Ch 10 Section 4 Flashcards
some liquid chemical substances have odors that are easily detected because some molecules at the upper surface of the liquid have enough energy to
overcome the attraction of neighboring molecules
a phase is any part of a system that has
uniform composition and properties
condensation is the process by which a
gas changes to a liquid
a gas in contact with its liquid or solid phase is often called a
vapor
if the temperature of the liquid remains constant and the cap remains closed, the rate at which molecules move from the liquid phase to the vapor phase remains
constant
near the beginning of the evaporation process, very few molecules are in the
gas phase, so the rate of condensation is very low
but as more liquid evaporates, the increasing number of gas molecules causes the
rate of condensation to increase
eventually, the rate of condensation equals the rate of evaporation, and a state of
equilibrium is established
equilibrium is a dynamic condition in which two opposing changes occur at
equal rates in a closed system
even though molecules are constantly moving between liquid and gas phases, there is no
net change in the amount of substance in either phase
vapor molecules in equilibrium with a liquid in a closed system exert a pressure
proportional to the concentration of molecules in the vapor phase
the equilibrium vapor pressure of the liquid is the pressure exerted by a vapor in
equilibrium with its corresponding liquid
the increase in equilibrium vapor pressure with increasing temperature can be explained in terms of the
kinetic-molecular theory for the liquid and gaseous states
increasing the temperature of a liquid increases the
average kinetic energy of the liquid’s molecules
this energy change increases the number of molecules that have enough energy to
escape from the liquid phase into the vapor phase
the resulting increased evaporation rate increases the number of
molecules in the vapor phase, which in turn increases the equilibrium vapor pressure
because all liquids have characteristic forces of attraction between their particles, every liquid has a specific
equilibrium vapor pressure at a given temperature
the stronger these attractive forces are, the smaller the percentage of liquid particles that can
evaporate at any given temperature is
a low percentage of evaporation results in a low
equilibrium vapor pressure
volatile liquids are liquids that
evaporate readily
volatile liquids have relatively weak forces of
attraction between their particles
ether is a typical
volatile liquid
nonvolatile liquids do not
evaporate readily
nonvolatile liquids have relatively strong attractive
forces between their particles (e.g. molten ionic compounds)
equilibrium vapor pressures can be used to explain and defined the concept of
boiling
boiling is the conversion of a liquid to a
vapor within the liquid as well as at its surface
if the temperature of the liquid is increased, the equilibrium vapor pressure
also increases
the boiling point of a liquid is the temperature at which the equilibrium vapor pressure of the liquid
equals the atmospheric pressure
the lower the atmospheric pressure is, the lower the
boiling point is
at the boiling point, all of the energy absorbed is used to evaporate the liquid, and the temperature remains
constant as long as the pressure does not change
if the pressure above the liquid being heated is increased, the temperature of the liquid will rise until the vapor pressure equals the
new pressure and the liquid boils once again→principle behind pressure cooker
the pressure cooker is sealed so that the steam pressure builds up over the surface of the boiling water, the pressure increases the boiling temp of h20, resulting in
shorter cooking time
vacuum evaporator causes boiling at lower-than-normal
temperatures
vacuum evaporators remove water from milk & sugar solutions. under reduced pressure, the water boils away at a temperature low enough to
avoid scorching milk or sugar
at normal atmospheric pressure, the boiling pt of water is
exactly 100 degrees celsius→normal boiling pt
the normal boiling pt of each liquid is the temperature at which the liquid’s equilibrium vapor pressure equals
760 torr
energy must be added continuously in order to keep a liquid
boiling
if you were to carefully measure the temperature of a boiling liquid and its vapor you would find that they are at the
same constant temperature
the temperature at the boiling point remains constant despite the
continuous addition of energy
the added energy is used to overcome the attractive forces between molecules of the liquid during the liquid-to-gas change and is
stored in the vapor as potential energy
a liquid’s molar enthalpy of vaporization is the amount of energy as heat that is needed to… one mole of liquid at the liquid’s…
vaporize; boiling point at constant pressure
