Bond Enthalpy: Definition, Calculations & Values

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  • 0:00 Definition
  • 1:10 Bond Enthalpy Values
  • 2:06 Enthalpy Change of Reaction
  • 3:12 Calculation Example 1
  • 4:26 Calculation Example 2
  • 5:54 Lesson Summary
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Lesson Transcript
Instructor: Nissa Garcia

Nissa has a masters degree in chemistry and has taught high school science and college level chemistry.

When a chemical reaction occurs, chemical bonds are broken and formed. Energy is required to break bonds when a chemical reaction occurs, and this is called the bond enthalpy. In this lesson, we will discuss the bond enthalpy in chemical reactions.


When we were growing up and we were playing with our own set of building blocks, we would piece things together and break them apart, depending on what we wanted to make. For housing and new developments, old structures sometimes need to be remodeled or totally demolished to build a new structure and design. Whether it's building blocks or houses, breaking things and putting them together to make something new is something that requires energy.

When it comes to chemical equations, just like in building blocks, to make something new, chemical bonds are broken so that new bonds can be formed. Breaking bonds require energy, and this energy is what we call bond enthalpy.

Bond enthalpy, also known as bond energy, is the energy that is needed to break a particular bond in a gaseous compound. The unit that expresses bond enthalpy is kilojoules per mole, or kJ/mol. For example, the energy required to break one bond between C and H in the compound methane, CH4, is 413 kJ/mol.

Bond Enthalpy Values

The bond enthalpy values is the value of the energy (the unit is in kJ/mol) required to break a particular chemical bond at a temperature of 298 K or 25 degrees Celsius. For each chemical bond, there are different values of bond enthalpies.

If the bond energy is high, then that means a high energy is needed to break this bond because the bond is strong and most likely, the molecule is stable and not very reactive. For instance, you spend more energy if you are trying to break apart something that is nailed together as compared to something that is taped together.

Double bonds have higher bond enthalpies than single bonds because double bonds are harder to break. So, it also follows that triple bonds generally have higher bond enthalpy values than double bonds and single bonds. Just like a stack of paper, if you try to tear it, it is very difficult, as compared to when you try to tear a single sheet of paper.

Enthalpy Change of Reaction

When bonds are broken and formed, there is an energy change involved, so the enthalpy of the reaction changes. The values of bond enthalpies of different bonds can be used to calculate the enthalpy change of a reaction. The enthalpy change of a reaction is the amount of energy absorbed or released in a reaction. The enthalpy change is indicated by the symbol deltaH and can be calculated by the following formula:

Enthalpy Change of Reaction Formula

The value of the enthalpy change of a reaction can determine if the reaction is endothermic or exothermic. If the enthalpy change of reaction is negative, then the reaction is exothermic, meaning energy is released and stronger bonds are formed than the ones that are broken.

Exothermic Reaction

If the enthalpy change of reaction is positive, then the reaction is endothermic, meaning, energy is needed or absorbed to break the bonds and weaker bonds are formed than the ones that are broken.

Endothermic Reaction

Calculation Example 1:

Let's go over some examples on how to calculate the enthalpy change of reaction:

Calculate the enthalpy change of the reaction of the reaction between hydrogen gas and chlorine gas to form hydrochloric acid. Is this reaction endothermic or exothermic?

Example 1 Reaction


It is best to draw the structural formula so we can see what the bonds present are in the chemical compound.

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