Chapter 1. Symbolic Descriptions

1.1 Symbolic Descriptions

Lecture X
Make It Or Break It: Bond Energies

Key Idea:
Breaking bonds requires energy.
Making bonds releases energy.

Slide development notes: It might be nice to have a picture/video of a burning log on this intro slide.
Should we call it something other than script?...)

Script for instructor:

  • This lesson is about the relationship between energy changes and the rearrangement of atoms. Atoms in molecules are bonded together. Energy is required to break these bonds. The opposite is also true: the same amount of energy is released when that same bond forms.
  • Consider a burning log. The wood is reacting with the oxygen in the air. The questions to consider are: Why do we see a flame? Why are the products of the reaction hot? Why is energy released even though the bonds holding the atoms together in the wood and oxygen are broken?

1.2 Bond Energy

A chemical bond is like a spring linking two atoms together to form an A-B molecule.

a-b molecule

Energy is always required to break bonds.

Slide development notes: This could be animated to show that energy is required to stretch the spring.

Script for instructor:

  • Imagine that the chemical bond between two atoms is like a spring.
  • How is energy involved in order to separate the atoms from one another? Is energy required or released? [required]
  • What happens if you stretch the spring and let go? [the atoms move toward each other to an "equilibrium" position.]
  • Is energy required or released? [released]
  • How can you prove that energy is released? [If the spring were hanging vertical and stretched downward, the spring would contract upward against gravity once released.]

Possible props: Magnetic marbles to show that energy is required to pull them apart, but that they attract one another well they are rolled so that they pass one another in close proximity.

1.3 Forming Chemical Bonods


Script for instructor:

  • The chemical bond is a form of potential energy. It is an attractive interaction between the positive charges on the nucleus of one atom with the negative charges on the electrons of the second atom.
  • More negative potential energy is more stable. You need to add energy to increase the potential energy and thereby separate the atoms.
  • The potential energy due to gravity provides a good analogy. Your potential energy is lowest and you are most stable standing on the ground. You need to expend energy to increase your potential energy to lift your body above the ground (e.g., by jumping).
  • The graph shows what happens as two atoms approach one another. The potential energy gets more negative due to attractive interactions as the distance between the atoms decreases. However, only to a point. If you try to push the atoms even closer, the positive charges on the nuclei of the two atoms repel one another. The potential energy increases sharply at short distances and the interaction becomes repulsive. You can determine the average bond length and the average bond enthalpy from the graph. Bond enthalpy is a measure of bond energy, the energy required to break a specific bond between two atoms.

1.4 Forming Molecules

2H(g) H2(g) 2O(g) O2(g)

Forming Molecules

Changes in PE result in changes in KE
since the total energy of an isolated system does not change.

(However, most systems are in contact with the surroundings!)

Slide development notes: Perhaps have the atoms moving randomly in straight lines rather than showing arrows. The longer arrows in the box on the right indicate faster average speeds. Also, note that the H atoms are moving faster than the O atoms in the box on the left. Likewise H2 is moving faster than O2.

Script for instructor:

  • Consider two different ways to arrange H and O atoms. The box on the left shows 8 H atoms and 4 O atoms all moving independently. The atoms are not bonded to one another so the potential energy of attraction is zero. The atoms are moving with an average kinetic energy proportional to the temperature. (Note that the average speeds of the H atoms are faster than the O atoms because the mass is less, as indicated by the length of the arrows..)
  • Now imagine that the atoms are allowed to rearrange to form H2 and O2 molecules as shown in the box on the right. Notice that there are still 8 H atoms and 4 O atoms. The potential energy is more negative because the atoms are attracting one another; bonds have formed.
  • If the box is isolated, energy is not exchanged with the surroundings. Thus, the total energy of the system (the contents in the box) remains constant. If the potential energy becomes more negative due to bond formation in the box on the right, the kinetic energy becomes more positive so that the sum PE + KE is the same as in the box on the left.
  • Higher kinetic energy means the molecules are moving faster, as indicated by the length of the arrows..) The temperature is much higher. The H2 and O2 molecules are hot!

1.5 Explosions

2H2(g) + O2(g) 2H2O(g)

Symbolic Description Explosions

In this reaction, H-H and O-O bonds are broken to form O-H bonds. The average bond strength of the products is stronger.

