Chemistry Chapter 12: Freezing Point Depression
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Questions and Answers

What happens to the freezing point of a solution compared to the pure solvent?

  • It is higher than the solvent's freezing point.
  • It remains unchanged.
  • It is lower than the solvent's freezing point. (correct)
  • It rises to the boiling point of the solvent.

    • Which factor affects the value of Kf in the freezing point depression equation?

  • The concentration of the solute only.
  • The size of the solute particles.
  • The type of solvent used. (correct)
  • The temperature at which the solution is measured.

    • What is the role of Le Chatelier’s principle in freezing point depression?

  • It explains why solutes increase freezing point.
  • It determines the initial temperature of the solvent.
  • It dictates the concentrations of solute in the solution.
  • It accounts for shifts in equilibrium when solutes are added. (correct)

    • What is the freezing point depression of a solution directly proportional to?

    <p>The molality of the solute.</p> Signup and view all the answers

    Why is calcium chloride preferred over sodium chloride for lowering freezing points in certain conditions?

    <p>It can depress the freezing point more effectively due to more ions produced.</p> Signup and view all the answers

    What temperature maximum is expected for freezing point depression when using NaCl?

    <p>18°C</p> Signup and view all the answers

    What occurs at the freezing point of a pure liquid?

    <p>A dynamic equilibrium is established between melting and freezing.</p> Signup and view all the answers

    In the equation ΔTsoln = Kf m, what does ΔTsoln represent?

    <p>The change in freezing point.</p> Signup and view all the answers

    What occurs to the phases of a substance above the critical point?

    <p>The liquid and gas phases become indistinguishable.</p> Signup and view all the answers

    In one-component systems, what does the phase rule simplify to when only one phase is present?

    <p>F = 3 - P</p> Signup and view all the answers

    Which component is NOT a variable in determining the degrees of freedom in a phase diagram?

    <p>Volume</p> Signup and view all the answers

    According to Raoult's law, which of the following statements is accurate regarding the partial vapor pressure of a substance?

    <p>It is proportional to its mole fraction and its vapor pressure when pure.</p> Signup and view all the answers

    In a two-component system at equilibrium, what happens to the degrees of freedom when pressure is constant?

    <p>F = 3</p> Signup and view all the answers

    What is the defining characteristic of a supercritical fluid?

    <p>It can be compressed like a gas yet has liquid-like density.</p> Signup and view all the answers

    What is the definition of molarity?

    <p>The number of moles of solute per liter of solution.</p> Signup and view all the answers

    What does the variable 'C' represent in the phase rule equation?

    <p>Number of components present in the system</p> Signup and view all the answers

    When three phases are in equilibrium, what does the phase rule indicate about the degrees of freedom?

    <p>F = 0</p> Signup and view all the answers

    Which of the following accurately describes an azeotrope?

    <p>A mixture of two liquids with a constant boiling point and composition.</p> Signup and view all the answers

    What does the partition coefficient (KD) measure?

    <p>The ratio of concentrations of a solute in two different phases.</p> Signup and view all the answers

    In which situation would solvent-solvent extraction be most effectively employed?

    <p>When the solute can exist in both liquid phases without reacting.</p> Signup and view all the answers

    How is molality defined?

    <p>The number of moles of solute per kilogram of solvent.</p> Signup and view all the answers

    What primarily dictates the colligative properties of a solution?

    <p>The number of solute particles</p> Signup and view all the answers

    What is the correct formula for calculating molality?

    <p>moles of solute/1 kg of solvent</p> Signup and view all the answers

    Which process is NOT applicable for separating immiscible liquids?

    <p>Distillation</p> Signup and view all the answers

    When extracting Compound X using toluene, what mass of compound X was recovered in the organic phase?

    <p>0.889 g</p> Signup and view all the answers

    If the vapor pressure above a solution is lower than that of the pure solvent, which of the following is true?

    <p>The solute molecules block solvent molecules from escaping.</p> Signup and view all the answers

    What does a low vapor pressure of a liquid indicate about its intermolecular forces?

    <p>They are very strong, making molecules escape easily.</p> Signup and view all the answers

    Which colligative property would most likely increase with the addition of nonvolatile solute?

    <p>Boiling point</p> Signup and view all the answers

    At the triple point, what phases can coexist?

    <p>Solid, liquid, and gas.</p> Signup and view all the answers

    Under ideal conditions, what is the expected change in enthalpy when an ideal solute is added to a solvent?

