Bond enthalpies
5.3 Bond enthalpies (3 hours)
Pause for thought
Be careful when you make up questions using average bond enthalpies to arrive at enthalpy changes for a reaction. Usually it will give results that are quite close to the literature values - but not always. Consider the case of using average bond enthalpies for determining the enthalpy of combustion of methane.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
Using the values from Section 11 of the IB data booklet the energy that has to be put in to break the bonds is 2652 kJ mol-1 (made up of 4 x C-H + 2 x O=O, i.e. (4 x 414) + (2 x 498) = 2652 kJ mol-1). The energy given out when the new bonds are formed is 3468 kJ mol-1 (made up of 2 x C=O + 4 x O-H, i.e. (2 x 804) + (4 x 463) = 3460 kJ mol-1). This means that overall the reaction is exothermic and the energy released is 808 kJ mol-1 or put another way ∆Hc(CH4) = – 808 kJ mol-1 .
The problem is that this does not agree with the value for the enthalpy change of combustion of methane which is given in Section 13 as – 891 kJ mol-1.
The astute student will pick up on one wrong assumption straightaway. Average bond enthalpies refer to the gaseous state. From the definition of enthalpy of combustion the water is in the liquid state. To correct for this we need to add the energy evolved when two moles of gaseous water condense to form two moles of liquid water. The enthalpy of vaporization of water is 44 kJ mol-1 so this brings our revised enthalpy of combustion value to – 896 kJ mol-1. This is still 5 kJ mol-1 different to the literature value so what other assumption is wrong?
The obvious answer is that average bond enthalpies have been used and these will vary slightly from compound to compound and probably account for the relatively small difference of 5 kJ . It is also worth looking at the average bond enthalpy value used for the C=O bond. Because carbon dioxide contains two C=O bonds originating from the single carbon atom the average bond enthalpy is not the same as for compounds that only contain one C=O bond. The current data booklet (for first exams in 2016) has the value of 804 kJ which is more or less correct for carbon dioxide whereas the old data booklet (for first exams in 2009) has the value of 746 kJ mol-1 which is of course very different and is the value when only one C=O bond is being referred to as in aldehydes and ketones.
Nature of Science
Measured energy changes can be explained using the model of breaking and forming bonds.
Agreement with empirical data depends on the sophistication of the model.
Data obtained based on the model can be used to modify theories where appropriate.
Learning outcomesAfter studying this topic students should be able to: Understand:
Apply their knowledge to:
| Clarification notesThe bond enthalpy values given in Section 11 of the data booklet should be used. International-mindednessThe pollution that causes stratospheric ozone depletion ('holes' in the ozone layer) particularly in the polar regions comes from a variety of countries and sources. International action and cooperation (e.g. The Montreal Protocol) have helped to lessen the problem of ozone depletion. |
Teaching tipsBond enthalpies follow on neatly from Hess's law. I get students to write the equation for the formation of methane from its elements and consider what has actually happened. The carbon is initially in the solid state so needs to be atomised to give carbon in the gaseous state. Hydrogen is already a gas but it is in the molecular form so also needs to be atomised to give gaseous hydrogen atoms. By knowing the enthalpy of atomisation of carbon and hydrogen and the enthalpy of formation of methane the energy evolved when four C-H bonds in the gaseous state are formed from their gaseous elemental atoms can be determined by using Hess's law - and hence the value for the C-H bond in methane can be calculated. There are several key points to stress about average bond enthalpies. Firstly the quantity of energy is exactly the same whether a bond is being broken or formed but the first process is endothermic and the second process is exothermic. Secondly bond enthalpies exclusively apply only to the gaseous state. If other states are involved then there will be addition enthalpy terms to take into consideration. Thirdly what is mean by average? For some bonds such as H-H or F-F there is no average as they only exist in a single compound (in these cases hydrogen gas and fluorine gas). For other bonds average means in similar compounds. This qualification should be included when students are asked to define average bond enthalpy. Stress also the variation in bond strengths by giving example of weak bonds (such as F-F) and strong bonds (such as O-H). This then leads on to explain why reactions are exothermic or endothermic. It is also worth stressing that generally the order of bond strength is triple > double > single. You will need to include a discussion about the O to O bond in both oxygen and ozone and how this enables the ozone layer to be in a steady state equilibrium. This actually neatly overlaps with one of the Higher Level applications in Topic 14.1. | Study guide
Pages 41 (& 33 for the ozone layer) QuestionsFor ten 'quiz' multiple choice questions with the answers explained see MC test: Bond enthalpies. For short-answer questions which can be set as an assignment for a test, homework or given for self study together with model answers see Bond enthalpy questions. Vocabulary listAverage bond enthalpy IM, TOK, 'Utilization' etc.See separate page which covers all of Topic 5 |
Teaching slides
Teachers may wish to share these slides with students for learning or for reviewing key concepts.
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Other resources
1. This is an interesting video produced by Ohio State University. Superficially it looks to be a straightforward tutorial on how to use average bond enthalpies to determine the enthalpy of combustion of propane. Ask students to look at it critically. They should notice that the result of -3361 kJ mol-1 obtained in the video by using average bond enthalpies is very different to the literature value of -2219 kJ mol-1 given in Section 13 of the IB data booklet. The tutorial ignores the definition of bond enthalpies and calculates the energy evolved when 5H2O(l) is formed by using just O-H bond enthalpies for the gaseous state i.e. it ignores the change of state from gaseous water to liquid water and the extra 4 x 44 kJ mol-1 of energy given out. Apart from the water problem the value for the C=O bond used is 1072 kJ mol-1. As we saw in ‘Pause for thought’ above, the IB data booklet value is 804 kJ mol-1 . By using 6 x 804 kJ mol-1 instead of 6 x 1072 kJ mol-1 for C=O and adding the factor of 4 x 44 kJ mol-1 for the change of state of the four moles of liquid water formed the answer comes out to -2229 kJ mol-1 which is close to the correct value of -2219 kJ mol-1. By getting your students to look critically at this video you can give them a good insight into TOK – they should not just trust the source of their knowledge even if it comes from a university! They should look at all sources of knowledge critically!
2. Defining average bond enthalpy the IB way from Richard Thornley, an IB Chemistry teacher at the International School of Genoa.
Defining average bond enthalpy