slightly increases the boiling point of the water through boiling point elevation, allowing the pasta to cook at a higher temperature, potentially improving texture. -
Why doesn't seawater freeze at $0°C$?
Solution: Seawater, which contains dissolved salts, has a lower freezing point than pure fresh water due to melting point depression caused by the dissolved ions. -
How can a chemist determine the purity of a solid sample?
Solution: A chemist can check the purity of a crystalline solid by measuring its melting point. A sharp, consistent melting point indicates high purity, while a wide range indicates the presence of impurities.