Chemistry/Physical Setting The Nature of Liquids Br. Jabreal

Aim: What is the nature of liquids? Do Now: Describe the arrangement of particles in a liquid, and draw a phase diagram of a liquid made up of molecules of the following substance: Definite volume and take the shape of their container.

A Model for Liquids According to kinetic theory, particles of liquids also have kinetic energy Allows liquid particles to flow past each other and allows liquids to conform to shape of their containers However, unlike gases, liquid molecules DO have attractive forces between each other

Evaporation Evaporation: Process by which a liquid changes to a gas. Would this be a physical or chemical change?

Evaporation Evaporation tends to take place at surface of liquids at ALL temperatures. During evaporation, only the molecules with a certain minimum kinetic energy can escape surface of the liquid Liquids evaporate faster when heated because … They are being supplied with more kinetic energy, allowing more particles to overcome the attractive forces keeping them in liquid state

Vapor Pressure Vapor Pressure: Measure of force exerted by a gas above a liquid in a closed system. Constantly, some particles are condensing and others are evaporating, but the force of the gas (vapor) particles colliding with the liquid’s surface is the vapor pressure Vapor pressure increases as temperature increases The increased supply of kinetic energy allows more particles to escape as gases, and more gas exerts more force

Equilibrium exists between the vapor (gas) and liquid Rate of evaporation eventually equals rate of condensation, although no net change in number of particles of liquid or gas Evaporation Liquid Vapor (Gas) Condensation

At equilibrium, amount of vapor and amount of liquid NOT necessarily equal, although their rates of conversion are.

Boiling Point When a liquid is heated to the point that particles throughout the entire sample have enough energy to escape as gas particles, the liquid is considered to be boiling. Boiling point: Temperature at which the vapor pressure of the liquid equals external pressure on the liquid

Boiling Point As pressure changes, so does the boiling point. Explain how/why based on the definition of the boiling point. At higher altitudes, what do you think the effect would be on the boiling point? How does this affect cooking? Normal boiling point refers to boiling point at standard pressure (1 atm or 101.3 kPa).

Table H Vapor pressure of Four Liquids

Table H: Vapor Pressure and Temperature Table H can be used to determine the boiling point of liquids at different pressures. Point where dotted line intersects curve is normal boiling point What’s the normal boiling point of ethanol? What’s the boiling point of water when the pressure is 70 kPa? What’s the boiling point of ethanoic acid at 85 kPa?

Practice At which temperature is the vapor pressure of ethanol equal to the vapor pressure of propanone at 35 oC? 35 60 82 95

Homework Section 13.2 Assessment Page 409 Questions 9, 10, 12 and 13 Question 75 (a-c)