Stronger bonds = larger negative PE

Slide development notes: Same as previous slide. The H2O molecules are really moving fast.

Script for instructor:

  • There is yet another way to arrange 8 H and 4 O atoms. They can bond together to form 4 H2O molecules. This is the same number of atoms.
  • When 4 H2 molecules and 2 O2 molecules rearrange to form 4 H2O molecules, what energy is associated with this change for an isolated system?
  • This reaction often leads to an explosion because the H2O molecules are moving extremely fast, creating a very high pressure. The temperature of these molecules is very high because the kinetic energy is large and positive.
  • If the sum of PE + KE is the same in both boxes, then the large kinetic energy of the H2O molecules indicates that the potential energy must be more negative. In other words, the average energy of attraction between atoms in H2O molecules is more negative than the average energy when the atoms are arranged as H2 and O2 molecules. The O-H bonds in H2O are stronger.
  • Show simulation: This is a good place to review the simulation showing this reaction. [Link]

1.6 Making and Breaking Bonds

How Much Energy Is Involved?

Making and Breaking Bonds

Script for instructor:

  • Consider the energy involved in the reaction between 2 H2 and O2 to form 2 H2O.
  • Energy is required to break the H-H bonds (+436 kJ/mol) and O=O bonds (+497 kJ/mol). Notice that the numbers are positive indicating that energy is required.
  • Energy is released when O-H bonds are formed (−463 kJ/mol). Notice that the number is negative indicating that energy is released.
  • Possible props: Model kit with 2 H2 and 1 O2 molecule built. Break all the bonds. Make 2 H2O molecules.

1.7 Combustion of Hydrogen

2H2(g) + O2(g) 2H2O(g)

Combustion of Hyrdrogen

Break 2 moles of H-H bonds: 2 mol (+436 kJ/mol)

Break 1 mole of O=O bonds: 1 mol (+498 kJ/mol)

Make 4 moles of O-H bonds: 4 mol (-463 kJ/mol)

rxn ~ -482 kJ or 241 kJ/mol of gaseous water formed

Script for instructor:

  • When H2 and O2 rearrange to form H2O molecules, the potential energy becomes more negative. The kinetic energy becomes more positive by the same amount.
  • When the system is placed in contact with the surroundings, the hot H2O molecules with high kinetic energy transfer energy to the surroundings until thermal equilibrium is reached. It is possible to use the energies required to make and break bonds in order to estimate the energy transferred to the surroundings:
  1. Calculate the energy required to break the bonds of 2 H2 molecules and an O2 molecule. The potential energy of the atoms is less negative.
  2. Calculate the energy released when 2 H2O molecules are formed (4 O-H bonds).
  3. Sum the values. The sum is the energy released due to the reaction.
  • Notice that the value is expressed as ΔHrxn which is called the "enthalpy of the reaction" or the "heat of reaction". The value calculated is for the reaction as written: 2 moles H2, 1 mole O2, and 2 moles H2O. The heat of reaction per 1 mole H2O is the value calculated divided by 2.

[rxn needs to be done in math type with superscript and subscript stacked (not staggered as shown)]

1.8 Energy of Activation

2H2(g) + O2(g) 2H2O(g)

Why do you need a spark?

Why Do You Need a Spark?

A spark is needed before the reaction can proceed. But once started, the reaction continues without additional input of energy.

Script for instructor:

  • You may have noticed that reactions do not necessarily starting on their own. For example, logs can exist for long periods of time without burning (reacting with oxygen). Until there is a spark.
  • Recall the spark in the animation. What was its role? [The spark broke a bond and the resulting atoms were very reactive.] In reality, a spark is needed to increase the kinetic energy of the molecules so that they collide with one another with more energy.
  • This is a bit confusing. You need to add energy to make the reaction happen, but we just calculated that a lot of energy is produced. The energy needed to get the reaction started is called the “activation energy.” Once this energy is added, no further energy is required because the reaction itself produces energy to continue the process of rearranging the atoms to form product molecules.

Possible demo: Produce soap bubbles filled with hydrogen and oxygen. Show that a spark cause the reaction to proceed. You do not need to keep adding heat. Show the electrolysis of water (the reverse reaction). When you add enough energy, you can observe H2 and O2 gas. As soon as you stop adding energy, the gas production stops. This is because energy is required to decompose H2O molecules to the elements.

