Topic 15.4: Spontaneity

15.4.1 Predict whether a reaction or process will be spontaneous by using the sign of ΔG.

Gibb's free energy simply states whether a process would be spontaneous or not. So if ΔG is a negative, then the process will be spontaneous.

15.4.2 Calculate ΔG for a reaction using the equation ΔG = ΔH - TΔS and by using values of the standard free energy change of formation, ΔG

Use the information given to calculate the change in Gibb's free energy of formation. All of these values are performed at standard conditions.

15.4.3 Predict the effect of a change in temperature on the spontaneity of a reaction using standard entropy and enthalpy changes and the equation  ΔG = ΔH - TΔS

It is easy to instantly tell whether a reaction is spontaneous or not simply by looking at the positive and negative symbols. If enthalpy change is positive while entropy is negative, then the reaction is non-spontaneous and vice versa.  

Topic 15.3: Entropy

15.3.1 State and explain the factors that increase the entropy in a system

The degree of disorder of a system is quantified by its entropy (S)

Two factors that affect the disorder of a system:

• State of matter

Solid don't move around as much as a gas particle.

• Number of moles

More particles, more chaos (More people at a party, More chaos and fun)

(After IB)

15.3.2 Predict whether the entropy change for a given reaction or process is positive or negative

• First, you check how many moles of molecules. In this case both sides of the equation are equal.
• Second, check the state of matter. The right hand side has a solid instead of an aqueous. Thus we could conclude that this reaction has a lowering entropy value.

Thus, in this equation, entropy value is decreasing.

15.3.3 Calculate the standard entropy change for a reaction using standard entropy values.

This is the equation for entropy change.

It is important to note that all entropy values are positive. With the entropy values increase in the order of solid < liquid < gas. As entropy depends on the temperature and pressure, tabulated entropy values refer to standard conditions.

Table 11 of the data booklet has a list of values for organic compounds.

Topic 15.2: Born-Haber cycle

15.2.1 Define and apply the terms lattice enthalpy and electron affinity

The first electron affinity is the enthalpy change when one mole of gaseous atoms attracts one mole of electrons. Values are shown in Table 7 of the IB data booklet.

The lattice enthalpy is the enthalpy change that occurs when one mole of a solid ionic compound is separated into gaseous ions under standard conditions


15.2.2 Explain how the relative sizes and the charges of ions affect the lattice enthalpies of different ionic compounds.

The ionic model assumes that the only interaction is due to the electrostatic forces between the ions. The energy needed to separate the ions depends the product of the ionic charges and the sum of the ionic radii

• An increase in the ionic radius of one of the ions decreases the attraction between the ions
• An increase in the ionic charge increases the ionic attraction between the ions.

15.2.3 Construct a Born-Haber cycle for group 1 and 2 oxides and chlorides, and use it to calculate an enthalpy change

A Born-Haber cycle is a larger and more complex version of the Hess's law. It uses two paths to calculate the bond enthalpy of a part of the cycle.

Route 1:

Step 1: Enthalpy of Formation

Route 2:

Step 1: Enthalpy change of atomization
Step 2: First Ionization Energy
Step 3: Half of Average Bond Enthalpy (The energy of one chlorine)
Step 4: Electron Affinity
Step 5: Lattice Enthalpy

In the end, both paths form the same substance. Thus, using calculations to find the true value of lattice enthalpy.


15.2.4 Discuss the difference between theoretical and experimental lattice enthalpy values of ionic compounds in terms of their covalent character

The bonding of sodium iodide is stronger than expected from a simple ionic model because the large and "squishy" iodide ion is distorted or polarized by the smaller sodium ion. This gives the compound some covalent character, which provides an additional contribution to the bonding. A covalent bond can be considered an extreme case of distortion, with the negative ion so polarized that we can consider two of the electrons from the iodide ion as shared with the "positive ion".

Topic 15.1: Standard enthalpy changes of reaction

15.1.1 Define and apply the terms standard state, standard enthalpy change of formation and standard enthalpy change of combustion.

The standard state of a substance is the pure form of the substance under standard conditions of 298 kelvins and 1.00 10^5 Pa.

Standard enthalpy change of formation is the energy change on the formation of one mole of substance from its constituent elements in their standard states.

• It gives a measure of the stability of a substance relative to its elements
• It can also be used to calculate the enthalpy changes of all reactions, either hypothetical or real

Standard enthalpy change of combustion is the heat evolved on the complete combustion of one mole of a substance.

• This data can be used to calculate the enthalpy change of all reaction
• They can also be used more directly to compare the heat output of different fuels.

15.1.2 Determine the enthalpy change of a reaction using standard enthalpy changes of formation and combustion.

Standard enthalpy changes of combustion

Using the Hess's law, we could find Hf by using the following equation. 

Hf = Total combustion of reactants - Total combustion of products

Standard enthalpy change of formation

Using the Hess's law, we could find Hf by using the following equation.

Topic 15: Energetics

Topic 15 of the IB HL Chemistry syllabus is the Periodicity. IBO recommends to spend 8 hours on this topic.

This topic has 4 sub-chapters: "Standard enthalpy changes of reaction", "Born-Haber cycle", "Entropy" and "Spontaneity". Each are separated with numerical values in order of mentioned.

These are advanced HL syllabus statements, it is recommended to bring a Casio Graphical Calculator instead of Texas. Casio Calculators have the periodic table installed already.