Skip to content

11 28 Unit 4 Lesson 7

Can we explain why hydrogen has such a high energy output per gram compared to other fuels?

Learning Objectives:

  • Describe the general relationship between bond energy and bond length
  • Evaluate if energy is absorbed or released when bonds form and when bonds are broken
  • Compare and contrast endothermic and exothermic reactions
  • Use the law of conservation of matter to balance chemical equations
  • Compare and contrast the combustion of octane to the combustion of hydrogen

Warm-up:

  • Attendance 
  • Open Student IMT Unit 4, make sure lesson 6 is complete. 
  • Write down the question for the day and complete the wonderings section for Lesson 7.  

IN-CLASS WORK:

  • Open L4.7 student sheet and make a copy  
  • Review L4.7 and complete all parts of the IMT for lesson 7.

OUT-OF-CLASS WORK:

  • Open IMT for Unit 4 and make sure all sections for lesson 7 are complete. 
  • Make sure student sheet L4.7 is complete and complete the check for understanding on Schoology.

L4.7 Student Sheet

*Lesson 4.7: Can we explain now why hydrogen has such a high energy output per gram compared to other fuels? 

RECAP

  1. Let’s review what we figured out about covalent bonds. 

    1. What happens to the electrons in the covalent bonds in molecules like CH4, CO2, H2, and H2O?

    2. The octet rule says that atoms form bonds to fill their valence shells and achieve stability.  Complete the table for H, O, N, C.

Element Name Element Symbol No. Valence Electrons No. Covalent Bonds
Hydrogen H 1 1
Oxygen O 6 2
Nitrogen N 5 3
Carbon C 4 4

  1. Let’s review what we figured out from our magnetic sphere investigations and from the bond energies and bond length charts.

    1. Atoms are composed of positively charged nuclei surrounded by a negatively charged cloud of electrons. When bonds form, the nuclei of one atom attract the electrons of another atom, pulling the atoms toward each other.  What is the definition of bond length?
    2. What is the definition of bond energy?
    3. Revisit data from our last class on bond energy and bond length. What is the general relationship between bond energy and bond length?

  2. Let’s review what we learned from the lesson where we observed the temperature changes for two different chemical processes.

    1. What do we call reactions that result in a decrease in temperature?
    2. What do we call reactions that result in an increase in temperature?

  3. Let’s recall what we learned about bond-breaking and bond-forming.  Think about a magnet coming together or being pulled apart.

    1. Does bond breaking require or release energy? Breaking bonds requires energy
    2. Is breaking bonds endothermic or exothermic? Breaking bonds is endothermic
    3. Does bond making require or release energy? Bond making releases energy because the particles have reached a lower state of energy by bonding.
    4. Is forming bonds endothermic or exothermic? Forming bonds is exothermic

  4. Below is a diagram of a bond forming and breaking of O2.  Images from: MolView

    1. What stays the same during bond formation and bond breaking? 
    2. What changes in bond formation and bond breaking?

MODELING and COMPUTATIONS

  1. To answer part of our unit question of “Why does burning hydrogen release more energy than burning octane?”, we must look at the chemical reactions and the bonds forming and breaking. 
    1. Using two model kits, in groups of four, model both chemical reactions.
Process Equation
Combustion of Hydrogen 2H2(l) + O2(l) → 2H2O (g)
Combustion of Octane 2C8H18 (l) + 25O2 (g) → 18H2O (g) + 16CO2 (g)
  1. (cont)
    1. In your model count and record how many of each element you have on each side of the equation.
    2. For each process, do you have the same number of each element on both the reactant and product side of the equation? 
    3. Matter cannot be created or destroyed.  This is the law of conservation of matter.  Chemical equations must follow this law. We call this balancing chemical equations.  
      1. Work with another two groups to model the balanced chemical equations for combustion.  Add to your model so that you have the same number count of each atom as products and reactants. Use this table to help you. Balanced Chemical Equations - ELEMENT count

-tx- | Process | Element | No. of each element
as the REACTANT | No. of each element
as the PRODUCT | | -------- | ------- | -------------------------------------- | ------------------------------------- | | Hydrogen | H | 4 | 4 | | ^^ | O | 2 | 2 | | Octane | H | 18 | 18 | | ^^ | O | 50 | 50 |

  1. Take and insert a photo of your group’s balanced chemical equations. 
  2. Use your models to complete the table by counting the number of each molecule in your balanced equation for each process.
Process Molecule No. of each Molecule
Hydrogen H2 2
^^ O2 1
^^ H2O 2
Octane C18H18 2
^^ O2 25
^^ H2O 18
^^ CO2 16
  1. We add a coefficient in front of each molecule in a balanced equation to indicate the number of each molecule. Coefficients are greater than one (2,3,4,5…). We do not use one as a coefficient in balanced equations. In the blank lines provided, add the coefficients to show that you have a balanced equation that obeys the law of conservation of mass. 

BALANCED CHEMICAL EQUATIONS

-tx- | Balanced Chemical Equations || | Process | Equation | | ---------------------- | --------------------------------------------------------- | | Combustion of Hydrogen | 2 H2 (l) + O2 (l) → 2 H2O (g) + energy | | Combustion of Octane | 2 C8H18 (l) + 25 O2 (g)→ 18 H2O (g) + 16 CO2 (g) + energy |

Practice Balancing: Use the PhET Simulation to balance these equations

More to practice, here’s the balancing equation practice sheet

NEXT STEPS:

  1. Reflect on today’s question: Can we explain now why hydrogen has such a high energy output per gram compared to other fuels?

  2. Open up the IMT for this unit, complete all boxes for lesson 7

  3. Make sure all parts of the L4.7 student sheet are complete & complete the check for understanding on schoology.**

Last update: June 5, 2023
Created: June 5, 2023