1.9 Energy of Activation

2H2(g) + O2 2H2O(g)

Do all bonds need to be broken?

Do all bonds need to be broken?

The rate of the reaction indicates that the barrier is only 200 kJ/mol, not 1370 kJ/mol.

Script for instructor:

  • Notice that in the calculation we did, we assumed that all the bonds broke and then new bonds were formed. This requires 1370 kJ/mol to break the bonds in 2 moles of H2 and 1 mole of O2. Then 1852 kJ/mol are released in forming O-H bonds in H2O molecules. The difference is ΔHrxn = +1370 − 1852 = −482 kJ/mol. Notice that the sign is "+" when energy is required and "−" when energy is released.
  • More generally, the enthalpy of a reaction can be estimated as:
  • ΔHrxn = sum of reactant bond energies − sum of the product bond energies
  • One final note: The activation energy is typically much lower than the energy required to break all the bonds. For example, rather than breaking the H2 and O2 bond, it is possible for the two molecules to approach each other and form 2 OH molecules. The “” indicates an unpaired electron. Thus, OH is unstable and very reactive causing the reaction to continue to products.

1.10 Average Bond Enthalpies (kJ/mol)

Bond Average bond enthalpies Bond Average bond enthalpies
C-C 348 kJ/mol C-F 484 kJ/mol
C=C 612 kJ/mol C-Cl 338 kJ/mol
CΞC 838 kJ/mol C-Br 276 kJ/mol
O-O 146 kJ/mol C-I 238 kJ/mol
O=O 498 kJ/mol H-H 436 kJ/mol
N-N 163 kJ/mol H-C 416 kJ/mol
N=N 409 kJ/mol H-O 463 kJ/mol
NΞN 949 kJ/mol H-F 565 kJ/mol

What patterns do you notice?

Script for instructor:

  • Chemists have created tables of average bond enthalpies for the purpose of estimating enthalpies of reactions.
  • This is a place where students can engage with the discussion questions on patterns of bond enthalpies. [Link]

1.11 Average Bond Enthalpies (kJ/mol)

Weak Average Strength Very Strong!
Cl-Cl 242 kJ/mol C-H 416 kJ/mol NΞN 946 kJ/mol
O-O 146 kJ/mol C-C 348 kJ/mol C=O* 743 kJ/mol
N-O 210 kJ/mol H-H 436 kJ/mol CΞC 838 kJ/mol
C-O 360 kJ/mol
O-H 467 kJ/mol
O=O 498 kJ/mol

*C=O bond enthalpy in CO2 is 803 kJ/mol!

Script for instructor:

  • Notice which bonds are strong and which bonds are weak. Common reactants in combustion reactions are molecules with C and H. Common products of combustion reactions and CO2 and H2O.
  • Explain why based on bond enthalpies. [C−C and C−H bonds in alkane molecules are weaker than O−H and C=O in H2O and CO2]

1.12 Combustion of Methane

2H2(g) + O2(g) 2H2O(g)

Combustion of Methane

Break 4 moles of C-H bonds: 4 mol (+416 kJ/mol)

Break 2 moles of O=O bonds: 2 mol (+498 kJ/mol)

Break 2 moles of C=O bonds: 2 mol (-803 kJ/mol)

Make 4 moles of O-H bonds: 4 mol (-467 kJ/mol)

rxn ~ -814 kJ/mol of CH4 combusted [Measured value: -802 kJ/mol]

Script for instructor:

  • Ask students to use bond enthalpies to calculate the enthalpy of reaction for the combustion of methane, CH4.
  • Compared the value calculated from average bond energies is a good prediction of the measured value. The calculated value is not exact because the bond energies are averages.

1.13 Combustion Reactions

Combustion Reactions

2H2(g) + O2 → 2H2O(g)

CH4(g) + 2O2(g) → 2H2O(g) + CO2(g)

2C4H10(g) + 13O2(g) → 10H2O(g) + 8CO2(g)

Which fuels burns to release the most heat per mole?

