Unit 10 States of Matter and Gases. Phase Changes A phase change is a change in the state of matter During a phase change there is no change in temperature.

Unit 10 States of Matter and Gases

Phase Changes

A phase change is a change in the state of matter

During a phase change there is no change in temperature – the KE is used to spread or bring together the molecules of the substance

During a phase change there is only a change in potential energy

Phase Changes

• Temperature

- Measure of average kinetic energy

- Doesn’t change during a phase change

• Energy causes molecules to spread out or come closer together

Phase Changes

Solid becomes a liquid

- Melting PointLiquid becomes a solid

- Freezing PointGas becomes a liquid

- Condensation PointLiquid becomes a gas

- Boiling Point

Phase Changes

Melting and Freezing Points are the SAME TEMPERATURE

Condensation and Boiling Points are the SAME TEMPERATURE

Phase Changes

• Normal Boiling Point

-Boiling point of a substance at 1 atm

-1 atm = 760 Torr = 760 mm Hg = 101.3 kPa

-Water is 100 0C

• Normal Freezing Point

-Freezing point of a substance at 1 atm

-Water is 0 0C

Heating Curves

Heating Curveplot of temperature versus time for a substance

- energy is added at a constant rate

- X-axis can also be time

Heating Curves

l

Flat lines•No change in temperature•Phase changes•2 phases exist at the same time•Change in potential energy

Sloped lines•Change in temp•Change in kinetic energy

Phase Diagrams

•A phase diagram shows states of matter at different temperatures and pressure.

•Lines represent where the phase changes happen•On lines – 2

phases exist

Phase Diagrams

Triple point Where all 3 lines intersectAll 3 phases exist at once

Critical point Where gas/liquid line endsSupercritical fluid exists in region beyond point

Phase Diagrams

Supercritical FluidHas the density of a liquid, so it can

dissolve thingsBehaves like a gasUsed to extract chemicals/flavors

(coffee, tea, spices)

Phase Diagrams

Intermolecular Forces

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Intermolecular forces are attractive forces between molecules.•Exist between separate molecules of a substance•Weaker than intramolecular forces•Affect melting point, boiling point, etc.

Intramolecular forces hold atoms together in a molecule•Occur between atoms inside a molecule•Ex: bond energy for ionic and covalent bonds

Intermolecular Forces

1. Hydrogen Bond (strongest of all Intermolecular forces)

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• Hydrogen in a polar molecule attracted to a highly electronegative atom (O, N, F) in another.

• IT IS NOT AN ACTUAL BOND.

Hydrogen Bond

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Intermolecular Forces

Types of Intermolecular Forces

2. Dipole-Dipole Forces

• Attractive forces between polar molecules

• Slightly pos. end of one attracts slightly neg. end of other.

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Orientation of Polar Molecules in a Solid

Types of Intermolecular Forces

3. London Dispersion Forces – van der Walls forces (weakest)

• Attractive forces that are temporary dipoles in nonpolar molecules

• Momentary imbalance of electrons

• Affects all atoms

• Only forces in noble gases and nonpolar molecules

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Types of Intermolecular Forces

3. London Dispersion Forces – van der Walls forces (weakest)

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SO

O

What type(s) of intermolecular forces exist between each of the following molecules?

HBrHBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules.

CH4

CH4 is nonpolar: dispersion forces.

SO2

SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules.

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