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Structure 1. Models of the particulate nature of matter - Introduction to the particulate nature of matter

Grade 12IBChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Matter is defined as any substance that possesses mass and occupies physical space. According to the Kinetic Molecular Theory (KMT), all matter is composed of discrete particles that are in continuous, random motion.

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The physical state of a substance (solid, liquid, or gas) is determined by the balance between the kinetic energy of the particles and the inter-particle forces (attractions) between them.

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Solids have a fixed shape and volume because particles are held in a rigid lattice by strong forces, only vibrating about fixed positions.

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Liquids have a fixed volume but no fixed shape; particles are close together but can move past one another due to weaker attractive forces compared to solids.

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Gases have no fixed shape or volume and are easily compressed. Particles are far apart and move rapidly in all directions, with negligible inter-particle forces (except during collisions).

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Temperature is a measure of the average kinetic energy (EkE_k) of the particles in a substance. Absolute temperature in Kelvin (KK) is directly proportional to the average kinetic energy: Ek∝TE_k \propto T.

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Phase changes (e.g., melting, boiling) occur at constant temperature. During these processes, added energy (latent heat) is used to overcome inter-particle forces rather than increasing the kinetic energy of the particles.

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Matter can be classified into pure substances (elements and compounds) and mixtures. Homogeneous mixtures (solutions) have uniform composition, while heterogeneous mixtures have non-uniform composition and distinct phases.

📐Formulae

T(K)=t(∘C)+273.15T(K) = t(^{\circ}C) + 273.15

Ek=12mv2E_k = \frac{1}{2}mv^2

Ek∝TE_k \propto T

Density (ρ)=mV\text{Density } (\rho) = \frac{m}{V}

💡Examples

Problem 1:

A sample of Neon gas is heated from 300 K300\text{ K} to 600 K600\text{ K}. Describe the change in the average kinetic energy of the particles.

Solution:

Since the absolute temperature TT has doubled (300 K→600 K300\text{ K} \rightarrow 600\text{ K}), the average kinetic energy (EkE_k) of the Neon atoms also doubles.

Explanation:

In accordance with the Kinetic Molecular Theory, the average kinetic energy of gas particles is directly proportional to the absolute temperature. Therefore, Ek,final=2×Ek,initialE_{k, \text{final}} = 2 \times E_{k, \text{initial}}.

Problem 2:

Calculate the temperature in Kelvin for the boiling point of ethanol, which is 78.37∘C78.37^{\circ}C.

Solution:

T(K)=78.37+273.15=351.52 KT(K) = 78.37 + 273.15 = 351.52\text{ K}

Explanation:

To convert from Celsius to Kelvin, the constant 273.15273.15 (often rounded to 273273 in introductory problems) is added to the Celsius value to align with the absolute temperature scale.

Problem 3:

During the melting of ice at 273 K273\text{ K}, energy is continuously supplied, yet the temperature does not rise. Explain this observation in terms of the particulate nature of matter.

Solution:

The energy supplied is used as latent heat of fusion to overcome the hydrogen bonds holding the H2OH_2O molecules in a rigid crystalline lattice.

Explanation:

Because the energy is being used to increase the potential energy of the particles (by breaking attractive forces) rather than their kinetic energy, the temperature remains constant until the phase change is complete.