Chapter 10 Liquids and Solids

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energy is required to overcome the intermolecular forces in the liquid Example: Enthalpy of vaporization of water H vap = 40.

1 The Three States (Phases) of Matter Chapter 10 Liquids and Solids The Phase Changes of Water Changes of State Evaporation and Condensation Enthalpy (Heat) of Vaporization, H vap The energy needed to vaporize 1 mol of a liquid at 1 atm pressure Vaporization is endothermic since energy is required to overcome the intermolecular forces in the liquid Example: Enthalpy of vaporization of water H vap = 40.7 kjmol -1 The large enthalpy of vaporization of water (due to hydrogen bonding) helps cool the surface of the Earth as well as the body through perspiration 1

Vapor Pressure of a Liquid in a Closed System Measurement of Vapor Pressure closed system (a) Initially molecules

increases the vapor starts to condense back into liquid and condensation rate increases Eventually the rate of

equilibrium is called the equilibrium vapor pressure or just the vapor pressure of the liquid P vapor = P atmopshere

intermolecular forces between the molecules of the liquid The stronger the forces, the lower the vapor pressure Polar

non-polar molecules that interact via weak dispersion forces have high vapor pressures and are said to be volatile

significant Place the following in order of increasing vapor pressure: CH 4 H 2 O NaCl C 10 H 22 He NH 3 Solids also

increases rapidly with temperature since more molecules have the kinetic energy required to escape the liquid Normal

2 Vapor Pressure of a Liquid in a Closed System Measurement of Vapor Pressure closed system (a) Initially molecules evaporate (at a constant rate at a given temperature) and the amount of liquid decreases (b) As the amount of vapor increases the vapor starts to condense back into liquid and condensation rate increases Eventually the rate of vaporization equals the rate of condensation and the system reaches equilibrium The vapor pressure (P vap ) at equilibrium is called the equilibrium vapor pressure or just the vapor pressure of the liquid P vapor = P atmopshere - P Hg Vapor Pressure and Intermolecular Forces The vapor pressure of a liquid is determined by the size of the intermolecular forces between the molecules of the liquid The stronger the forces, the lower the vapor pressure Polar molecules that interact via stronger hydrogen bonding and dipole-dipole forces have low vapor pressures Small non-polar molecules that interact via weak dispersion forces have high vapor pressures and are said to be volatile However, non-polar molecules with high molecular weights have low vapor pressures because dispersion forces become significant Place the following in order of increasing vapor pressure: CH 4 H 2 O NaCl C 10 H 22 He NH 3 Solids also have vapor pressures, but they are typically much lower than liquids Vapor Pressure and Temperature Vapor pressure increases rapidly with temperature since more molecules have the kinetic energy required to escape the liquid Normal Boiling Point (nbpt) Temperature at which the vapor pressure of a liquid equals 1 atm or 760 mmhg Example: The normal boiling point of water is 100 C 2

Quantitative Dependence of P vap on Temperature Hvap 1 ln( Pvap ) C R T This is a simple linear equation of the form y = mx + b so if you measure P vap at

1/T will yield a straight line with a slope of H vap /R and an intercept of C (a constant characteristic of a given liquid) The slopes are always negative

strong hydrogen bonding between molecules Vapor Pressure of Liquid Nitric Acid ln(p vap ) 7 6 5 4 ln(p vap ) = -4640(1/T) + 19.

3 Quantitative Dependence of P vap on Temperature Hvap 1 ln( Pvap ) C R T This is a simple linear equation of the form y = mx + b so if you measure P vap at different temperatures a plot of ln(p vap ) vs. 1/T will yield a straight line with a slope of H vap /R and an intercept of C (a constant characteristic of a given liquid) The slopes are always negative consistent with H vap being positive (endothermic) The smaller the slope, the lower H vap and the more volatile the liquid Water has a large H vap due to the strong hydrogen bonding between molecules Vapor Pressure of Liquid Nitric Acid ln(p vap ) ln(p vap ) = -4640(1/T) If we know H vap and P vap at one temperature we can calculate P vap at a given temperature or the temperature at a given P vap Since the constant, C does not depend on temperature we can solve for C at two temperatures T 1 and T 2 : /T (K -1 ) Melting and Freezing 3

Enthalpy (Heat) of Fusion, H fus The energy needed to melt 1 mol of a solid at 1 atm pressure Melting is endothermic since energy is required to

02 kjmol -1 Normal Melting Point (nmpt) Temperature at which a solid melts at pressure of a 1 atm or 760 mmhg Example: The normal melting point of

fusion and melting point is related to the strength of the intermolecular or interatomic forces in the solid: Note that the vapor pressure of ice

melting point at 1 atm but still remains a liquid for some time due its inability to immediately reorganize its structure into the solid Changes of

4 Enthalpy (Heat) of Fusion, H fus The energy needed to melt 1 mol of a solid at 1 atm pressure Melting is endothermic since energy is required to overcome the intermolecular forces in the solid Example: Enthalpy of fusion of ice H fus = 6.02 kjmol -1 Normal Melting Point (nmpt) Temperature at which a solid melts at pressure of a 1 atm or 760 mmhg Example: The normal melting point of water is 0 C At the normal melting point of a substance, the vapor pressures of the solid and the liquid are equal to 1 atm: A solid s enthalpy of fusion and melting point is related to the strength of the intermolecular or interatomic forces in the solid: Note that the vapor pressure of ice increases with temperature at a faster rate than the vapor pressure of water Supercooling This occurs when a liquid is rapidly cooled below it melting point at 1 atm but still remains a liquid for some time due its inability to immediately reorganize its structure into the solid Changes of state do not always occur exactly at the melting or boiling points! However, eventually the solid does form, releasing energy and bringing the temperature back up to melting point where the remainder of the liquid freezes 4

