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AQA Chemistry A-level 3.1.8: Born-Haber Cycles Notes
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AQA Chemistry A-level 3.1.8: Born-Haber Cycles Notes

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Questions and Answers

Why do gases generally have greater entropy compared to solids and liquids?

  • Gases have higher density
  • Gases are more ordered
  • Gases have lower energy content
  • Gases have particles that are further apart (correct)
  • What does entropy measure in chemistry?

  • Temperature
  • Disorder (correct)
  • Particle size
  • Energy content
  • What happens to entropy as temperature increases?

  • Entropy decreases
  • Entropy increases (correct)
  • Entropy becomes negative
  • Entropy remains constant
  • What change in entropy occurs when a substance melts or evaporates?

    <p>Sudden increase in entropy</p> Signup and view all the answers

    What kind of entropy value do spontaneous reactions possess?

    <p>Positive entropy value</p> Signup and view all the answers

    How does Gibbs Free-Energy help determine entropy without measuring effects on surroundings?

    <p>By allowing determination of ∆S values directly</p> Signup and view all the answers

    What is lattice dissociation enthalpy?

    <p>The energy required to break apart an ionic lattice into its constituent ions in a gaseous state</p> Signup and view all the answers

    What is the definition of atomisation enthalpy?

    <p>The energy required for the formation of a mole of gaseous atoms</p> Signup and view all the answers

    What does the enthalpy of electron affinity measure?

    <p>Enthalpy change when adding electrons to atoms</p> Signup and view all the answers

    What type of substances are lattice enthalpies used for?

    <p>Ionic substances</p> Signup and view all the answers

    How are Born-Haber cycles related to Hess's Law?

    <p>They represent reactions as vectors</p> Signup and view all the answers

    Which term describes the energy required to form an ionic lattice from its constituent ions?

    <p>Lattice formation enthalpy</p> Signup and view all the answers

    What happens to the sign of the enthalpy change if the cycle counters the direction of the arrow?

    <p>It becomes negative</p> Signup and view all the answers

    How is enthalpy of solution defined?

    <p>The enthalpy change when an ionic solid is dissolved in water to infinite dilution</p> Signup and view all the answers

    What is enthalpy of hydration defined as?

    <p>The enthalpy change when gaseous ions are dissolved in water</p> Signup and view all the answers

    Why is enthalpy of hydration nearly always negative?

    <p>Because water has a positive region that attracts negative ions</p> Signup and view all the answers

    In the perfect ionic model, what assumption is made about the ions?

    <p>They are perfectly spherical and show no covalent character</p> Signup and view all the answers

    How are enthalpies of solution and hydration combined to form an energy cycle?

    <p>Through a similar idea to Hess's Law</p> Signup and view all the answers

    Study Notes

    Entropy and Thermodynamics

    • Gases generally have greater entropy compared to solids and liquids due to the higher disorder and randomness of their molecular arrangements.
    • Entropy measures the disorder or randomness of a system in chemistry, typically denoted as 'S' and measured in units of J/K.
    • As temperature increases, entropy also increases, indicating a greater degree of disorder or randomness in the system.
    • When a substance melts or evaporates, its entropy increases due to the increased disorder of its molecular arrangement.

    Thermodynamic Properties

    • Spontaneous reactions possess a negative entropy change, indicating an increase in disorder or randomness.
    • Gibbs Free-Energy (ΔG) helps determine entropy without measuring effects on surroundings by considering the energy available to do work.

    Enthalpy and Lattice Energy

    • Lattice dissociation enthalpy is the energy required to separate one mole of an ionic solid into its constituent gaseous ions.
    • Atomisation enthalpy is the energy required to completely break down a mole of a substance into its individual atoms.
    • Enthalpy of electron affinity measures the energy change when an electron is added to an atom or molecule.
    • Lattice enthalpies are used for ionic substances, which involve the energy required to form an ionic lattice from its constituent ions.

    Thermochemical Cycles

    • Born-Haber cycles are related to Hess's Law, which states that the total enthalpy change in a reaction is equal to the sum of the enthalpy changes of the individual steps.
    • The energy required to form an ionic lattice from its constituent ions is described by the lattice enthalpy.
    • If the cycle counters the direction of the arrow, the sign of the enthalpy change is reversed.

    Enthalpy of Solution and Hydration

    • Enthalpy of solution is the energy change when one mole of a substance dissolves in a solvent.
    • Enthalpy of hydration is the energy change when a mole of a substance is surrounded by water molecules, typically resulting in a negative value due to the stabilization of the solute.
    • Enthalpies of solution and hydration are combined to form an energy cycle, allowing the calculation of lattice enthalpies.

    Ionic Model Assumptions

    • In the perfect ionic model, it is assumed that the ions interact only through electrostatic forces, ignoring any covalent character or van der Waals forces.

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    Explore detailed notes on Born-Haber cycles in AQA Chemistry A-level 3.1.8, which are essential for determining enthalpy changes in ionic substances. Learn how to use Born-Haber cycles similar to Hess's Law cycles to calculate lattice enthalpies.

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