    <p>Enthalpy remains unchanged</p> Signup and view all the answers

    Which liquid will boil at a lower temperature if both are at the same external pressure?

    <p>The liquid with a higher vapor pressure.</p> Signup and view all the answers

    What happens to the vapor composition when a more volatile liquid is boiled?

    <p>The vapor will be richer in the more volatile component.</p> Signup and view all the answers

    When adding 20.0 g of Br2 to 40.0 g of CH2Cl2, which of the following is necessary for calculating molality?

    <p>Mass of the solvent in grams</p> Signup and view all the answers

    Why do liquids with higher vapor pressures require less heat to boil?

    <p>They have weaker intermolecular forces, allowing easier molecule escape.</p> Signup and view all the answers

    Which of the following describes osmotic pressure in relation to colligative properties?

    <p>It increases with the addition of solute particles.</p> Signup and view all the answers

    Which statement accurately defines a characteristic of ideal solutes?

    <p>They introduce disorder without affecting enthalpy.</p> Signup and view all the answers

    What does the boiling point of a liquid depend on?

    <p>The vapor pressure relative to the external pressure.</p> Signup and view all the answers

    How can the composition of vapor over boiling liquid be analyzed?

    <p>By condensing the vapor and analyzing its components.</p> Signup and view all the answers

    What would happen if a liquid with low vapor pressure is subjected to an increase in temperature?

    <p>It would require significantly more heat to reach its boiling point.</p> Signup and view all the answers

    When analyzing a liquid mixture's phase diagram, what can you infer about the composition of vapor above the boiling liquid?

    <p>It is richer in the more volatile component than the initial mixture.</p> Signup and view all the answers

    In Raoult's law, what condition must be met for the vapour pressure of a solvent to be described accurately?

    <p>The solvent must be nearly pure</p> Signup and view all the answers

    Which of the following pairs of liquids is commonly cited as closely resembling an ideal mixture?

    <p>Hexane and heptane</p> Signup and view all the answers

    What happens to the partial vapour pressure of a liquid when its mole fraction is doubled, according to Raoult's Law?

    <p>It is doubled</p> Signup and view all the answers

    When plotting the vapour pressure of two components in a mixture, how does the vapour pressure of the more volatile liquid compare to the less volatile liquid?

    <p>It is higher than the less volatile liquid at pure conditions</p> Signup and view all the answers

    The total vapour pressure of a mixture is derived from which of the following?

    <p>The sum of the vapour pressures of both components</p> Signup and view all the answers

    What occurs to the vapour pressure of a solute as its mole fraction decreases in a dilute solution?

    <p>It decreases linearly</p> Signup and view all the answers

    If a liquid has a high vapour pressure at a given temperature, what does this imply about its molecular behavior?

    <p>Its molecules escape easily from the surface</p> Signup and view all the answers

    What is the expected graphical relationship when plotting the partial vapour pressure of component A against its mole fraction in an ideal mixture?

    <p>Straight line</p> Signup and view all the answers

    Study Notes

    Course Information

    • The course is Physical Chemistry I (CHY2018).
    • Course materials are available at utechonline.utech.edu.jm and on Scoology.com (access code: 4RDC-F4TD-C9475).
    • Assessment includes:
      • Test 1 (Week 6): 15%
      • Test 2 (Week 12): 15%
      • Tutorial quizzes: 20%
      • Laboratory: 20%
      • Final examination: 30%

    Unit 1: Phase Equilibria

    • Students should be able to:
      • Review colligative properties of solutions
      • Understand the Theory of Ideal Solutions
      • Understand one-component systems (vapour pressure diagrams)
      • Understand the qualitative relationship between boiling point, latent heat of vaporisation and intermolecular forces.
      • Understand two-component systems (mixtures of two miscible liquids)
      • Apply Raoult's Law to the vapour pressures of miscible liquids
      • Understand phase diagrams, fractional distillation, azeotropic mixtures, and eutectic systems.
      • Understand partition coefficients
      • Understand immiscible solutions
    • Recommended text: Elements of Physical Chemistry (5th) by Peter Atkins

    What is Phase Equilibria?

    • The study of equilibrium between or within different states of matter (solid, liquid, gas).

    Colligative Properties of Solutions

    • Properties that depend on the concentration of solute particles but not on their identity.
    • In liquid solutions, solute molecules displace solvent molecules, reducing solvent concentration.
    • Colligative properties depend only on the ratio of solute to solvent, not the properties of either.
    • Colligative properties are independent of the nature of the solute.

