Chapter 17

Part 3

Intermolecular forces vs. Intermolecular forces vs. Intramolecular forcesIntramolecular forces

What happens when the water What happens when the water evaporates?evaporates?

• Intermolecular attractions (van der Waals forces) are broken.

• Intramolecular attractions (bonds) are not broken.

alcohol vs. water

How do the vapor pressures, rates of evaporation, and van der Waals forces compare?

How would the boiling point of alcohol compare to water?

Names Ethanol, Ethyl alcohol, rubbing alcohol

Molecular formula C2H6O Boiling point 78.4°C

Why is it called rubbing alcohol?

Substances with weaker Van der Waals Forces

1. Are easier to evaporate

2. Have higher vapor pressure

3. Be more volatile

4. Have lower boiling points

Equilibrium

• A state of equilibrium is the most stable state for a reversible system.

LeChatlier’s Principle

• If a stress is placed on a system in equilibrium the system will tend to readjust so that the stress is reduced.

• 3 Stresses are changing the:– Concentration– Temperature– Pressure

• LeChatlier’s Principle = “Do the opposite”

I smell gasoline

C8H18(l) ↔ C8H18(g)

C8H18(l) ↔ C8H18(g)

• Can is sealed

• Contents exert certain pressure due to “vapor pressure”

• Can is opened

• Pressure decreases

• Reaction shifts to increase the pressure

• More liquid goes into the vapor state

2NO2(g) ↔N2O4(g)brownbrown colorlesscolorless

How does applying LeChatlier’s Principle explain that this reaction is exothermic?

Rule: An increase in temperature will always shift a reaction in the endothermic direction.

What other rule could we use?

Note: The rule for temperature applies “do the opposite” already. So don’t do the opposite of the rule.

H2O(l) H2O(g)

Water Vapor Pressure Table Temperature Pressure (°C) (mmHg)

Temperature Pressure (°C) (mmHg)

Temperature Pressure (°C) (mmHg)

0.0 4.6 5.0 6.5 10.0 9.2 12.5 10.9 15.0 12.8 15.5 13.2 16.0 13.6 16.5 14.1 17.0 14.5 17.5 15.0 18.0 15.5 18.5 16.0 19.9 16.5

19.5 17.0 20.0 17.5 20.5 18.1 21.0 18.6 21.5 19.2 22.0 19.8 22.5 20.4 23.0 21.1 23.5 21.7 24.0 22.4 24.5 23.1 25.0 23.8 26.0 25.2

27.0 26.7 28.0 28.3 29.0 30.0 30.0 31.8 35.0 42.2 40.0 55.3 50.0 92.5 60.0 149.4 70.0 233.7 80.0 355.1 90.0 525.8 95.0 633.9 100.0 760.0

Is the reaction above exothermic or endothermic?

Predict the effect of the following changes on the reaction:

2SO3(g) ↔ 2SO2 (g) + O2 (g) ∆H = +197.78 kJ

(a) Increasing the temperature of the reaction.

Predict the effect of the following changes on the reaction:

2SO3(g) ↔ 2SO2 (g) + O2 (g) ∆H = +197.78 kJ

(b) Increasing the pressure on the reaction.

Predict the effect of the following changes on the reaction:

2SO3(g) ↔ 2SO2 (g) + O2 (g) ∆H = +197.78 kJ

(c) Adding more O2.

Predict the effect of the following changes on the reaction:

2SO3(g) ↔ 2SO2 (g) + O2 (g) ∆H = +197.78 kJ

(d) Removing O2.

Homework

Worksheet 1 Chapter 17.