Physical Science Lesson 7 the Structure and Properties of Matter

The Structure and Properties of Matter The Properties of a Substance as Effect of Intermolecular Forces of Attraction

Objective  At the end of the lesson, lesson, you should be able able to explain the effect of intermolecular intermolecular forces forces on the properties of substances. Recall the different types of intermolecular forces of attraction:   

London dispersion forces (LDF) forces  (LDF) – present in all molecules Dipole – dipole forces – present in polar molecules Hydrogen bonding – present in molecules with a hydrogen bonded to an electronegative electrone gative atom (O, N, or F)

How are these intermolecul intermolecular ar forces related to the observable properties of different substances?

Learn about it! The properties of molecules depend on the type and strength of their intermolecular intermolecular forces of attraction. Properties Dependent on IMFA   Phase at room room temperature temperature  When molecules molecules have strong intermolecula intermolecularr forces of attraction, attraction, they are are packed close together. They often exist as condensed phase (solid or liquid) at room temperature.  When molecules molecules have weak intermolecular intermolecular forces forces of attraction, attraction, they are far apart apart from each other. They often exist as gas at room temperature.  Solubility Solubility  refers  refers to the ability of a substance to dissolve in a given amount of solvent at a specified temperature. Recall the rule “like “ like dissolves like.” like .” When the solute and the solvent both exhibit the same intermolecular forces of attraction, they form a solution.

Examples  Water and ethanol are polar substances. They both exhibit LDF, dipole - dipole forces, and hydrogen bonding. When mixed together, they form a solution.

On the other hand, hexane and water do not have similar intermolecular forces of attraction. The only attractive forces present in hexane are the relatively weak London dispersion forces. These forces cannot significantly disrupt the strong hydrogen bonding among water molecules. Therefore, hexane and water form a heterogeneous mixture.

Learn about it!  Melting point  Melting point is the temperature at which the substance changes from solid to liquid. Stronger intermolecular forces means greater amount of energy is needed to break the attractive forces between molecules. Substances with stronger IMFA have higher melting points compared to those with weaker IMFA.  Boiling point 

Boiling point is the temperature at which the substance changes from liquid to gas. Similar to melting point, stronger intermolecular forces means greater amount of energy is needed to break the attractive forces between molecules. Substances with stronger IMFA have higher boiling pointscompared to those with weaker IMFA.  Surface Tension Surface Tension is the tendency of a fluid to acquire the least possible surface area. Molecules with stronger intermolecular forces will exert greater cohesive forces and acquire less surface area (higher surface tension) than those with weaker IMFA. Viscosity  Viscosity is the measure of a fluid’s resistance to flow. Molecules with stronger intermolecular forces have greater resistance to flow, and thus, higher  viscosity  compared to those with weaker IMFA. Vapor Pressure  Vapor Pressure is the pressure exerted by a vapor in equilibrium with its liquid phase in a closed system. Molecules with stronger intermolecular forces have less tendency to escape as gas, and thus lower vapor pressure compared to those with  weaker IMFA.

Learn about it! Strengths of Intermolecular Forces  When comparing the strengths of intermolecular forces, check the difference in molecular weight. If the difference is too large (>1000), then generally, the molecule  with greater molecular weight has stronger intermolecular forces. Otherwise, you may use the following strategy: 1. Check which molecule exhibits hydrogen bonding. This molecule will have stronger intermolecular forces. 2. Check which molecule is polar. Polar molecules have stronger intermolecular forces. 3. Compare London dispersion forces. More massive molecules have stronger intermolecular forces.

Key Points

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The properties of molecules depend on the type and strength of their intermolecular forces of attraction. “Like dissolves like.” When the solute and the solvent both exhibit same intermolecular forces of attraction, they form a solution.  When comparing properties, stronger intermolecular forces result in higher  boiling and melting points, higher viscosity, higher surface tension, and lower  vapor pressure. Increasing strengths of IMFA: London dispersion forces, Dipole-dipole forces, H bonding

Question 1  Which of the following properties is not influenced by the type of intermolecular forces? Correct!

1molar mass Molar Mass is the sum of all the atoms that make up the molecule. It affects the strength of the intermolecular forces present between molecules. Next question

2 boiling point3melting point4solubility  Question 2  Which phase of matter has the strongest IMFA? Correct!

4solid Molecules with strong intermolecular forces of attraction often exist as solid at room temperature. Next question

1liquid2plasma3gas Question 3  What happens to the boiling point as you increase the strength of IMFA? Correct!

4It increases. Stronger intermolecular forces between molecules require more energy so that a phase transition  would occur. Next question

1It becomes zero.2It remains the same.3It decreases.

Rate the following fluids in order from lowest viscosity to highest viscosity: Hot air Ketchup Milk Cold air A.

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Ketchup>milk>hot air>cold air

B.

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Ketchup>milk>cold air>hot air

C.

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Hot air>cold air>milk>ketchup

D.

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Cold air>hot air>milk>ketchup

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