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Ch 12 Chem 1 Notes

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    Chapter 12: Phase Changes and Equilibrium

    Grade Level Indicator

    Describe how a physical, chemical, or ecological system in

    equilibrium may return to the same state of equilibrium if the

    disturbances it experiences are small. Large disturbances may

    cause it to escape equilibrium and eventually settle into some

    other state of equilibrium.

    Essential Question

    How do various factors combine to cause a substance to be a given

    phase of matter?

    I can...

    list and define all phase changes (boiling, freezing, etc.)

    relate phase changes to energy transfers and attractive forces

    explain how the concept of dynamic equilibrium relates to phase

    changes

    discuss the relationship between temperature, vapor pressure,

    and the boiling point of a substance

    analyze heating and cooling curves and use them to determine

    the amount of energy associated with phase change

    compare molar heat of fusion and molar heat of vaporization,

    and explain the difference in their magnitudes

    analyze and interpret phase diagrams

    Key Words

    equilibrium dynamic equilibrium

    phase condensation

    evaporation boiling

    freezing sublimation

    deposition Le Chateliers principle

    vapor pressure volatile

    nonvolatile freezing point

    boiling point molar heat of vaporization

    molar heat of fusion phase diagram

    triple point critical point

    critical temperature critical pressure

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    Chapters 12: Phase Changes and Equilibrium

    Section 1: Mathematics of Phase Changes

    Different phases exist because of the relationship between a substances temperature,

    pressure, and intermolecular forces

    Every phase change is accompanied by a change in energy

    Energy is either released in the process(exothermic), or absorbed in theprocess(endothermic)

    The energy change associate with these processes can be thought of in both thequalitative and quantitative sense

    Qualitatively, this can be determined by applying a little logic

    SAMPLE EXERCISE: For each of the following processes, determine if the process isendothermic or exothermic

    a) H2O(s) H2O(l) _________________________

    b) H2O(g) H2O(l) _________________________

    c) CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g) _________________________

    Quantitatively, the energy change associated with a phase change is found using the

    molar heat of fusion and the molar heat of vaporization

    The molar heat of fusion is the amount of heat energy(in kJ) that must be absorbed tomelt 1 mole of a substance, or the amount of heat released when 1 mole of a substance

    freezes

    The molar heat of vaporization is the amount of heat energy(in kJ) that must beabsorbed to vaporize(evaporate) 1 mole of a substance, or the amount of heat released

    when 1 mole of a substance condenses

    The three important points to remember with molar heats of fusion and vaporization

    are:

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    1) Heat of fusion is used with melting and freezing; heat of vaporization is used with

    evaporating and condensing

    2) Each substance has its own heat of fusion and heat of vaporization

    The heat of fusion of water is different than the heat of fusion of pudding

    3) The heats of fusion and vaporization and fusion are the number of kJ for every one

    mole of the substance: if there is more or less than 1 mole, this must be taken intoaccount

    SAMPLE EXERCISE 1: What quantity of energy is needed to melt 500.0 grams of

    ice, H2O(l)?

    SAMPLE EXERCISE 2: How much heat is given off when 37.0 g of C6H6(g) isconverted to C6H6(l)?

    Substance Heat of Fusion Heat of Vaporization

    Water, H2O 6.02 kJ/mol 40.7 kJ/mol

    Butane, C6H6 5.0 kJ/mol 24.0 kJ/mol

    Ethanol, C2H5OH 5.02 kJ/mol 38.56 kJ/mol

    Oxygen, O2 0.22 kJ/mol 3.41 kJ/mol

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    Chapters 12: Phase Changes and Equilibrium

    Section 2: Phase Changes

    The phase of a sample of matter(solid, liquid, gas) depends in part on the relationship

    between kinetic energy

    What happens to a sample of H2O when the temperature reaches 0C(at a pressure of 1

    atm)?

    What happens to a sample of H2O when the temperature reaches 100C(at a pressure of

    1 atm)?

    From this, it is seen that at the temperature of the phase change, the two phases exist in

    dynamicequilibrium

    Dynamic equilibrium exists when two opposite processes occur at identical rates

    At a constant temperature(and pressure of 1 atm)) of 0C, H2O is melting and freezing

    at the same rate

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    H2O(s) + heat energy H2O(l)

    H2O(l) H2O(l) + heat energy

    So while ice is constantly melting, and water is constantly freezing, the amount of eachremains constant

    H2O(s) + heat energy H2O(l) At a constant temperature(and pressure of 1 atm) of 100C, H2O is evaporating and

    condensing at the same rate

    H2O(l) + heat energy H2O(g)

    H2O(g) H2O(l) + heat energy

    So while water is constantly evaporating, and steam is constantly condensing, the

    amount of each remains constant

    H2O(l) + heat energy H2O(g)

    Equilibrium is a part of all virtually every chemical process; at some point in a reaction,a dynamic equilibrium is reached in which the rate of the forward process equals the rate

    of the reverse reaction

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    Chapters 12: Phase Changes and Equilibrium

    Section 3: Mathematics of Phase Changes II

    This gives a way to calculate the amount of energy involved in, for example, themelting of 25.00 g of ice, H2O(s)

    But think about what would need to happen if this sample of ice is initially at atemperature of -10.00C

    Would the energy initially added to the ice cause the ice to melt?

