Liquids

Chapter 10

Review: Gases

Indefinite shape

Indefinite volume

Take the shape and volume of

container

Particles are far apart

Particles move fast

Low Density

Easy to expand and compress

Review: Solids

Definite shape

Definite volume

Particles close together, fixed

Particles move very slowly

High density

Hard to expand/compress

Liquids: in between

Closer to properties of solidsSlow diffusionHigh attraction between particlesMedium amount of energy

Forces of Attraction

Intramolecular forces: Hold atoms together within a moleculecovalent and ionic bonds

Intermolecular forces: Hold molecules to each other3 types

Dipole-Dipole Attraction

Dipole: molecule with a separation of charge (polar covalent)

Due to differences in electronegativity~1% as strong as a covalent bond

Hydrogen Bond

Very strong dipole-dipole attraction (10% of a covalent bond’s strength)

Occurs when H is bonded to O, N, F in a very polar bond

O H

3.5 2.1Gives water its unusual properties

H-Bonding Affects Boiling Points

Strong attraction requires much energy to overcome, so water is a liquid at normal temperatures

London Dispersion Forces

Occur in all substances-polar and nonpolar

Due to formation of instantaneous dipoles as electrons moving around nucleus concentrate on 1 side of molecule or atom

This induces a dipole in neighboring atoms or molecules

These are the weakest intermolecular forces

These are the only forces of attraction in nonpolar substances

Importance of Water

Covers 70% earth’s surfaceNecessary for reactions in living cellsModerates earth’s temperatureCoolant for engines & nuclear power

plantsTransportationGrowth medium for many organisms

Properties of Water

ColorlessTastelessAt 1 atm, water freezes at 0°C and

vaporizes completely at 100°C Liquid phase occurs from 0-100°C

Special Properties of Water

Surface TensionLiquids tend to form a “skin” making the

surface less penetrable by solids

Unequal attraction at surface, mainly down

Equal attraction in all directions

Surface Tension

Detergents can interfere with the attractions and will cause the paperclip to fall

Adhesion Attraction of the surface of a liquid to the

surface of a solid• Depends on the material

• Water is attracted to glass• Mercury is not

No adhesion

Cohesion

Molecular attractions within a material(water molecules to water molecules, for

example)Here, cohesion

causes water to

form beads; ad-

hesion causes it

to stick to the web

Capillary Action

A liquid rises in a narrow tube when it breaks the surface tensionMovement of water through paper

Ice Floats!Molecules in a liquid have more movement

than a solid and more energy (particles move apart

Generally, solids are more dense than their ir liquids

Liquid water

Ice

When water becomes fixed points in a solid, hydrogen bonds hold molecules in place

Gives ice an open hexagonal structure

Greater volume means lower density than aliquid

Lower K.E. causes distance between molecules to be less

Higher K.E. causes distance between molecules to be more

Max. density at 4 C

Phase Changes of Water

At 1 atm, water freezes at 0°C and vaporizes completely at 100°C

Liquid phase occurs from 0-100°CChanges from one phase to another will

either require energy or release energy Solid Liquid Liquid Gas Solid Gas

Melting/Freezing

Vaporization/Condensation

Sublimation/Deposition

From Solid to LiquidAs energy is added, K.E. increases

Solid warms up

At 0°C, solid begins to melt and temperature remains at 0 until all solid is turned to liquid

When all is liquid, temperature begins to rise

From Liquid to Gas

As heat is added, K.E. increases (increase in temperature)

At 100C, bubbles form in liquidTemperature remains the same until all

liquid is converted to a gas.Once all is a gas, added energy causes

temperature to increase

Ice melting/water vaporizing

When a substance is in phase, increasing the energy increases the temperature

When a substance is changing phase, increasing the energy does not increase the temperature but is used to break forces of attraction between molecules

Phase diagramTemperature vs. Energy

Heating a Solid

Melting a Solid

Heating a Liquid

Vaporizing a Liquid

Heating a Gas

Calculating EnergiesEnergy is measured in calories or Joules

1.00 cal = 4.184 JThe amount of energy needed to change

states depends on:1. Type of matter

2. Quantity of matter

Type of matter

Molar heat of fusion (Hfusion)= energy needed to melt 1 mole of a substance

Molar heat of vaporization (Hvap)= energy needed to vaporize 1 mole of a substance

