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.