liquids · •evaporation happens when atoms or molecules escape from the liquid and turn into a...

liquids notebook.notebook

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November 07, 2012

Liquids

How can you tell the state of matter by looking at the picture?Liquids and solids are often referred to as condensed phases because the particles are very close together.

Liquids and gases are called fluids because they can be made to flow, or move. In any fluid, the molecules themselves are in constant, random motion, colliding with each other and with the walls of any container.


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November 07, 2012

When the ice melts and the crystal lattice breaks down, the cages collapse and the molecules move closer together, taking up less space. Consequently, a given weight of water occupies more volume as ice than as liquid. • In other words, ice is less dense than water. Therefore, ice floats on liquid water. • Also, a full, closed container of water will break as it freezes because the ice must expand. A water pipe may break if it freezes in winter because of this unusual property of water.

Cohesive forces at work that hold the liquid molecules together.


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November 07, 2012

Evaporation is all about the energy in individual molecules, not about the average energy of a system. The average energy can be low and the evaporation still continues.

You might be wondering how that can happen when the temperature is low. It turns out that all liquids can evaporate at room temperature and normal air pressure. • Evaporation happens when atoms or molecules escape from the liquid and turn into a vapor. • Not all of the molecules in a liquid actually have the same energy.

The energy you can measure with a thermometer is really an average of all the molecules in the system. There are always a few molecules with a lot of energy and some with barely any energy at all. • The molecules with a lot of energy are able to build up enough power to become a gas. Once they reach that energy level, they can leave the liquid. When the molecule leaves, it has evaporated.


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November 07, 2012

Evaporation Ordinary evaporation is a surface phenomenon some molecules have enough kinetic energy to escape. If the container is closed, an equilibrium is reached where an equal number of molecules return to the surface. The pressure of this equilibrium is called the saturation vapor pressure.


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Vapor Pressure


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November 07, 2012

Factors influencing the rate of evaporation

• Concentration of the substance evaporating in the air.If the air already has a high concentration of the substance evaporating, then the given substance will evaporate more slowly.

• Concentration of other substances in the air: If the air is already saturated with other substances, it can have a lower capacity for the substance evaporating.

• Flow rate of air: This is in part related to the concentration points above.

*If fresh air is moving over the substance all the time, then the concentration of the substance in the air is less likely to go up with time, thus encouraging faster evaporation. *This is result of the boundary layer at the evaporation

surface decreasing with flow velocity, decreasing the diffusion distance in the stagnant layer.

• Concentration of other substances in the liquid (impurities): If the liquid contains other substances, it will have a lower capacity for evaporation.

• Temperature of the substance: If the substance is hotter, then evaporation will be faster.

• Intermolecular forces: The stronger the forces keeping the molecules together in the liquid state, the more energy one must get to escape.

• Surface area : A substance which has a larger surface area will evaporate faster as there are more surface molecules which are able to escape.

• Pressure: In an area of less pressure, evaporation happens faster because there is less exertion on the surface keeping the molecules from launching themselves.


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November 07, 2012

Evaporation vs Boiling Ordinary evaporation is a surface phenomenon since the vapor pressure is low and since the pressure inside the liquid is equal to atmospheric pressure plus the liquid pressure , bubbles of water vapor cannot form.

But at the boiling point, the saturated vapor pressure is equal to atmospheric pressure, bubbles form, and the vaporization becomes a volume phenomena.


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November 07, 2012

For a system consisting of vapor and liquid of a pure substance:The equilibrium state is directly related to the vapor pressure of the substance, as given by the

ClausiusClapeyron equation

where P1, P2 are the vapor pressures at temperatures T1, T2 respectively, ΔHvap is the enthalpy of vaporization , and R is the universal gas constant .

• The rate of evaporation in an open system is related to the vapor pressure found in a closed system. If a liquid is heated, when the vapor pressure reaches (equals) the ambient pressure the liquid will boil.


