Energetics and kinetics flashcards

Energetics covers how energy changes during chemical reactions, focusing on enthalpy changes measured under standard conditions. Students must understand exothermic and endothermic reactions, Hess's law, bond enthalpies, and how to calculate enthalpy changes from calorimetry data. Lattice enthalpies and Born-Haber cycles extend this to ionic compounds. Kinetics examines how quickly reactions occur, linking rate to collision frequency and activation energy. The Maxwell-Boltzmann distribution explains how temperature affects the proportion of particles with sufficient energy to react. Catalysts provide an alternative reaction pathway with a lower activation energy. Students must also understand how concentration, pressure, surface area, and temperature influence reaction rate, and be able to interpret rate equations, rate constants, and reaction orders from experimental data.

1. Card 1

   What is the definition of standard enthalpy change of formation?

   The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions (298 K, 100 kPa).

2. Card 2

   State Hess's law.

   The total enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same. This is a consequence of conservation of energy.

3. Card 3

   How do you calculate enthalpy change using average bond enthalpies?

   Enthalpy change = sum of bond enthalpies broken minus sum of bond enthalpies formed. Breaking bonds is endothermic; forming bonds is exothermic.

4. Card 4

   What does a negative enthalpy change tell you about a reaction?

   A negative enthalpy change means the reaction is exothermic; energy is released to the surroundings and the products are at a lower enthalpy than the reactants.

5. Card 5

   What is activation energy?

   Activation energy is the minimum energy that colliding particles must possess for a reaction to occur, i.e. to overcome the energy barrier and form products.

6. Card 6

   How does a catalyst increase reaction rate?

   A catalyst provides an alternative reaction pathway with a lower activation energy, so a greater proportion of particles have sufficient energy to react at a given temperature.

7. Card 7

   What does the Maxwell-Boltzmann distribution show?

   It shows the spread of molecular energies in a gas sample. The area under the curve beyond the activation energy represents the fraction of particles able to react.

8. Card 8

   What is the rate equation and what does the order of reaction mean?

   Rate = k[A]^m[B]^n. The order with respect to each reactant shows how rate depends on that concentration; overall order is m + n. k is the rate constant.

9. Card 9

   How does increasing temperature affect the rate constant k?

   Increasing temperature increases k because more particles exceed the activation energy. The relationship is described by the Arrhenius equation: k = Ae^(-Ea/RT).

10. Card 10

    What is lattice enthalpy and what does its magnitude indicate?

    Lattice enthalpy is the enthalpy change when one mole of an ionic lattice is formed from gaseous ions. A larger magnitude indicates stronger ionic attractions in the lattice.

11. Card 11

    What is a Born-Haber cycle used for?

    A Born-Haber cycle applies Hess's law to ionic compounds, allowing calculation of lattice enthalpy indirectly from other measurable enthalpy changes.

12. Card 12

    How do you determine the order of reaction from experimental data?

    Compare experiments where one concentration changes while others are fixed. If doubling concentration doubles rate, order is first order; if rate quadruples, it is second order.

• Students often confuse bond breaking (endothermic) with bond forming (exothermic) when calculating enthalpy changes from bond enthalpies, leading to a sign error in the final answer.
• A common error is assuming that a faster reaction always has a larger rate constant, forgetting that rate also depends on concentration and that k only changes with temperature or the presence of a catalyst.
• Many students incorrectly state that a catalyst is consumed in a reaction or that it lowers the activation energy of only the forward reaction, when in fact it lowers the activation energy of both the forward and reverse reactions equally.

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