Superheating This occurs when a liquid is rapidly heated above its boiling point at 1 atm but

which rapidly expand and burst before reaching the surface, blowing the liquid out of the out

the formation of large bubbles Solid turns directly into a gas or a gas turns directly into a

The energy needed to sublime 1 mol of a solid at 1 atm pressure Sublimation is endothermic since

5 Superheating This occurs when a liquid is rapidly heated above its boiling point at 1 atm but still remains as a liquid Sublimation and Deposition Bubbles of hot vapor form in the liquid which rapidly expand and burst before reaching the surface, blowing the liquid out of the out the container This is called bumping and can be controlled by using boiling chips which prevent the formation of large bubbles Solid turns directly into a gas or a gas turns directly into a solid without passing through the liquid state Examples: Enthalpy (Heat) of Sublimation, H sub The energy needed to sublime 1 mol of a solid at 1 atm pressure Sublimation is endothermic since energy is required to overcome the intermolecular forces in the solid Example: Enthalpy of sublimation of iodine H vap = 28.7 kjmol -1 The enthalpy of sublimation is the sum of the enthalpy of fusion and the enthalpy of vaporization: H sub = H fus + H vap carbon dioxide dry ice iodine Heating Curves 5

It is possible to calculate the total amount of energy required to convert at solid at some initial temperature to another phase (either a liquid or a gas) at a given temperature by summing up the

6 It is possible to calculate the total amount of energy required to convert at solid at some initial temperature to another phase (either a liquid or a gas) at a given temperature by summing up the energies required at each stage This requires calculating the amount of energy required to heat a particular phase to a given temperature using its specific heat capacity: q = s x m x T where: s = specific heat capacity (Jg -1 ºC -1 ) q = energy required (J) m = mass of sample (g) T = temperature change (ºC) = T final -T initial Cooling Curves Energy is released when a vapor condenses at the boiling point: The energy released when 1 mol of a gas condenses into liquid at 1 atm pressure = - H vap Energy is released when a liquid freezes at the melting point: The energy released when 1 mol of a liquid freezes into a solid at 1 atm pressure = - H fus Energy is released when a solid deposits from a gas at the sublimation point: The energy released when 1 mol of a gas sublimes into a solid at 1 atm pressure = - H sub 6

Phase Diagrams Show which states exist for a closed system as a function of temperature and pressure supercritical fluid region The Phase Diagram of Water T m = normal melting point (0 C, 1 atm) T b

0060 atm) T c = critical temperature above which the vapor cannot be liquefied at any pressure (374 C) P c = critical pressure required to produce liquefaction at the critical temperature (218 atm) T

external pressure increases due to ice being less dense than water A liquid boils at the temperature where the vapor pressure of the liquid equals the external pressure so as we go higher in

7 Phase Diagrams Show which states exist for a closed system as a function of temperature and pressure supercritical fluid region The Phase Diagram of Water T m = normal melting point (0 C, 1 atm) T b = normal boiling point (100 C, 1 atm) T 3, P 3 = triple point where all three states can coexist simultaneously ( C, atm) T c = critical temperature above which the vapor cannot be liquefied at any pressure (374 C) P c = critical pressure required to produce liquefaction at the critical temperature (218 atm) T c, P c = critical point (374 C, 218 atm) Supercritical Fluid Region Other Features The solid/liquid boundary line has a negative slope indicating that the melting point of water decreases as the external pressure increases due to ice being less dense than water A liquid boils at the temperature where the vapor pressure of the liquid equals the external pressure so as we go higher in altitude, the external pressure goes down and so does the boiling point At temperatures and pressures higher than the critical point, a substances exists as a supercritical fluid, which has properties in between that of a liquid and a gas For example, it can diffuse through solids like a gas, and dissolve materials like a liquid! The Phase Diagram of Carbon Dioxide The solid/liquid line has a positive slope since the solid is denser than the liquid T 3, P 3 = triple point (-56.6 C, 5.1 atm) T c, P c = critical point (31 C, 72.8 atm) At 1 atm the solid sublimes at (-78 C) leading to it commonly being referred to as dry ice 7

8 Astronomical Applications Carbon dioxide ice also sublimes on the surface of the planet Mars which has a maximum surface temperature of 20 C, a minimum surface temperature of -140 C and an average surface temperature of -53 C and a surface pressure of atm The Planet Mars has Polar Caps like the Earth 8

The North Polar Cap of Mars during Winter The outer carbon

Residual North Polar Cap of Water Ice during Summer Titan the

containing hydrocarbons (mostly methane, CH 4 and ethane, C 2

9 The North Polar Cap of Mars during Winter The outer carbon dioxide cap sublimes into the atmosphere during Spring The Residual North Polar Cap of Water Ice during Summer Titan the largest moon of Saturn Has a cold, nitrogen atmosphere containing hydrocarbons (mostly methane, CH 4 and ethane, C 2 H 6 Phase Diagram of CH 4 Under the surface conditions on Titan (pressure 1.6 atm, temperature 94 K), the methane phase diagram indicates that liquid methane should be present on the surface Not to scale! 9

10 Liquid Erosion Features Hydrocarbon lakes? Cassini Orbiter deploying Huygens Probe Parachuted into Titan s Atmosphere Jan 14 th 2005 The Surface of Titan A slushy mixture of water ice and liquid hydrocarbons 10

Chapter 11. Liquids and Intermolecular Forces

Chapter 11. Liquids and Intermolecular Forces Chapter 11 Liquids and Intermolecular Forces States of Matter The three states of matter are 1) Solid Definite shape Definite volume 2) Liquid Indefinite shape Definite volume 3) Gas Indefinite shape Indefinite