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12 2 Unit 4 Lesson 9

Can we measure the energy released from the combustion of different fuels?

Learning Objectives:

  • Understand the concept of calorimetry and how it is used to quantify energy
  • Apply endothermic and exothermic reactions to calorimetry
  • Analyze energy when doing calorimetry
  • Calculate energy of combustion using q = mcΔT
  • Calculate moles using mass in grams
  • Honors - compare bond energies with total enthalpy of reaction

Warm-up:

  • Attendance 
  • Open Student IMT Unit 4, make sure lesson 8 is complete. 
  • Write down the question for the day and complete the wonderings section for Lesson 9.  

IN-CLASS WORK:

OUT-OF-CLASS WORK:

  • Open IMT for Unit 4 and make sure all sections for lesson 9 are complete. 
  • Make sure student sheet L4.9 is complete and complete the two Edpuzzles on schoology before our next class.
  • Review - balancing equations, go here to practice, also more here Balancing Equations

L4.9 Student Sheet

*Lesson 4.9: Can we measure the energy released from the combustion of different fuels? 

RECAP

  1. Let’s review balancing equations, click here

  2. How many carbon atoms are in a mole of methane? How many hydrogen atoms are in a mole of methane? 

  3. How many particles are found in 1 mole of methane?

  4. Calculate the molar mass of methane, CH4, including correct units.

Can we find the energy released in a reaction using bong energy

Calculate the bond energy from the reactants and products and then combine them to determine the overall energy released from two combustion reactions. 

Bond Bond Energy (kJ/mol)
C - C 347
C - H 413
C - O 358
C = O 799
O = O 495
O - H 467
  1. Using the models, balanced equations, and chart above, fill in the chart with bond energies to determine the overall energy for each of the reactions.

METHANOL COMBUSTION | What bonds are being BROKEN? | What is the bond energy? | How many of each? | What is the Total energy in this bond type? | TOTAL energy required to break the bonds in the balanced equation: | | ---------------------------- | ------------------------ | ----------------- | ------------------------------------------- | ------------------------------------------------------------------ | | C - H | 413 | 3 | 1239 | 2064 | | C - C | 347 | 0 | | ^ | | O - H | 467 | 1 | 467 | ^ | | C - O | 358 | 1 | 358 | ^ | | O = O | 495 | 0 | | ^ | | | | | | |

What bonds are being MADE?

What is the bond energy? 

How many of each?

What is the Total energy in this bond type?

TOTAL energy released from forming bonds in the balanced equation:

O - H

467

C = O

799

ETHANOL COMBUSTION

What bonds are being BROKEN?

What is the bond energy? 

How many of each?

What is the Total energy in this bond type?

TOTAL energy required to break the bonds in the balanced equation: 

C - H

413

C - C

347

C - O

467

O - H

358

O = O

495

What bonds are being MADE?

What is the bond energy? 

How many of each?

What is the Total energy in this bond type?

TOTAL energy released from forming bonds in the balanced equation:

O - H

467

C = O

799

  1. Using the equation, find the overall reaction energy for the combustion of methanol.

Overall reaction energy = energy absorbed for breaking bonds – energy released from forming bonds

  1. Divide your answer by the coefficient in front of methanol in your balanced equation.  This will tell you how much energy was released per mole of methanol.

  2. From calculations from above, find the total energy needed to break the bonds in the reactants.

  3. Using the equation, find the overall reaction energy for the combustion of ethanol.

Overall reaction energy = energy absorbed for breaking bonds – energy released from forming bonds

  1. Divide your answer by the coefficient in front of ethanol in your balanced equation.  This will tell you how much energy was released per mole of ethanol.

  2. By looking at your answers, which fuel releases more energy per mole - methanol or ethanol?  

  3. Give two reasons why is ethanol the preferred additive to gasoline instead of methanol?

How can we measure the energy released in a chemical reaction? 

Introduction: 

Using common chemistry lab techniques, we can measure the amount of energy released from a chemical reaction by placing a beaker of water above the reaction and recording the change in temperature before and after. If we assume that all of the energy from the reaction goes into the water, then the energy released from the reaction is approximately equal to the energy absorbed by the water. This is due to the law of conservation of energy. We will record the mass of the water and the temperature before and after the reaction to determine the total energy absorbed. This method of determining the amount of energy released from a reaction is known as calorimetry. It’s also the same process that food scientists use to determine the amount of energy in calories in the foods you eat.  

When we know the mass, specific heat (how well something holds heat), and change of temperature of the system or the surroundings, then we can calculate the energy transferred using the equation: 

q = mcsΔT  

q = heat in joules

m = mass in g

cs = the specific heat capacity

ΔT = the change in temperature in oC

Because the object and surroundings are in contact, the energy transferred between the object and surroundings is equal and is measured using the equation –q = +q.

The –q represents the releasing energy, which indicates an exothermic process (Look at the picture below).  An exothermic process is where an object loses energy to the surroundings and the surroundings gain the same amount of energy according to the law of conservation of energy.   

The +q represents gaining energy, which indicates an endothermic process (Look at the picture object gains the same amount of energy according to the law of conservation of energy.   

The amount of energy transferred from an object to the surrounding or from the surrounding to an object is equal in magnitude due to the law of conservation of energy, but not in sign. 

Practice Problems for q = mcsΔT

  1. How much energy is needed to change the temperature of 50.0 g of H2O by 150C? 

  2. How many grams of water can be heated from 20.0oC to 75.0oC using 12500.0 Joules?

  3. What is the final temperature after 840 Joules is absorbed by 10.0g of water at 25.0oC?

  4. What is the specific heat of aluminium if the temperature of a 28.4 g sample of aluminum in increased by 8.10C, when 207J of heat is added? 

  5. What is the specific heat of silicon if the temperature of a 4.11 g sample of silicon is increased by 3.80C when 11.1 J of heat is added? 

  6. How much heat must be added to an 8.21g sample of gold to increase it’s temperature by 6.20C? The specific heat of gold is 0.13 j/g 0C

NEXT STEPS:

  1. Reflect on today’s question: Why does burning one mole of gasoline result in more energy than burning one mole of hydrogen?

  2. Open up the IMT for this unit, complete all boxes for lesson 9

  3. Make sure all parts of the L4.9 student sheet are complete & complete the check for understanding on schoology. 

REVIEW & REINFORCE:

Use the following sections of the textbook to help you review materials that were covered in this lesson

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