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Adhesive forces increase the surface tension of the liquid. The surface tension reduces the surface area of the liquid, thereby pulling the liquid up the tube. The liquid climbs until adhesive and cohesive forces are balanced by gravity on the liquid. Why soda or water rise up in a straw?.

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Adhesive forces increase the surface tension of the liquid. The surface tension reduces the surface area of the liquid, thereby pulling the liquid up the tube.

The liquid climbs until adhesive and cohesive forces are balanced by gravity on the liquid

Why soda or water rise up in a straw?

How Plants and trees get water and dissolved nutrients from the soil?

Capillary Action

The rise of liquids up in narrow tubes is called capillary action


a. The surface tension reduces the surface area of the liquid, thereby pulling the liquid up the tube.

  • Both viscosity and surface tension decrease with increasing temperature.

  • Viscosity increases with increasing temperature while surface tension decreases.

  • Viscosity decreases with increasing temperature while surface tension increases.

  • Both viscosity and surface tension increase with increasing temperature.


a. The surface tension reduces the surface area of the liquid, thereby pulling the liquid up the tube.

  • Both viscosity and surface tension decrease with increasing temperature.

  • Viscosity increases with increasing temperature while surface tension decreases.

  • Viscosity decreases with increasing temperature while surface tension increases.

  • Both viscosity and surface tension increase with increasing temperature.


b. The surface tension reduces the surface area of the liquid, thereby pulling the liquid up the tube.

  • Viscosity increases as intermolecular forces increase while surface tension decreases.

  • Viscosity decreases as intermolecular forces increase while surface tension increases.

  • Both viscosity and surface tension increase as intermolecular forces increase.

  • Both viscosity and surface tension decrease as intermolecular forces increase.




4 phase changes
4) which force is most important.Phase Changes


Energy changes associated with changes of state
Energy Changes Associated with Changes of State which force is most important.

  • The heat added to the system at the melting and boiling points goes into pulling the molecules farther apart from each other.

  • The temperature of the substance does not change during the phase change.

Heating Curve for 1.00 mol of water at constant pressure of 1 atm (Ti = -25 oC, Tf = 125 oC)


“C” which force is most important.: is the heat capacity; “s” : is the specific heat; DT = Tf – Ti

unit depends on the unit of “C” or “s”


Energy changes associated with changes of state1
Energy Changes Associated with Changes of State which force is most important.

  • Heat of Fusion or Enthalpy of fusion ( ): Energy required to change a solid at its melting point to a liquid.

  • Heat of Vaporization or Enthalpy of vaporization ( ): Energy required to change a liquid at its boiling point to a gas.

  • Heat of Sublimation or Enthalpy of Sublimation ( ): Energy required to change a solid at its melting point to a gas.


  • evaporation, endothermic which force is most important.

  • melting (or fusion), endothermic

  • sublimation, endothermic

  • melting (or fusion), exothermic


  • evaporation, endothermic which force is most important.

  • melting (or fusion), endothermic

  • sublimation, endothermic

  • melting (or fusion), exothermic


Give it some thoughts which force is most important.

  • We use cubes of ice to cool water, how this works?

  • We feel cool when we step out of a swimming pool or a warm shower. Why?

  • Refrigeration, How?


Sample Exercise which force is most important.

Calculating DH fro Temperature and Phase Changes

Calculate the enthalpy change upon converting 1.00 mole of ice at -25 oC to water vapor (steam) at 125 oC under a constant pressure of 1 atm. The specific heats of ice, water, and steam are 2.09, 4.18, 1.84 J/g.K, respectively. For water DHfus = 6.01 and DHvap = 40.67 kJ/mol

Note:

DHvap >> DHfus Why?


Strategy: Consider both temperature and phase changes which force is most important.

AB: -25 to 0 oC (solid)

BC: 0 (solid) to 0 oC (liquid)

CD: 0 (liquid) to 100 oC (liquid)

DE: 100 (liquid) to 100 oC (vapor)

EF:100 (vapor) to 125 oC (liquid)

s= 2.09 (ice), 4.18 (liq), 1.84 (vap) J/g.K, DHfus = 6.01 and DHvap = 40.67 kJ/mol


Practice Exercise which force is most important.