the magnitude of the molar enthalpy of vaporization is a measure of the
attraction between particles of the liquid
the stronger this attraction (between particles of a liquid) is, the more energy that is required to overcome it, which results in a
higher molar enthalpy of vaporization
each liquid has a characteristic
molar enthalpy of vaporization
water has an unusually high molar enthalpy of vaporization due to the extensive
hydrogen bonding in liquid water
the high molar enthalpy of vaporization of water makes water a very effective
cooling agent
at the higher temperature (distribution of kinetic energies of molecules in a liquid at 2 different temps), a greater portion of the molecules have the kinetic energy required to
escape from the liquid surface and become vapor
the physical change of a liquid to a solid is called
freezing
freezing involves a loss of energy in the form of …by the liquid and can be represented by the following reaction:
heat; liquid→solid + energy
the normal freezing point is the temperature at which the solid and liquid are in
equilibrium at 1 atm pressure
at the freezing point, particles of the liquid and the solid have the same
average kinetic energy
energy loss during freezing is a loss of
potential energy that was present in the liquid
at the same time energy decreases, there is a significant increase in particle order because the solid state of a substance is much more
ordered than the liquid state, even at the same temp
melting, also occurs at
constant temperature
as a solid melts, it continuously absorbs energy as heat, represented by following equation:
solid + energy→liquid
for pure crystalline solids, the melting point and freezing point are
the same
at equilibrium, melting and freezing proceed at
equal rates
the following general equilibrium equation can represent these states:
solid + energy ⇄ liquid
the temperature of a system containing ice and liquid water will remain at 0 degrees celsius as long as
both ice and water are present
the temperature of a system containing ice and liquid water will persist no matter what the
surrounding temperature
adding energy in the form of heat to such a system (liquid water and ice) shifts the equilibrium to the
right
that shift increases the proportion of liquid water and decreases that of
ice
only after all the ice has melted will the addition of energy increase the
temperature of the system
a solid’s molar enthalpy of fusion is the amount of energy as heat required to …one mole of solid at the solid’s
melt; melting point
the energy absorbed increases the solid’s potential energy as its particles are pulled apart, overcoming the
attractive forces holding them together
there is a significant decrease in particle order as the substance transforms from
solid to liquid
similar to molar enthalpy of vaporization, the magnitude of the molar enthalpy of fusion depends on the
attraction between the solid particles
at sufficiently low temperature and pressure conditions, a liquid
cannot exit
under such conditions ( sufficiently low temperature and pressure conditions) a solid substance exists in equilibrium with its
vapor instead of its liquid
equation for solid existing in equilibrium with vapor:
solid + energy ⇄vapor
sublimation is the change of state from a solid
directly to a gas
deposition is the change of state from a gas
directly to a solid
a phrase diagram is a graph of pressure versus temperature that shows the conditions under which the
phases of a substance exists
a phase diagram reveals how the states of a system change with
changing temperature or pressure
in water phase diagram, curve ac indicates the temperature and pressure conditions at which ice and water vapor can
coexist at equilibrium
in water phase diagram, curve AD indicates the temperature and pressure conditions at which ice and liquid water
coexist at equilibrium
because ice is less dense than liquid water, an increase in pressure lowers the
melting point
the triple point of a substance indicates the temperature and pressure conditions at which the solid, liquid, and vapor of the substance can
coexist at equilibrium
the critical point of a substance indicates the
critical temperature and critical pressure
the critical temperature is the temperature above which the substance cannot
exist in the lqiuid state
critical temperature of water is
373.99 degrees celsius
above water’s critical temperature, water cannot be liquefied, no matter how much
pressure is applied
the critical pressure is the lowest pressure at which the substance can exist as a
liquid at the critical temperature
the critical pressure of water is
217.75 atm
in phase diagram of water , slope of line ad shows that ice melts at a higher temperature with
decreasing pressure
below the triple point, the temperature of sublimation decreases with
decreasing pressure
