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Capillary Action

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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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slide1

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

slide2

a.

  • 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.
slide3

a.

  • 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.
slide4

b.

  • 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.
slide5

The polarity of the substance is necessary to determine which force is most important.

  • Viscosity and surface tension are not related to adhesive and cohesive forces.
  • adhesive forces
  • cohesive forces
slide6

The polarity of the substance is necessary to determine which force is most important.

  • Viscosity and surface tension are not related to adhesive and cohesive forces.
  • adhesive forces
  • cohesive forces
energy changes associated with changes of state
Energy Changes Associated with Changes of State
  • 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)

slide9

“C”: 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
  • 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.
slide11

evaporation, endothermic

  • melting (or fusion), endothermic
  • sublimation, endothermic
  • melting (or fusion), exothermic
slide12

evaporation, endothermic

  • melting (or fusion), endothermic
  • sublimation, endothermic
  • melting (or fusion), exothermic
slide13

Give it some thoughts

  • 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?
slide14

Sample Exercise

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?

slide15

Strategy: Consider both temperature and phase changes

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

slide16

Practice Exercise

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

slide17

What Happens?

Pgas = 0

vacuum

?

Imagine t = 0

After some time (t)

slide18

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

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

Value of boiling point depends on the external pressure. External pressure higher than 1 atm causes the water to boil at a temperature higher than 100 oC

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).
slide21

[2]

, 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

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

slide23

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

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