    Therefore, to calculate the energy needed to melt 25.00 grams of ice that had an initial

    temperature of -10.00C, it is also necessary to calculate the amount of energy needed toget the ice to its melting point at 0.00C

    Unless told otherwise, it is always assumed that the temperature for a phase change isthe normal(at 1 atm) temperature for that phase change

    To help organize and plan the work for these questions, heating curves and cooling

    curves are very helpful

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    To see how this works, consider the amount of energy needed to take 25.00 grams of

    ice, H2O(s) and melt it all. That is, convert it all to H2O(l).

    H2O(s) H2O(l)

    Heat of Vaporization, H2O: 40.67 kJ/mol

    Heat of Fusion, H2O : 6.008 kJ/mol

    Specific Heat, H2O(s) : 2.092 J/gK

    Specific Heat, H2O(l) : 4.184 J/gK

    Specific Heat, H2O(g) : 1.841 J/gK

    Note the specific heat(c) also depends on the phase of a substance

    SAMPLE EXERCISE 1: Calculate the enthalpy change upon converting

    18.02 grams of ice at -25.0C to water vapor(steam) at 125C undera constant pressure of 1 atm.

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    56.0 kJ or 56,000 J

    SAMPLE EXERCISE 2 : What is the energy change during the process in

    which 100.0 g of water at 50.0C is cooled to ice at -30.0C? Is

    this an endothermic or exothermic process?

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    60.6 kJ or 60,600 J

    Chapters 12: Phase Changes and Equilibrium

    Section 4: Vapor Pressure and Vapor Pressure Diagrams

    Imagine, if you will, a sealed bottle of water: what are the H2O(l) molecules at the

    surface of the liquid constantly doing?

    How will this cause the pressure inside the bottle do change?

    When this dynamic equilibrium has been reached, the constant pressure inside the bottleis known as the equilibrium vapor pressure, although it is more commonly just called

    vapor pressure

    Vapor pressure is normally defined as the pressure exerted by a vapor in equilibrium

    with its liquid or solid phase

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    In the chart above, the curves represent the vapor pressures of variousliquids---------------------------------------------------------------------------------------------------

    -------------------------------------------------------------------------

    Answer the following questions based on the vapor pressure diagram above:

    1) What is the relationship between temperature and vapor pressure?

    2) What is the relationship between atmospheric pressure and the vapor pressure of the

    liquid at the liquids boiling point?

    3) What is the normal boiling point(boiling point at 1 atm) for diethyl ether? For ethyl

    alcohol?

    4) Which of these three substances has the strongest intermolecular forces? Which hasthe weakest? Middle?

    Chapters 12: Phase Changes and Equilibrium

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    Section 5: Phase Diagrams

    Shown above is the phase diagram for water; this shows the characteristic appearance of

    a phase diagram

    All phase diagrams have essentially the same appearance, with only a few smalldifferences

    In looking at the graph, what conditions determine the phase(solid, liquid, or gas) of a

    substance?

    So when it is said water boils at 100C, is this always true?

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    Notice the diagram is separated into three regions; each of these regions represents one

    of the phases of matter

    Which region goes with which phase?

    The same phase goes with the same region in every single phase diagram you will eversee

    Water at 0.5 atm and -20C would be at what phase?

    Water at 0.5 atm and 50C would be at what phase?

    So water at 0.5 atm and -20C would undergo what phase change when the pressure

    remains unchanged and the temperature increases to 50C?

    The lines between the phases regions show the pressure and temperature conditions at

    which a given phase change will occur

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    At any set of temperature and pressure data that are a point along a line, this is where a

    phase change occurs, and the two phases exist in equilibrium

    What happens where all three of these line segments come together?

    The other point to be aware of is the critical point: this point represents the criticaltemperature and the critical pressure

    Beyond the critical point, the liquid and the gas phases are indistinguishable

    Critical temperature is the highest possible temperature at which a substance can still bea liquid

    The critical pressure is the pressure at the critical temperature

    SAMPLE EXERCISE: Describe any changes in the phases present when H2O is kept at1000C while the pressure is increased from 6.0 x 10-3 atm to 10 atm.

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