For Water

(Hfusion) = 80.0 cal/g = .334 kJ/g

(H vap) = 540. cal/g = 2.26 kJ/g

Finding Energy in a Phase Change

Change from a solid liquid q = mHfusion

Change from a liquid gasq = mHvap

q = energy (cal or J)m = mass (g)Hfusion =heat of fusion (cal/g)

q = energy (cal or J)m = mass (g)Hvap =heat of vaporization

Example

How much heat in calories is needed to melt 15.0 g of water?

q = mHfusion

15.0 g water x 80.0 cal = 1.20 x 103 cal 1 g water

Energy and Being In Phase

When all of a substance is in one phase, (e.g. all liquid), the amount of energy required to cause a temperature change depends on:1. type of substance

2. amount of substance

3. range of temperature change

Type of Matter

Specific Heat (c): Energy required to change the temperature of 1 gram of a substance by 1 Celsius degreecg = specific heat of a gas

cl = specific heat of a liquid

cs = specific heat of a solid

For Water

cg = .480 cal/gC or 2.01 J/gC

cl = 1.00 cal/gC or 4.18 J/gC

cs = .500 cal/gC or 2.09 J/gC

Finding Energy When In Phase

q = mcT

T = Tfinal – Tinitial

q = energy (cal or J)m = mass (g)c = specific heat (cal/gC)T =change in temperature (C)

Example

How much energy is required to heat 50.0 g of water from 20.0 C to 85.0 C?

q = mcl TT = 85.0 – 20.0 = 65.0 Cq = (50.0g)(1.00 cal/gC)(65.0 C)q = 3,250 cal

Phase changes

In Phase

Phase diagramTemperature vs. Energy

Phase Change Problems1. Draw graph.

2. Mark start and stop points.

3. Every corner means a new equation is needed.

4. Flat sections will use q = mH ( no T means no slope).

5. Find each energy (q1, q2, q3..).

6. Add all energies to get the total energy.

Phase Changes

Vaporization (evaporation): molecules of a liquid escape the liquid’s surfaceRequires energy to overcome intermolecular

forces

Molecules with enough energy to evaporate

Maxwell Boltzman distribution

To evaporate, a particle must:Be at the surfaceHave sufficient energyBe moving in the right direction

Not at surface

Moving in the wrong direction Not enough

energy

I’m free!!

Evaporation produces Vapor Pressure

1. A closed container with a vacuum has a liquid added to it.

2. Molecules begin to evaporate

3. Some particles are recaptured by the liquid

4. Eventually rate of particles leaving = rate of being recaptured

Equilibrium Vapor PressureWhen rate of evaporation = rate of

condensation the pressure becomes constant (Equilibrium vapor pressure)

Vapor pressure and temperature

As temperature increases, more particles evaporate

Vapor Pressure

The vapor pressure of substances varies greatly, depending on the strength of the forces of attraction between particles.

Volatile liquids evaporate easilySmall particle sizeOnly London dispersion forces of attraction

BoilingOccurs when the equilibrium vapor

pressure reaches atmospheric pressureOnly then can a bubble maintain itself

anywhere in the liquidAir Pressure

Particle of gas exerts pressure on surrounding molecules, pushing them out of the way, and therefore up against air pressure

This is the difference between evaporation and boiling

As atmospheric pressure changes, so does the equilibrium vapor pressure necessary for boiling to occur

Therefore, the boiling point depends on the pressure

Vapor Pressure vs. Temperature can give us the boiling point of a substance at any pressure

The b.p. at 1 atm is called the normal boiling point.

P vs. T Phase Diagram

We can expand our graph to see the effect of pressure on melting/freezing and sublimation/deposition

For a “normal” substance:Increasing pressure

raises the melting point (AD)

Important Points on Diagram A: Triple point: temperature

& pressure at which solid, liquid and gas coexist at equilibrium

B: Critical Point: indicates critical temp and pressure

Critical Temp: temp above which substance cannot exist in a liquid state

Critical Pressure: lowest pressure required for substance to exist as a liquid at the critical temperature

Phase Diagram for Water

Line AB slants backwards.

Increasing the pressure lowers the melting point.

1 atm

0°C 100°C