    Colligative Properties of Solutions (continued)

    • 'Colligative' means 'depending on the collection'.
    • The properties are associated with changes in the entropy or disorder of the solvent.
    • The increase in disorder is independent of the identities of the species used.
    • It is dependent on the number of solute particles, not their chemical identities.
    • For example, a 0.01 mol kg-1 aqueous solution of any non-electrolyte will have the same boiling point, freezing point, and osmotic pressure.
    • Molality (m) is defined as the number of moles of solute per kilogram of solvent.

    Activity

    • Calculate the molality of a solution containing 20.0 g of Br2 in 40.0 g of CH2Cl2.

    Colligative Properties of Solutions (Characteristics)

    • Ideal solute: has no effect on the enthalpy of the solution. 
    • Ideal solute: impacts the entropy by introducing disorder.
    • Entropy increases when components mix.
    • Solute can modify solution properties.

    Colligative Properties of Solutions (List)

    • Vapor pressure depression
    • Boiling point elevation
    • Freezing point depression
    • Osmotic pressure

    Vapour Pressure Depression

    • When a non-volatile solute is added to a solvent, the vapor pressure of the solvent above the solution is lower than the vapor pressure above the pure solvent.
    • This is because solute molecules reduce the surface area available for solvent molecules to escape into the gas phase.
    • The reduction in vapor pressure is proportional to the number of solute particles.

    Vapor Pressure Depression (Raoult's Law)

    • Raoult's Law states that the vapor pressure of the solvent is directly proportional to the mole fraction of the solvent in the solution.
      • P(solvent) = X(solvent) * P°(solvent)
      • Where P°(solvent) is the vapor pressure of the pure solvent

    Vapor Pressure Depression (Important Points)

    • The chemical nature of the solute does not affect vapor pressure depression, but the number of solute species does.
    • The mole fraction of the solvent must include all the species in the solution.

    Boiling Point Elevation

    • The normal boiling point of a liquid is the point where its vapor pressure equals 1 atm.
    • When a non-volatile solute is present, the solution has a lower vapor pressure.
    • To reach 1 atm, the boiling point of the solution must be raised.
    • Boiling point elevation is calculated using Raoult's Law, expressing the amount of solute particles as a molality instead of a mole fraction.

    Normal Melting and Boiling Points

    • Normal melting and boiling points occur when pressure = 1 atm.
    • These points are found by a horizontal line drawn across the phase diagram at 1 atm pressure.

    Boiling Point Elevation (Calculation)

    • Molality is used to express solute concentration in boiling point elevation calculations because it is temperature-independent.
    • The increase in boiling point is directly proportional to the solute concentration (in molality).
    •  ΔT(solution) = K(b) m(solute)
      • K(b) is the boiling point elevation constant.

    Freezing Point Depression

    • The freezing point of a pure liquid is the temperature where molecules cluster to form a crystal lattice.
    • At the freezing point, a dynamic equilibrium exists between the rate of freezing and melting.
    • Adding a solute to a solution creates a higher concentration in the liquid phase.
    • According to Le Chatelier's principle, the system shifts towards melting to correct the concentration gradient, causing a lower freezing point for the solution compared to the pure solvent.

    Freezing Point Depression (Calculation)

    • The freezing point depression of a solution is proportional to the solute's molality.
    • ΔT(solution) = K(f) m(solute)
      • K(f) is the freezing point depression constant.

    Freezing Point Depression (Applications)

    • Used in everyday applications, such as salting roads to lower ice point.
    • Used in antifreeze for cars to prevent water freezing in winter.
    • Calcium chloride (CaCl2) can be used in colder temperatures.

    Osmotic Pressure

    • Osmosis is the movement of solvent across a semipermeable membrane from a region of low solute concentration to a region of high solute concentration.
    • Applied pressure to prevent the flow of water is osmotic pressure.
    • Osmotic pressure is the minimum pressure required to prevent the inward flow of water across a semipermeable membrane. 
    • Osmotic pressure is a measure of the tendency of a solution to take in water by osmosis.

    Osmotic Pressure (Calculation)

    • The equation for calculating osmotic pressure is
      • Π = MRT
        • Π is osmotic pressure.
        • M is molar concentration
        • R is the ideal gas constant,
        • T is temperature in Kelvin.