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November 07, 2012

Boiling Point

The boiling point is defined as the temperature at which the saturated vapor pressure of a liquid is equal to the surrounding atmospheric pressure .

• For water, the vapor pressure reaches the standard sea level atmospheric pressure of 760 mmHg at 100°C or 1atm or 760 torr or 101.3kPa.

• Since the vapor pressure increases with temperature, it follows that for pressure greater than 760 mmHg (e.g., in a pressure cooker), the boiling point is above 100°C and for pressure less than 760 mmHg (e.g., at altitudes above sea level), the boiling point will be lower than 100°C. Table H Regents level shows this.

As long as a vessel of water is boiling at 760 mmHg, it will remain at 100°C until the phase change is complete. Rapidly boiling water is not at a higher temperature than slowly boiling water. The stability of the boiling point makes it a convenient calibration temperature for temperature scales .


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November 07, 2012

However, surface area does effect the rate of evaporation. They have a direct relationship!


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November 07, 2012

Entropya measure of disorder.


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November 07, 2012

Standard Temperature and Pressure (STP)

Definition: 273 K (0 degrees Celsius) and 1 atm pressure.

STP is often used for measuring gas density and volume.

Pressure Pressure is defined as a force over an area.

In Chemistry, pressure is often measured in: kilopascals (kPa)millimeters of mercury (mm of Hg)atmospheres (atm).

For convenience sake, a standard atmospheric pressure has been set at 101.3 kPa = 760 mm of Hg =1.0 atm.

101.3 kPa = 760 mm of Hg =1.0 atm.

You must be able to convert from one unit to another using dimensional analysis.


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November 07, 2012

The instruments that measure air pressure are called barometers, from Greek words for weight and measure.

• Evangelista Torricelli invented the mercury barometer in 1643 and today's mercury barometers are much like those of the 17th century.

A glass tube from which the air has been removed is inserted into a dish of mercury. • The air pressing down on the mercury in the dish forces some of the mercury up into the glass tube. • At the Earth's surface, air pressure pushes the mercury about 30 inches (approx. 760 mm) up into the tube.


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November 07, 2012

Liquid column gauges consist of a vertical column of liquid in a tube whose ends are exposed to different pressures. • The column will rise or fall until its weight is in equilibrium with the pressure differential between the two ends of the tube. • A very simple version is a Ushaped tube halffull of liquid, one side of which is connected to the region of interest while the reference pressure (which might be the atmospheric pressure or a vacuum) is applied to the other. • The difference in liquid level represents the applied pressure.

Any fluid can be used, but mercury is preferred for its high density (13.534 g/cm³) and low vapour pressure. For low pressure differences well above the vapour pressure of water, water is a commonlyused liquid (and "inches of water " is a commonlyused pressure unit).

dHg = 13.5951 g / cm3

If you are using water as the liquid you must multiply by 13.6 to get your answer because water's density is only 1 g / cm3

Open Ended Manometers


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November 07, 2012

The opentube manometer is device that can be used to measure pressure. The opentube manometer is used to measure the pressure of a gas in a container. The pressure of the gas is given by h (the difference in mercury levels) in units of torr or mmHg. Atmospheric pressure pushes on the mercury from one direction, and the gas in the container pushes from the other direction. In a manometer, since the gas in the bulb is pushing more than the atmospheric pressure, you add the atmospheric pressure to the height difference:

Pgas > Patm Gas pressure = atmospheric pressure + h (height of the mercury)

Pgas < Patm Gas pressure = atmospheric pressure h (height of the mercury)


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November 07, 2012

You could be asked about a closedtube manometer. Closedtube manometers look similar to regular manometers except that the end that’s open to the atmospheric pressure in a regular manometer is sealed and contains a vacuum. In these systems, the difference in mercury levels (in mmHg) is equal to the pressure in torr.

open manometerPgas > Patmclosed manometer

open manometerPgas < Patm


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November 07, 2012

Pgas=740mmHg (25.4 x 0.500in) =727.4mmHg

Note closed manometer Pgas=45mmHg (This is not possible, bad picture)

Pgas=(1.06 x (760mmHg/1 atm)) + 65mmHg =870mmHg


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November 07, 2012

When the vapor pressure of ethanol is 522 mm of Hg, the temperature of the ethanol is

• As the strength of intermolecular forces increases, surface tension increases.

