Chapter 17 Part 3

Intermolecular forces vs. Intramolecular forces

What happens when the water evaporates? Intermolecular attractions (van der Waals forces) are broken. Intramolecular attractions (bonds) are not broken.

Demo Squirt Water and Alcohol on chalkboard » VDWF Rate of Evap BP VP Alcohol __________________________________ Water

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?

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 C 8 H 18(l) ↔ C 8 H 18(g)

C 8 H 18(l) ↔ C 8 H 18(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

browncolorless 2NO 2 (g) ↔N 2 O 4 (g) brown colorless 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.

H 2 O(l)  H 2 O(g) Is the reaction above exothermic or endothermic?

Predict the effect of the following changes on the reaction: 2SO 3 (g) ↔ 2SO 2 (g) + O 2 (g) ∆H = kJ (a) Increasing the temperature of the reaction.

Predict the effect of the following changes on the reaction: 2SO 3 (g) ↔ 2SO 2 (g) + O 2 (g) ∆H = kJ (b) Increasing the pressure on the reaction.

Predict the effect of the following changes on the reaction: 2SO 3 (g) ↔ 2SO 2 (g) + O 2 (g) ∆H = kJ (c) Adding more O 2.

Predict the effect of the following changes on the reaction: 2SO 3 (g) ↔ 2SO 2 (g) + O 2 (g) ∆H = kJ (d) Removing O 2.

Homework  Worksheet 1 Chapter 17.