    Osmotic Pressure (Other important information)

    • Measurements of freezing point depression and osmotic pressure are commonly used for determining the molar mass of solutes with low water solubility(e.g.large biomolecules).
    • Vapor pressure depression and boiling point elevation are less sensitive to concentration changes

    Activity

    • Calculate the boiling point and freezing point for a 0.501 m glucose solution.

    Phase Diagram

    • Represents the phase changes of a substance as a function of temperature and pressure.
    • Solid, liquid, and gas phases are shown.
    • Important points, such as the triple point (where all three phases coexist) and the critical point (where the liquid and gas phases become indistinguishable)

    Phase Diagram (Specific Points)

    • Triple point: The temperature and pressure where all three classical phases (solid, liquid, gas) coexist simultaneously. .
    • Critical point: The point where the liquid-gas phase boundary ends, and above which there is no liquid-gas phase boundary. At this point, the critical temperature (Tc) and critical pressure (Pc) are reached.

    Phase Diagram (Specific Substances)

    • Water and carbon dioxide phase diagrams have different shapes and are not ideal.

    Phase Rule

    • Degrees of freedom (F) refer to the variables (pressure, temperature, and composition) that can be changed independenly without changing the number of phases.
    • Components (C) relate to the minimal amount of independent species needed to identify the composition of all existing phases within a system.
    • The phase rule: F = C - P + 2 (where P is the number of phases)

    Azeotropes

    • Liquid mixtures having fixed boiling points when they are heated.
    • They show a maximum/minimum in the boiling point curve.
    • Azeotropes indicate differences in interactions between molecules.

    Raoult's Law

    • The partial vapor pressure of a substance in a liquid mixture is proportional to its mole fraction in the mixture and its vapor pressure when pure
    • P(substance)=X(substance) P°(substance)
    • Where;

    Ideal Solutions

    • Hypothetical solutions in which Raoult's law holds throughout their entire composition range.

    Ideal Solutions (Important Factors)

    • Components should have comparable molecular shapes and intermolecular forces.
    • Benzene and toluene mixtures are good examples.

    Limitations of Raoult's Law

    • Ideal solutions are theoretical concepts, and all real solutions deviate from Raoult's law to some extent.

    Limitations of Raoult's Law(Deviations from Ideality)

    • Variations usually small for excess solvents; deviate more for concentrated solutes.
    • Environment unlike that of pure solute; unlikely that vapor pressure is directly related.

    Examples of Ideal Mixtures

    • Mixtures of similar substances, such as hexane and heptane, benzene and methylbenzene, and propan-1-ol and propan-2-ol.

    Vapour Pressure/Composition Diagrams

    • Partial vapor pressure of each component is proportional to its mole fraction.

    Boiling Point/Composition Diagrams

    • Boiling point at which the vapour pressure equals the external pressure
    • Liquid with higher vapor pressure boils at lower temperature
    • Mixture boiling point curve will be a single line( ideal mixtures)

    Problem

    • Calculate the partial vapor pressure of A in a mixture
    • Calculate the partial vapor pressure of B in a mixture
    • Calculate the total vapor pressure of the liquid

    Definitions

    • Colligative properties: Solution properties depending on concentration, not identity.
    • Phase diagram: Graphic representation of phase changes.
    • Triple point: Temperature/pressure where all three phases coexist.
    • Molarity: Moles of solute/litre of solution.
    • Molality, Moles of solute/kg of solvent.
    • A Azeotrope is a constant boiling point and composition mixture of two liquids
    • Raoult's Law: Vapor pressure of solvent is proportional to its mole fraction.
    • Immiscible mixtures: Components don't dissolve in each other
    • Partition coefficent: Ration of solute concentration in two immiscible phases
    • Solvent-solvent extraction: Separating immiscible mixtures by using different solvents
    • Solid-phase extraction: Separating immiscible mixtures by using different solid phases
    • Plasma: Ionized gas
    • Supercritical fluid: Substance above its critical point.
    • Superfluid: A state of matter with zero viscosity.

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    CHY2018 Unit 1 Lecture - Phase Equilibria PDF

    Description

    Test your understanding of freezing point depression and its related concepts in chemistry. This quiz covers key principles such as the effect of solutes on freezing points, Le Chatelier’s principle, and the phase rule. Prepare to explore how specific solutes like calcium chloride and sodium chloride affect solutions!

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