ΔH ~ -241 kJ/mol of H2O

ΔH ~ -814 kJ/mol of CH4

ΔH ~ -2653 kJ/mol of C4H10

Which fuel burns to release the most heat per gram?

~-121 kJ/mol

~-51 kJ/g

~-46 kJ/g

Script for the instructor:

  • Let’s compare the combustion of three gases: hydrogen, methane, and butane. The gases are contained in a balloon. A small spark will cause the balloon to pop, the gases will mix with oxygen, and products will form.
  • Which fuel releases the most heat per mole?
  • We calculated the values for the combustion of H2 and CH4. Now do the calculation for C4H10. Predict which reaction will transfer the most heat.
  • Show demo: Either perform this as a demo or show the video. [Link]
  • How do you know that the balloons contained roughly equal moles of gas? [Pext, T, and V are approximately the same. Therefore, n = # of moles is equal.]
  • Are the balloon equivalent in mass? Explain. [Butane is the heaviest because it has the largest molar mass. Hydrogen is the smallest because it has the smallest molar mass.]
  • Which fuel releases the most heat per gram? Do the calculation. [hydrogen]
  • Why might heat per gram be important? [If the fuel needs to be moved around, more energy will be required to move the heavier gas.]
  • Why might heat per liter be important? [Gaseous molecules occupy a lot of space. In contrast, the liquid fuel we put in our cars takes up much less volume.]

1.14 Calories in a Donut

C6H12O6(s) + 6O2(g) ? 6CO2(g) + 6H2O(g)

Break 7 moles of C-H bonds: 7 mol (+416 kJ/mol)

Break 7 moles of C-O bonds: 7 mol (+360 kJ/mol)

Break 5 moles of C-C bonds: 5 mol (+348 kJ/mol)

Break 5 moles of O-H bonds: 5 mol (+467 kJ/mol)

Break 6 moles of O=O bonds: 6 mol (+498 kJ/mol)

Make 12 moles of C=O bonds: 12 mol (-803 kJ/mol)

Make 12 moles of O-H bonds: 12 mol (-467 kJ/mol)

?H°rxn ~ -2745 kJ ~ -2745 kJ /mol of glucose

[Measured value: -2538 kJ/mol]

Script for instructor:

  • Show the calculation for the calories in a donut. Assume the donut is all glucose, a form of sugar. The slide shows the energy required to break all the bonds in the sugar molecule and the energy released to form the product molecules.
  • This is a great opportunity to discuss the common misconception that breaking bonds releases energy.

Show video: A small piece of donut is heated gently and then placed in liquid oxygen. The reaction is spectacular.

  • Why might a person conclude that breaking bonds releases energy? [The donut breaks apart and there is a lot of heat, light, sparks]
  • How would you explain to someone that this is not the correct way to think about the reaction? [Breaking bonds requires energy. The energy released is due to the formation of strong bonds in the product molecules. The product molecules are gases, so they are not visible. This is what gives the appearance that it is the decomposition of the donut that releases energy. Not true!

1.15 Calories in a Donut

C6H12O6(s) + 6O2(g) ? 6CO2(g) + 6H2O(g)

?Hrxn (measured) = -2538 kJ/mol of glucose

1 kcal = 4.184 kJ
?Hrxn= -2538 kJ/mol of glucose = 607 kcal/mole

1 Food Calorie = 1 kcal
Molar mass of glucose = 180 g/mol
Food Calories per gram = 3.4 Cal/g

Glazed donut weights ~60g
Total Food Calories in a glazed donuts ~200 Food Cal

Script for instructor:

  • Show the conversion from kJ/mol for the reaction to Food Calories.
  • This same reaction occurs in your body stepwise (fortunately) when you eat a donut. You eat a donut and breathe in oxygen. You breathe out carbon dioxide and water. The energy from the reaction is transferred to your body to keep you warm, to allow you to move, and make new tissue, etc., and to send nerve signals.

1.16 Wrap-up points

  • Breaking chemical bonds always requires energy.
  • Chemical reactions proceed via a transition state which is a lower energy state than that corresponding to fully atomized reactants.
  • Average bond enthalpies can be used to estimate the enthalpies of chemical reactions.
  • Combustion reactions are often very exothermic due to the large negative enthalpy of the C=O bonds in CO2.
  • Review the wrap-up points