Changes of state
2.6
Particle theory can help to explain the changes of state. Melting, boiling, freezing and condensing are processes that depend on changing the forces between the particles. In melting and boiling the strength of the forces between particles becomes less. The distance between particles increases and the arrangement becomes more random. The particles move more so more energy is needed from the surroundings.
In freezing and condensing the strength of the forces remains the same. The distance between particles decreases and the arrangement become less random. The particles move less so less energy is needed.
The amount of energy needed to change state from solid to liquid and from liquid to gas depends on the strength of the forces between the particles of the substance.
KEY INFORMATION
The strength of the forces between particles depends on the nature of the particles involved, on the type of bonding and on the structure of the substance. The stronger the forces between the particles, the higher the melting point and boiling point of the substance. 2
Look at the data for substance W in the table. Describe what is happening to the particles and the forces between them at these temperatures: –28 °C, –18 °C, –14 °C, 25 °C, 38 °C, 42 °C, 46 °C
3
Identify the states of the substances in these equations: KNO3 (aq) + AgI (s) a KI(aq) + AgNO3 (aq) heat ZnO(s) + CO2 (g) b ZnCO3 (s)
4
Suggest why iron has high melting and boiling points.
5
Ethanol has a boiling point of 78.4 °C. Propane has a boiling point of –42 °C. Suggest why.
Figure 2.16 Particle model of evaporation
These diagrams are only diagrams that show a model. For example, in the model of a solid the individual spheres represent particles that are not, themselves, solids. It is only when lots of these particles are arranged closely in a regular pattern that they together represent a solid.
DID YOU KNOW?
HIGHER TIER ONLY
In chemical equations, the three states of matter are shown as (s), (l) and (g), with (aq) for aqueous solutions.
Limitations of models This simple model is limited as: • there are no forces represented between the spheres • all the particles are represented as spheres • the spheres are represented are solid and inelastic. This means that the change in forces and collisions between particles cannot be represented fully. However, it is a useful model to show spatial arrangement, both regular and random. 6
Explain the limitations of the particle theory when considering the process of condensing.
Google search: 'melting office graph'
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5/24/16 8:51 PM