• The stronger the intermolecular forces the higher the viscosity, molecules are closer together and get tangled up and become resistant to flow.

• Volatility is directly related to a substance's vapor pressure and intermolecular forces.

The higher the vapor pressure of a liquid at a given temperature, the higher the volatility and the lower the normal boiling point of the liquid .


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November 07, 2012

Vapor pressure increases with increasing temperature • At 100°C the vapor pressure of water is 760 torr (1 atm) or equal to the atmospheric pressure on the liquid (in an open container)

• At this temperature, interior bubbles will not collapse and the water boils

• At high altitudes (i.e. up in the Mountains) the air pressure is less than at sea level. Thus, water will boil at a lower temperature (the vapor pressure needed to support a bubble is lower at high altitude). Therefore, cooking times are longer for things that need to be boiled (e.g. boiled eggs take longer to cook at high altitudes).


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November 07, 2012

Equilibrium can exist not only between the liquid and vapor phase of a substance but also between the solid and liquid phases, and the solid and gas phases of a substance.

A phase diagram is a graphical way to depict the effects of pressure and temperature on the phase of a substance:

Can an equilibrium exist between solid liquid and gas at the same time?


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November 07, 2012

Triple Point Phase Diagram is a way to look at the effects of pressure and temperature on the states of matter.


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November 07, 2012

The curves indicate the conditions of temperature and pressure under which equilibrium between different phases of a substance can exist

• The vapor pressure curve is the border between the liquid and gaseous states of the substance • For a given temperature, it tells us the vapor pressure of the substance

• The vapor pressure curve ends at the critical point . The temperature above which the gas cannot be liquefied no matter how much pressure is applied (the kinetic energy simply is too great for attractive forces to overcome, regardless of the applied

pressure)

• The line between the gas and solid phase indicates the vapor pressure of the solid as it sublimes at different temperatures

• The line between the solid and liquid phases indicates the melting temperature of the solid as a function of pressure

• For most substances the solid is denser than the liquid this can be seen by looking at the slope of the line between solid and liquid states.

• An increase in pressure usually favors the more dense solid phase

• Usually higher temperatures are required to melt the solid phase at higher pressures

• The "triple point" is the particular condition of temperature and pressure where all three physical states are in equilibrium

• Regions not on a line represent conditions of temperature and pressure where only one particular phase is present

• Gases are most likely under conditions of high temperature

• Solids are most likely under conditions of high pressure


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November 07, 2012

A + B > C + 20kJ (exothermic)

C + 20kJ >A + B (endothermic)Enthalpy


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November 07, 2012

• The frozen state of water (ice) is actually less dense than the liquid state, thus, the liquid state is more compact than the solid state

• Increasing pressure, which will favor compactness of the molecules, will thus favor the liquid state Increasing pressure will thus lower the temperature at which

the solid will melt

• The melting curve slopes to the left, unlike most compounds

• At 100 °C the vapor pressure of water is 760 torr or 1 atm, thus at this temperature water will boil if it is at 1 atm of pressure

• At pressures below 4.58 torr, water will be present as either a gas or solid, there can be no liquid phase


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November 07, 2012

Above a certain temperature, the critical temperature, and above a certain pressure (the critical pressure), the boundary between gas and liquid disappears. Above this point, called the critical point, the substance acts like a gas in some ways and like a liquid in some ways.

The substance is now is a fourth state called the supercritical state.

The supercritical state is important to polymers because carbon dioxide in the supercritical state is a useful solvent in which to carry out certain polymerization reactions.

liquids · •evaporation happens when atoms or molecules escape from the liquid and turn into a...

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