What is the enthalpy change during the process in which 100.0 g of water at 50.0 oC is cooled to ice at -30.0 oC? Use the specific heats and enthalpies of phase change given in the previous example.

Answer:

-20.9 kJ – 33.4 kJ -6.27 kJ = -60.6 kJ


What Happens? which force is most important.

Pgas = 0

vacuum

?

Imagine t = 0

After some time (t)


If no molecules exist in the gas phase, there is zero pressure

At any temperature, some molecules can escape from the surface of a liquid by evaporation

vacuum

As more molecules escape the liquid, the pressure exerted by the vapor in the space above the liquid will begin to increase

After some time

After some to time, liquid and vapor reach a state of dynamic equilibrium: liquid molecules evaporate and vapor molecules condense at the same rate. The gas pressure remains constant as long as the temperature remains constant. This constant pressure is called the“vapor pressure”

Equilibrium vapor pressure over a liquid. At equilibrium molecules enter and leave the liquid at the same rate


Explanation of vapor pressure on the molecular level

Depends on the strength of the attractive forces pressure

The average K.E of surface molecules > energy needed to escape the surface, molecules evaporate

The average K.E of surface molecules < energy needed to escape the surface, molecules do not evaporate

The weaker the attractive forces the larger is the vapor pressure

As the temperature increases the vapor pressure increases

Explanation of Vapor Pressure on the Molecular Level

The distribution of average kinetic energies of surface molecules as function of temperature


Volatility

In pressure cooker, water boils at temp> 100 oC and therefore food gets cooked fast

Volatility

In an open container, liquids (such as gasoline) that evaporate readily are said to be volatile

Vapor Pressure, and Temperature

  • The boiling point of a liquid is the temperature at which its vapor pressure equals the external pressure.

  • The normal boiling point is the temperature at which its vapor pressure is 760 torr (1 atm).


[2] External pressure higher than 1 atm causes the water to boil at a temperature higher than 100

, At P1 and T1

, At P2 and T2

[3]

Subtracting Eq. [3] from Eq. [2], we get

If we know (P2, T2); (P1, T1) we can calculate DHvap

The Clausius-Clapeyron Equation

[1]

, At any P and T

Slope = Dlnp / D(1/T)

lnp2

Dlnp

lnp1

D(1/T)

1/T2

1/T1


Phase diagrams
Phase Diagrams External pressure higher than 1 atm causes the water to boil at a temperature higher than 100

Phase diagrams display the state of a substance at various pressures and temperatures and the places where equilibria exist between phases.

Let us have a closer look


Critical point (B): the temperature and the pressure are called the critical temperature and the critical pressure. It is the highest temp. and pressure at which a liquid can exist. Above the critical point the liquid and vapor are indistinguishable from each other. The greater the attractive force the higher is the critical point

Solid-Liq equilibrium

Liq-gas equilibrium

Solid-liq-gas equilibrium

Solid lines: are called the co-existence curves or the interface lines. Each point along the AB, AD, and AC lines, is the B.P, M.P, and sublimation point at a given pressure, respectively.

The normal B.P, M.P, and sublimation points are those along AB, AD, and AC at pressure of 1 atm, respectively.


Phase diagram of water and carbon dioxide
Phase Diagram of Water and Carbon Dioxide called the critical temperature and the critical pressure. It is the highest temp. and pressure at which a liquid can exist. Above the critical point the liquid and vapor are indistinguishable from each other. The greater the attractive force the higher is the critical point

No normal B.P or normal F.P

Normal F.P

The critical point

P=5.11 atm

T=-56.4 oC

Carbon dioxide cannot exist in the liquid state at pressures below 5.11 atm; when heated at 1 atm it does not melt but sublimes.

The critical point

P=217.7 atm

T=374.4 oC

(T.P)

(T.P)

(T.P)

Normal B.P

Normal sublimation point

Note the high critical temperature and critical pressure: these are due to the strong hydrogen bonding between water molecules.

The low critical temperature and critical pressure for CO2 make supercritical CO2 a good solvent for extracting nonpolar substances (such as caffeine).

The slope of the solid–liquid line is negative. This means that M.P decreases with increasing pressure (liq is more compact than ice


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