The kinetic molecular theory is a model to explain physical properties of matter (solids, liquids and gases) and changes of state.

The kinetic molecular theory is a scientific model that helps us understand the physical properties of matter, such as solids, liquids, and gases, and how they change from one state to another.

According to this theory, all matter is made up of tiny particles, called molecules or atoms, that are in constant motion. The motion of these particles is related to their temperature and the amount of energy they possess.

• In a solid, the particles are tightly packed and vibrate in fixed positions, which gives the solid its shape and rigidity.
• In a liquid, the particles are still close together but can move around more freely, which allows the liquid to flow and take the shape of its container.
• In a gas, the particles are far apart and move randomly in all directions, which makes the gas expand to fill its container.

The kinetic molecular theory also explains how changes of state occur. For example, when a solid is heated, the particles gain energy and begin to vibrate more rapidly, eventually breaking free from their fixed positions and becoming a liquid. When a liquid is heated even further, the particles gain even more energy and begin to move around more rapidly, eventually breaking free of each other completely and becoming a gas.

One of the key assumptions of the kinetic molecular theory is that the particles in a gas are in constant, random motion and move independently of one another. This leads to some important consequences for the physical properties of gases, such as their pressure, volume, and temperature.

For example, the theory predicts that the pressure of a gas is directly proportional to the number of particles in the gas and their average speed. It also predicts that the volume of a gas is inversely proportional to its pressure, meaning that as the pressure of a gas increases, its volume decreases, and vice versa. Additionally, the theory predicts that the temperature of a gas is directly proportional to the average kinetic energy of its particles.

The kinetic molecular theory has some limitations and assumptions that do not always hold true in real-world situations. For example, the theory assumes that the particles in a gas do not interact with each other, which is not always the case. Additionally, the theory assumes that the particles are point masses with no size or volume, which is also not entirely accurate. However, despite these limitations, the kinetic molecular theory is still a useful tool for understanding the behavior of matter at a molecular level.

Distinguish the different states of matter.

One of the most significant applications of the particle model of matter is in the study of thermodynamics. Thermodynamics is the study of energy and its transformations, and the particle model of matter provides a framework for understanding the relationship between the energy of particles and their temperature.

Changes of state, such as melting, freezing, boiling, and condensation, occur at a molecular level due to changes in the temperature and pressure of a substance. At the molecular level, matter is made up of tiny particles, such as atoms, molecules, or ions, that are constantly in motion.

When the temperature of a substance is increased, the motion of its particles also increases, causing them to vibrate more rapidly and with greater energy. As a result, the distance between particles increases, and the substance expands. This is why most substances, except for water, expand when heated and contract when cooled.

When a substance is heated to its melting point, the added energy causes its particles to overcome the attractive forces holding them together in a fixed position, and the substance starts to change from a solid to a liquid. During melting, the temperature of the substance remains constant until all of the solid has melted into a liquid. This is because the added energy is used to break the intermolecular forces between the particles, rather than increasing the temperature.

When a substance is heated further, its particles continue to gain energy and move more rapidly, causing them to break away from the surface of the substance and enter the surrounding environment. This is known as evaporation or boiling. During boiling, the temperature of the substance remains constant until all of the liquid has been converted to a gas. This is again because the added energy is used to break the intermolecular forces between the particles, rather than increasing the temperature.

When a substance is cooled, the opposite occurs. Its particles lose energy and move more slowly, causing the attractive forces between them to become stronger. At the freezing point, the particles are held so tightly that they lock into a fixed position, causing the substance to change from a liquid to a solid.

Use state symbols (s, l, g and aq) in chemical equations.

State symbols are used in chemical equations to indicate the physical state of reactants and products. Here are the most common state symbols:

• (s) for solid
• (l) for liquid
• (g) for gas
• (aq) for aqueous (dissolved in water)

Examples of chemical reactions with state symbols:

1. Formation of water from hydrogen gas and oxygen gas:

2H₂(g) + O₂(g) → 2H₂O(l)

2. Dissociation of table salt in water:

NaCl(s) → Na⁺(aq) + Cl^–(aq)

3. Combustion of propane gas:

C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(g)

4. Precipitation reaction between silver nitrate solution and sodium chloride solution:

AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)

5. Reaction of calcium oxide with water:

CaO(s) + H₂O(l) → Ca(OH)₂(aq)

Sublimation

Sublimation is a physical change in which a substance transitions directly from the solid phase to the gas phase without passing through the intermediate liquid phase. This process typically occurs when a solid substance is heated under specific conditions of temperature and pressure, causing its molecules to gain enough energy to break free from their structured lattice and move freely as gas molecules.

To better understand sublimation, let’s consider an example involving dry ice, which is the solid form of carbon dioxide (CO₂). When dry ice is exposed to room temperature and atmospheric pressure, it begins to sublimate. During this process, the CO₂ molecules in the solid form absorb heat energy and gain enough kinetic energy to overcome the intermolecular forces holding them together in the solid lattice. As a result, they break free and transition directly into the gaseous state, bypassing the liquid phase.

CO₂(s) → CO₂(g)

You may have seen this phenomenon in action during a theatrical performance or a Halloween party where dry ice is used to create a low-lying fog effect. When dry ice is placed in water or exposed to air, it sublimates rapidly, producing a dense cloud of cold CO₂ gas that hugs the ground. This gas is heavier than air, which causes it to sink and create the desired fog effect.

Questions

1. What is the kinetic molecular theory?
2. How does the kinetic molecular theory explain the physical properties of matter?
3. What are the different states of matter and how do they differ?
4. How do changes of state occur according to the kinetic molecular theory?
5. What are the key assumptions of the kinetic molecular theory?
6. What are some limitations of the kinetic molecular theory?
7. What is thermodynamics and how is it related to the particle model of matter?
8. How do particles behave when a substance is heated or cooled?
9. What are state symbols and how are they used in chemical equations?
10. What is sublimation and how does it occur?

Answers