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.

Topic 5.4: Bond enthalpies

5.4.1 Define the term average bond enthalpy

The standard molar enthalpy change of bond dissociation is the energy change when 1 mole of bonds is broken. At its standard state at 297 degree kelvins and 1 atmospheric pressure.

This is in the data booklet.

5.4.2 Explain, in terms of average bond enthalpies, why some reactions are exothermic and others are endothermic

In an exothermic reaction, the amount of energy required to break the bonds of the reactants is less than the amount of energy released when the bonds form in the products.

This suggests that the bonds in the reactant are weaker than those in the product (and that the product is therefore more stable).

In an endothermic reaction the reverse is true. The reactants have stronger bonds than the products.

Topic 5.3: Hess's law

5.3.1 Determine the enthalpy change of a reaction that is the sum of two or three reactions with known enthalpy change

The enthalpy for a reaction depends only on the difference between the enthalpy of the products and the enthalpy of reactions. It is independent of the route by which the reaction may occur.

The enthalpy change for a reaction is the sum of the individual enthalpy changes for each step.

H1 = H2 + H3 = H4 + H5 + H6

Hess's law is particularly useful for determining the enthalpy change for a reaction that is difficult to measure directly.

It is important to realize that a reaction can be reversible and the enthalpy simply changes sign. Furthermore, take notice on the different moles. If a reaction requires two moles, simply times the reaction enthalpy by two.

These are a few examples:

Topic 5.2: Calculation of enthalpy changes

5.2.1 Calculate the heat energy change when the temperature of a pure substance is changed

The heat energy change or enthalpy change is dependent on three factors:

1. The temperature change
2. The mass that changes temperature
3. The specific heat capacity of the mass

The final product is measure in kJ (energy).

Temperature change will simply be a difference in temperature (Celsius or Kelvin)

Mass is in Kg (In aqueous solutions, it will be the mass of water which is the same as it's volume in dm^3)

C is the specific heat capacity and in an aqueous solution this will be 4.18 kJ kg^-1 K^-1

The final equation is:

To change the final equation to enthalpy change, you need the equation to divide by the number of moles of the limiting reagent.

5.2.2 Design suitable experimental procedures for measuring the heat energy changes of reactions

The enthalpy changes of reaction in solution can be calculated by carrying out the reaction in an insulated system, for example, a polystyrene cup. The heat released or absorbed by the reaction can be measured from the temperature change of the water.

This additional statement is extremely important:

Errors due to heat loss to the surroundings can be minimized by plotting the temperature rise against time and then extrapolating the graph back to estimate the temperature rise for an instantaneous reaction.

5.2.3 Calculate the enthalpy change for a reaction using experimental data on temperature changes, quantities of reactants and mass of water

Using this equation, you can plot all the information and retrieve the enthalpy change.

Enthalpy change = [(Mass of water x 4.18 kJ kg^-1 K^-1 x temperature changes) = heat change]  / [(quantites of reactants / Molar mass of reactants) = moles]


5.2.4 Evaluate the results of experiments to determine enthalpy changes

This is similar to the syllabus statement 5.2.3 except you will be given data instead. Use the formula as a guideline to find what's important in the table.

Finding the moles would most likely be in the data booklet. Double check results with data booklet as well

Topic 5.1: Exothermic and endothermic reactions

5.1.1 Define the terms exothermic reaction, endothermic reaction and standard enthalpy change of reaction.

These words are often used to describe the energy changes that take place during a chemical reaction.

Reactions that release heat energy are called exothermic reactions. These causes a rise in temperature because chemical bonds are broken

When heat energy is taken in form the surroundings by the chemicals, causing a temperature drop, this is called an endothermic reaction. Energy is required because the bonds made exceed the energy levels of the bonds broken.

The absolute enthalpy stored is very difficult to measure. Usually, the enthalpy change of reactants compared to the products is measured.

If this is carried out in the lab at atmospheric pressure (101 kPa) and 298 degree kelvins, then this is called the standard enthalpy change of reaction.

A release of energy is better because the products are more stable than the reactants.


5.1.2 State that combustion and neutralization are exothermic processes.

Combustion and Neutralization are both exothermic processes. The energy required to hold the organic substance, acids and alkali is much higher than salts and atmospheric gases.

The products of combustion are carbon dioxide and water, which is much lower than the organic substances.

The products of neutralization are salts and water, which is much lower than acid and alkali substances.

5.1.3 Apply the relationship between temperature change, enthalpy change and the classification of a reaction as endothermic or exothermic.

Temperature change

- Positive change is exotheric - release heat
- Negative change is endothermic - takes in heat

Enthalpy Change

- Positive change is endothermic - takes in energy
- Negative change is exothermic - release energy

The ideas do not contradict because energy is equal to heat energy, unless the energy released is in a different form such as sound.


5.1.4 Deduce, from an enthalpy level diagram, the relative stabilities of reactants and products, and the sign of the enthalpy change for the reaction.

A substance with too much energy isn't as stable as substances with less energy.

This is how a enthalpy level diagram should look like for exothermic and endothermic

Topic 5: Energetics

Topic 5 of the IB HL Chemistry syllabus is the Energetics. IBO recommends to spend 8 hours on this topic.

This topic has 4 sub-chapters: "Exothermic and endothermic reactions", "Calculation of enthalpy changes", "Hess's law" and "Bond enthalpies". Each are separated with numerical values in order of mentioned.

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